text:
Anna Shiryaeva
email: anja.shiryaeva@gmail.com

Vladimir Sabelnikov
email: vladimir.sabelnikov@onera.fr

Critical analysis of classical turbulent combustion experiments on the basis of RANS simulations

The purpose of the work is to analyze the consistency of experimental data concerning classical turbulent combustion tests and their applicability for CFD programs validation. Efforts are directed to development and verification of a computationally efficient method for numerical simulation of turbulent combustion in a high-speed flow, based on RANS equations and taking into account turbulence-combustion interaction (TCI). Computations are fulfilled with zFlare module from application package EWT-TsAGI [1] which is based on in-house code ZEUS (TsAGI) [2] and intended to describe the flows of ideal compressible gases mixtures with variable heat capacities and nonequilibrium chemical reactions. Several kinetics models for H2 and hydrocarbon fuel combustion in air together with two-parameter turbulence models based on the Boussinesq hypothesis are implemented into the program. The paper describes some results of zFlare verification, as well as detailed analyses of experimental data. Five classical turbulent combustion experiments are regarded: Evans et al. experiments [3-4], Burrows & Kurkov [5], A.D.Cutler [6-7], Cheng et al. [8] and Moreau et al. backward-facing step combustor [9]. In addition, several approaches for TCI based on methods of Partially Stirred Reactor model (PaSR, [10]) have been studied and compared.

INTRODUCTION

Measurements in combustion chambers with supersonic flow are very expensive and usually do not provide sufficient accuracy due to the interaction of a large number of different physical effects. Therefore, the main experiments for validation are supersonic reacting jets that flow into the open space. Their advantage is complemented by good accessibility for optical measurements and visual observation, as well as a reduction in the number of measurement points due to axial symmetry. The number of available experimental works with supersonic flows with chemical reactions that could be used for program validation is very small [START_REF] Chomiak | Flame liftoff in diesel sprays[END_REF][START_REF] Hell | [END_REF]. From simple configurations, the authors know three experiments with supersonic axisymmetric jets, in which different measuring technologies were used.

The first studies were carried out by J. Evans et al. and are presented in [3][4]. These data are widely used for the validation of CFD programs so far -e.g., [4,[13][14][15][16][17][18]. In these experiments, pressure measurements were made by the Pitot tube and sampling of gas followed by concentrations measurement. That is why it is this test that was selected by the authors to be first to verify the program in view of the seeming simplicity and a large amount of experimental data. Unfortunately, the use of contact methods exerts a local influence on the combustion processes by introducing disturbances into the flow, cooling the flow and catalytic effects on the surface of the probe.

Another experimental work on the study of the free flow of a supersonic jet of hydrogen into a co-flow of air with M > 1 was carried out by T. Cheng et al. and generalized in [START_REF] Tedder | [END_REF]. In these experiments visual methods were used to measure temperature and concentration: spontaneous Raman scattering (SRS) and laser induced fluorescence (LIF). This work is very widely used now for CFD programs validation -e.g., [START_REF] Hell | [END_REF][18][19][START_REF] Moule | Modelisation et Simulation de la Combustion dans LES Ecoulements Rapides. Applications aux Superstatoreacteurs[END_REF][START_REF] Baurle | [END_REF][22]. Both measurement technologies are based on the detection of incoherent light. The emission of light in all directions at once leads to a very weak signal, especially in combustion chambers with limited access for optical investigations. As a consequence, these measurement technologies have limited usefulness in case of outflows into confined space.

One more series of similar experiments was fulfilled by A.Cutler et al. [6][START_REF] Cutler | AIAA Paper[END_REF]. In these works, a combination of non-contact methods such as coherent anti-Stokes Raman spectroscopy (CARS) and Rayleigh scattering was used to study the supersonic jet of hydrogen flowing into the air stream. But, although the setup was intended to study supersonic combustion, the measurements have so far been limited to a non-reactive flow [23][START_REF] Bivolaru | AIAA Paper[END_REF].

There is also, of course, a number of experimental studies devoted to the study of flows in a combustion chamber of a high-speed ramjet [START_REF] Tedder | [END_REF][26][27][28], but flows of this type are too complex to be used for validation and are not considered at this stage of the work. From the experimental data of this type only M. Burrows and A. Kurkov experiment with hydrogen burning in a plane near-wall jet differs in classical simplicity. However, this experiment is of the same period as Evans, Shexnayder and Beach experiment (70s of the XX century), and it is characterized by the same problemsthe absence of non-contact measurements of concentrations, the uncertainty in the formulation of the boundary conditions and the strong influence of gas dynamics on the flame structure.

The present study is aimed to improve the known and to develop new physical models of turbulent combustion in high-speed combustion chambers within RANS approach and their implementation in computer programs. The emphasis is on the assessment of turbulence-combustion interaction (TCI) influence account. As demonstrated in the work, TCI account applied to high-speed simple configuration flows had very little influence on the flow compared to other effects and didn't help to improve results. Thus an experiment was chosen where TCI influence was earlier [29] proved to be significant and PaSR based approach permitted to improve results. So, a low-speed experiment was used for studying PaSR based methods.

PHYSICAL AND NUMERICAL METHODS

All the computations were carried out with the use of zFlare program which is based on in-house code ZEUS (TsAGI) [START_REF] Moretti | [END_REF]. This program is focused on the flows of a of ideal compressible gases mixture with variable heat capacities and nonequilibrium chemical reactions. Due to high complexity of equation systems, only explicit finite volume numerical schemes are now implemented in the code. For RANS computations, a second order TVD scheme with van Leer limiter was adopted. Steady RANS computations were conducted using relaxation method. To accelerate it, local time step was employed. Steady solutions were successfully obtained on all computational grids. Two twoparameter turbulence models based on the Boussinesq hypothesis were implemented, tested and compared: (k-) SST Menter [START_REF] Petrova | Turbulence-chemistry interaction models for numerical simulation of aeronautical propulsion systems[END_REF] and (q-) Coakley [START_REF] Menter | [END_REF]. The first one is a widely recognized model for aerodynamic problems while the second one was developed primarily for high speed near-wall flows.

Several combustion kinetics models for hydrogen in air: Moretti [32], ONERA [33], [34] and hydrocarbon fuel in air were introduced into the program. In addition, a method for accounting for turbulence-combustion interaction has been introduced and is being developed. In this paper, an approach based on methods such as Eddy Dissipation Concept Model (EDC, Magnussen [START_REF] Vincent-Randonnier | Proceedings of the Combustion Institute[END_REF][START_REF] Magnussen | On the structure of turbulence and a generalized eddy dissipation concept for chemical reaction in turbulent flow[END_REF]) and Partially Stirred Reactor model (PaSR, [10]) is chosen to describe TCI.

In these methods it is assumed that, at high Reynolds numbers, chemical reactions occur mainly in the fine structures associated with the smallest turbulent vortices. These structures are non-uniformly distributed in space, and their volume is a fraction of the total volume occupied by the gas. Thus, regions containing fine structures seem to be immersed in the surrounding flow. It is in thin structures the molecular mixing, molecular heat transfer and chemical reactions occur most intensively. In PaSR models, fine structures are considered to be a reactor in which the reaction proceeds continuously. Molecular diffusion brings to the thin structures a fresh mixture from the space surrounding the fine structures, and also removes reaction products from the fine structures. Also, due to molecular diffusion, there is a heat exchange between the fine structures and the surrounding space. The main parameters of the model are the volume fraction of fine structures * and the characteristic residence time of the gas in reactor * . The state of gas in thin structures is described by a system of differential equations for a homogeneous reactor.

The most famous among PaSR class models is considered to be Magnussen EDC ("Eddy Dissipation Concept") model [START_REF] Vincent-Randonnier | Proceedings of the Combustion Institute[END_REF][START_REF] Magnussen | On the structure of turbulence and a generalized eddy dissipation concept for chemical reaction in turbulent flow[END_REF]. In this model, the reaction rate in fine structures is considered to be much larger than in the surrounding space. The state in thin structures is quasistationary and is described by an algebraic system of equations for the stationary state of a homogeneous reactor [START_REF] Ertesvåg | [END_REF]. The main parameters of Magnussen model are expressed through turbulence parameters -turbulence kinetic energy k, turbulence energy dissipation rate ε:

K k * 4 / 3 2 * ; 7 . 9
, where / is average kinematic molecular viscosity, V is the cell volume.

One can see that * is assumed to be equal to the characteristic lifetime of the smallest turbulent vortices One should note PaSR model shortcomings, associated with the lack of the fine structures state history account. Fine structures do not have sensitivity to what happens in them upstream of the point under consideration, and also do not consider the development in time. It is a very significant drawback, because under the same conditions of the averaged field, different states might be realized in the fine structures, with or without combustion, depending on the prehistory of the reactor, i.e. hysteresis effects are present. Moreover, there is no uniqueness in the choice of the initial state in the fine structures. In order to resolve these ambiguities, an extended version of PaSR model was proposed by V.A. Sabelnikov -"Extended PaSR (EPaSR)" [START_REF] Chomiak | Flame liftoff in diesel sprays[END_REF], in which it was proposed to abandon the quasistationary treatment of the fine structures and solve a partial differential equation system that takes into account the composition variation in time as well as convective and the diffusion transport of the fine structures in space. The model includes an additional set of differential equations for describing the state in the fine structures, which almost doubles the dimension of the final system of equations in comparison with the case without taking TCI into account. These equations are analogous to the equations for the average flow parameters with the addition of a term taking into account the contribution of the exchange between the fine structures and the surrounding space. This model was previously successfully applied [19,29] to the modeling of both high-speed and essentially subsonic flows with combustion.

A simplified version of EPaSR model was also proposed [START_REF] Sabelnikov | [END_REF]. In order to reduce costs it was proposed to abandon the description of convective and diffusion transport of the fine structures over space. Hence the fine structures are described by ordinary differential equations that take into account only local fine structures composition variation in time. It is a nonstationary version of PaSR model. The system of equations for reactor is no longer solved until the stationary state is reached. Instead only one cycle of Newton's iterations is done with the internal time step equal to the value of the physical step in time. But this model has the same disadvantage as the stationary variant of PaSR model, concerning isolation of the fine structures at an arbitrary point of flow.

Another new original TCI model of the same class -Generalized PaSR (GPaSR) -was proposed by V. A. Sabelnikov and was implemented and applied in practice in current work for the first time. The motivation for creating this model was the need for minimal modifications of the original PaSR model to approximate the spatial development of fine structures. The model does not require the solution of additional differential equations to describe the transfer of parameters of thin structures over space. However, convective transport along thin structures is taken into account approximately. GPaSR is a kind of compromise between the accuracy of the EPaSR model and the relative computational simplicity of PaSR.

TEST CASE 1: EVANS-SCHEXNAYDER-BEACH EXPERIMENT

Evans et al. [START_REF] Tedder | [END_REF][9] experiments were chosen for the models validation and program verification due to apparent simplicity and large number of experimental data. In the experiment [9] combustion of hydrogen in a round supersonic jet with Mach number M=2 which expired into the supersonic air flow with Mach number M=1.9 containing water vapor, was studied. In these experiments, contact measurements were done: Pitot tube for pressure and gas sampling. The scheme of the experiment is given in Fig. 1. Data [9] is still widely used for CFD programs validation. Unfortunately, the data is not sufficient -there are no details about the geometry of the nozzles, about the turbulence level, about how uniform was the flow at the heater exit, about the boundary layers. This lack of information doesn't permit to get correct profiles at the nozzles exit section. Calculations were initialized from cross-section x/d=0.33 where profiles from [9] are prescribed.

There were many unsuccessful attempts in earlier authors' works to reach a good agreement with the experimental data. This work was intended to concentrate on TCI account with the aim to improve results due to this effect. The first results obtained with EDC [START_REF] Vincent-Randonnier | Proceedings of the Combustion Institute[END_REF][START_REF] Magnussen | On the structure of turbulence and a generalized eddy dissipation concept for chemical reaction in turbulent flow[END_REF] method led to ignition failure in the calculations. It should be noted that the EDC Magnussen model was developed and is used mainly for subsonic flows, but the flow in this problem is essentially supersonic. Under such conditions characteristic chemical reactions time becomes comparable with 030078-3 characteristic flow time, which is not taken into account in EDC model. This effect can be considered on the basis of PaSR model [10]. The difference between PaSR and EDC model is mainly in the basic model values determinationfine structures volume fraction * and characteristic residence time in reactor * . The obtained with PaSR results had very little difference from the results obtained without TCI. It could be explained by large values of reactor fraction obtained in calculations (about 0.9 in the region of essential heat release). A more thorough study might be needed in order to improve the formulas for * estimation. At the moment, the calculations were organized with a number of fixed * values in the entire region during all the calculation for understanding influence of the model. These results (see Fig. 2) demonstrated that at The lack of experimental data gives a lot of arbitrariness in calculations. Various entry profile modifications were regarded (turbulent and gas-dynamic parameters, boundary layers), the influence of chemical kinetics, turbulence and diffusion models was studied. After that a series of different nozzles calculations was fulfilled with the aim to get a proper profile at the nozzles exit section. But it all didn't lead the authors to satisfactory agreement with experimental data both for concentrations and Pitot pressure distributions. On the contrary, improving one distribution we could spoil another one. An example of results, obtained with some nozzles geometry regarded, given in Fig. 3. Green lines there stand for the last calculation and violet -for the initial calculation with no TCI and prescribed profiles from [9] at x/d=0.33. One can notice some improvements in N2 profiles and Pitot away from the axis, but axial Pitot profiles became worse. Notable is that the difference reachable by TCI account is comparable with the difference obtained by other effects regarding. Besides, the largest shift for TCI account compared with no TCI corresponded to almost nocombustion flow. In general, none of the considered multiple modifications made it possible to significantly approach the experiment in the mass concentrations and Pitot pressure distributions simultaneously. A detailed analysis of the experimental data [START_REF] Tedder | [END_REF][9] and several tens of computational works by other authors, as well as multiple parametric calculations showed the insufficiency of this experimental data base for qualitative validation. Concluding, TCI account applied to this simple configuration didn't help to improve results and seems to be negligible compared to other effects.

TEST CASE 2: A.D. CUTLER EXPERIMENT

Therefore, it was decided to first consider the flow of such class without combustion and adjust the turbulence model under the cold flow conditions. Validation of mixing prediction capabilities is of primary importance, because the code is to be used for the simulation of combustion. The experiments of supersonic non-reacting coaxial jet by A. D. Cutler et al. [6][START_REF] Cutler | AIAA Paper[END_REF] conducted at NASA Langley Research Center were chosen for this purpose. Results are to be presented for the simulation of a supersonic coaxial helium jet, since this test case was developed with the validation of CFD software in mind, particularly the validation of codes to be used in the simulation of supersonic combustion.

Remarkably, that in all other authors' numerical simulation of the same experiment there is a common tendency that the calculated maximum position is too close to the lower wall. In some works certain complicated models for variable turbulent Shmidt and Prandtl numbers or models for TCI account were proposed and tuned in order to improve results remaining 2D task formulation. A lot of efforts were undertaken by the authors with the aim to obtain results closer to experimental data, including variations of different gas-dynamic, turbulent parameters, diffusivity properties, TCI account with PDF-flamelet models [START_REF] Shiryaeva | Development and application of numerical technology for simulation of different combustion types in high-speed viscous gas turbulent flows[END_REF][START_REF] Shiryaeva | [END_REF] or PaSR approach. But nothing influenced sufficiently the exit profiles. Only the account of 3D effects gave the desired shift off the lower wall for the profiles extremum position. Some narrowing of the channel due to boundary layers growth in z direction was modeled. It was realized by imposing variable channel width z(x) with dz/dx<0. One computational cell was again left in z direction so that computational costs weren't increased. As a result, a strong increase in boundary layer thickness on the lower wall along the channel was caused by a positive pressure gradient and channel shock wave structure. As a result, the extremums of the exit profiles have shifted in desirable direction, away from the lower wall.

TEST CASE 4: T.S. CHENG

The tuned in A.D. Cutler experiments modeling program was applied to modeling of flows with combustion -T. S. Cheng et al. supersonic burner [START_REF] Tedder | [END_REF] experiments. A hydrogen-fuel jet surrounded by an annular jet of hot vitiated air at Mach 2 exited into ambient air. A set of measurements gathered in 1994 by Cheng et al. [START_REF] Tedder | [END_REF] include data for temperature and major species, using ultra-violet Raman scattering and laser induced fluorescence techniques. This experiment is therefore relevant for numerical code validation in supersonic combustion, and this work is largely referenced in the literature. The finest grid includes about 350000 cells with the use of wall functions. Grid convergence was obtained. The flow in the nozzles was also simulated (see Fig. 56with the flow field pattern from the calculations). In the majority of computational studies by other authors the flow is usually simulated starting from the first section available from experiment in which they impose the measured profiles (x/D=0.85). Generally this approach isn't sufficient to account for the specific features of boundary conditions. We will concentrate on the work of J. Moule [19] where extended partially stirred reactor model (EPaSR) of the flame in a LES framework was applied to the full task simulation including the flow in the nozzles for the heated air and fuel. A correct description of the flame main characteristics was obtained.

030078-7

On the basis of approach without TCI, the account parametric study of this experiment was fulfilled with the aim to study sensibility of obtained results on different conditions: the walls' temperature distribution, total temperature and pressure at the entry of each nozzle, turbulence parameters, turbulent Shmidt and Prandtl numbers, gas composition, etc. The obtained results are compared with experimental data and with the work [19], fulfilled with RANS approach without TCI as well as with LES approach with EPaSR. The best conditions were chosen and after that an attempt to apply PaSR was fulfilled with these conditions.

Concerning the full set of results (some of which are given in Fig. 7) one can see that the agreement with the experiment is not perfect but taking into consideration the level of experimental asymmetry, it could be concluded that the further improvements seem to be doubtful, because the difference between one half of experimental data and computations isn't larger than the discrepancy between the two halves of experimental data. The authors' results are not worse than the computations [19] fulfilled using LES with EPaSR model for TCI account. Also worth noting is that LES approach with a complicated model for TCI account from [19] doesn't give definite improvements compared with RANS no TCI approach from the same work.

An attempt to take TCI into account with the PaSR approach in this task led to the same conclusions as earlier in Evans et.al. and Burrows & Kurkov experiments modeling: the obtained with TCI account results had very little difference from the results obtained without it. Concluding, TCI account applied to high-speed simple configuration flows modeling did't help to improve results and seems to be negligible compared to other effects.

TEST CASE 5: MAGRE & MOREAU EXPERIMENT

Next experimental data were chosen with the aim to study and validate PaSR approaches under significantly different conditions of very low-speed flow. The configuration of channel with backward-facing step, presented in Fig. 8, was chosen [9]. The backward-facing step configuration is a widespread tool to stabilize the combustion at high velocities and is generally used to evaluate the performance of turbulence models in the prediction of separated flows. This configuration is interesting for several reasons. Firstly, simple 2D geometry allows performing many calculations in a reasonable time. This property is particularly relevant in the context of model validation, where many tests are necessary. Secondly, the existence of experimental measurements in both nonreactive and reactive flows is a vital advantage. In addition, this flow was extensively studied numerically in the past. There are some databases that can be used for validation of results, including the most recent with the use of EPaSR method [29].

The geometry of combustion chamber used in [9] is shown in Fig. 8. Air preheated to 520 K passes through a long duct upstream of the step. The methane is injected at about 1 m upstream of the step. Then, it mixes with air preheated to 520 K. The injection velocity of the gas is typically around 50 m/s. The grid was refined in the X and Y directions at the corner of the step and in the recirculation region. The finest mesh is made of 250000 rectangular cells, contains about 150 cells per step height and 140 cells per channel width, "wall functions" are used in the first near-wall cells. A methane/air mixture is injected at T = 525 K. Inlet profiles were obtained after numerical simulation of a 2D channel before the chamber. The calculation of this channel was performed with inlet velocity u = 53 m/s. At outlet, the pressure is set to P = 1 bar. During numerical simulation the walls were assumed to be adiabatic. In the experiment, the walls were cooled by water, but the wall temperature was not measured. Assumption of adiabatic walls overestimated the temperature near walls in case of reactive flow. In the outlet and inlet sections reflecting boundary conditions were applied. First of all flow a simulation of the flow without combustion was fulfilled. Here particular emphasis was placed upon correct description of recirculation region behind the step. The exact determination of mean reattachment length Xr is difficult due to fuzzy character of the eddy edge. The mean experimental reattachment length was found to be Xr/h=4.3±0.2h [9]. In our calculations we got Xr/h < 4.85, that is almost the same as in [29]. Vertical profiles of the Favre-averaged streamwise velocities are compared with experimental data and calculations [29] in Fig. 9. The comparison of mean streamwise velocity shows a satisfactory agreement between the experimental and simulation data. Mesh convergence was obtained. Largest differences with the experimental measurements are observed in the lower part of the chamber (Y < 0 m). However, one should take into account that this area is difficult to cover experimentally because of a complicated particle seeding.

In reactive case, we use the same computational domain, grid, boundary conditions and physical models as in the calculation of nonreactive flow described above. In this experiment combustion is realized by the wave mechanism in premixed methane with air mixture. So the kinetics model should give firstly correct values of the laminar flame propagation speed rather than correct induction period. This created difficulties in the choice of the kinetics model. But it was found that the chosen model caused undamping waves in the flame front surroundings and led to flow instabilities. These oscillations might be additionally sustained by reflective boundary conditions. Unfortunately, we couldn't reach mesh convergence for combustion case due to very strong instabilities development on fine meshes. The stationary solution was calculated by time averaging in URANS calculation with global time stepping after the oscillating regime was reached. The convergence is reached by the averaged solution when it doesn't more change. The results obtained by the authors with no TCI account are of the same quality of agreement with experimental data as calculations in [29]. The primary focus here was on studying PaSR approach for TCI account in low-speed conditions. The results of application of various PaSR-based approaches are demonstrated in Fig. 1011. In Fig. 10 fields of averaged temperature obtained in no TCI approach and with EPaSR method are given, one can see notable changes: the flame front is smoother in last case. In Fig. 11 several vertical temperature profiles obtained in the calculations without TCI approach and with the use of PaSR, EPaSR and GPaSR methods are compared with each other, with experimental data and computations [29]. One can see that TCI account here makes the profiles smoother which allows to come closer to experimental data and thus to improve results compared to no TCI account. PaSR overestimates the averaged temperature in the first cross-section. This is associated with the PaSR shortcomings concerned with the fact that no flow prehistory is accounted for and local state in the fine structures (T*, Yi*) is defined exclusively by the averaged parameters and no exchange between neighboring portions inside the fine structures is accounted for. Modification GPaSR is capable of regarding exchange between the reactors in neighboring cells due to convective transport account. Herein only incoming fluxes are taken into account, because it is these fluxes that matter. The results obtained with

030078-10

GPaSR in the first cross-sections are closer to the most complicated model of this class (EPaSR) due to this convective transport inside the fine structures. On the basis of presented results one can say that EPaSR gives the best results. GPaSR is quite close to EPaSR, but it needs further analysis and tuning. It seems to be very promising due to computational economy compared to EPaSR. Based on the application of PaSR class methods for TCI account to simulation of simple configuration experiments on supersonic combustion, it was shown that taking into account the TCI does not allow us to approach the experimental data in this case. Accounting for other gas-dynamic effects is comparable with the TCI, and often even allows to achieve a much better agreement with the experiment.

To adjust the PaSR class methods, numerical simulations of essentially subsonic flow without combustion, as well as flow with combustion without TCI are performed. Different PaSR class methods are compared. The models have been adjusted. GPaSR model for accounting TCI appears to be a very promising compromise between the gain in computational costs (PaSR) and the accuracy of the model (EPaSR). The quality of results obtained in this test with EPaSR and GPaSR models is on the same level.

K

  in the EDC model. Another version of the same class is PaSR model from[START_REF] Schetinkov | Physics of gas combustion[END_REF], in which different expressions are proposed for the main parameters of the model: chem is the characteristic time scale of chemical processes, which is usually estimated by the formula laminar flame front thickness and velocity.

1 *FIGURE 1 .FIGURE 2 .

 112 FIGURE 1. Evans et al. experiment: scheme (a), computational mesh (b) and temperature field in calculation without TCI account (c)

FIGURE 3 .

 3 FIGURE 3. N2 mass fraction (a, c) and Pitot pressure (b, d) distributions: axial (a, b) and in section x/d=15.5 (c, d). Calculation 1 -from the section x/d=0.33, Calculation 2 -including flow simulation in nozzles

FIGURE 5 .

 5 FIGURE 5. Cheng experiment numerical simulation: Mach number (M) field

FIGURE 6 .

 6 FIGURE 6. Cheng experiment numerical simulation: temperature (T) field

FIGURE 7 .

 7 FIGURE 7. Cheng experiment numerical simulation: temperature distributions in sections x/D=10.8 (a) and x/D=32.3 (b) and H2O mole fraction distributions in sections x/D=10.8 (c) and x/D=32.3 (d)

FIGURE 8 .

 8 FIGURE 8. Magre & Moreau experiment: scheme of A3C test bench along with optical diagnostics (a), computational grid near the step (b) and backward-facing step dimensions (c)

FIGURE 9 .FIGURE 10 .

 910 FIGURE 9. Magre & Moreau experiment numerical simulation: vertical profiles of averaged streamwise velocity in nonreactive backward-facing step flow in sections x=0.06 m (a), x=0.1 m (b), x=0.25 m (c)

FIGURE 11 .

 11 FIGURE 11. Magre & Moreau experiment numerical simulation: vertical profiles of averaged temperature in sections x=0.1 m (a), x=0.25 m (b), x=0.34 m (c) and streamwise velocity in the same sections: x=0.1 m (d), x=0.25 m (e), x=0.34 m (f)in reactive backward-facing step flow

ACKNOWLEDGMENTS

The research was supported by the Ministry of Education and Science of Russian Federation ("megagrant", agreement No. 14.G39.31.0001). The authors are profoundly grateful to the assistance of Prof. A. D. Cutler and R. W. Pitz, who supplied them with the experimental data, Prof. V. Vlasenko who inspired and supported the work, Dr. A. Troshin for the constructive comments and ideas and all the colleagues for cooperation.

The results are presented in details in another paper and report at the same conference ("Large Eddy Simulation of a Supersonic Coaxial Helium-Air Jet" by Alexey Troshin et al.). Concluding in two words, the experimental data is very precise and the obtained results are on the same level as other CFD works. Thus the program zFlare was validated for no-combustion cold flow.

TEST CASE 3: BURROWS & KURKOV EXPERIMENT

In Burrows-Kurkov experiment [5] hydrogen was injected parallel to the vitiated air flow. Composition measurements were done at the exit plane of the test section. The scheme of the experiment and parameters of the fuel jet and air flow are given in Fig. 4. The results obtained in pure mixing case without combustion are in good accordance with experiment and other authors calculations. When combustion was simulated, the influence of kinetic scheme was found to be insignificant. On the contrary, the influence of the initial turbulent parameters at the entry was found to be appreciably visible [START_REF] Shiryaeva | Development and application of numerical technology for simulation of different combustion types in high-speed viscous gas turbulent flows[END_REF].


domain: spi phys

text:
Santiago Casu

Bastian Geiger

Rainer Kiemel

Jean-Yves Lestrade

Jérome Anthoine

Development and characterization of a catalyst for the decomposition of hydrogen peroxide

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. Introduction

Since the 1960's, most of the orbital propulsion hot gases thrusters are based on hydrazine as a storable propellant. Because of its very high toxicity, hydrazine was included in ECHA's candidate list in 2011 and might be prioritized for inclusion in Annex XIV of REACH at any time. Therefore, the search for less toxic or "green" alternatives for hydrazine has become of great interest in the past few years. The most promising substitutes for hydrazine, like ammonium dinitramide (AND) or hydroxylammonium nitrate (HAN) both show higher density impulses [START_REF] Gohardani | Green Space Propulsion: Opportunities and Prospects[END_REF][START_REF] Masse | AF-M315E Propulsion System Advances and Improvements[END_REF] but suffer from very high combustion temperatures and require extremely expensive alloys and manufacturing processes for the realization of the combustion chamber. Hydrogen peroxide (H 2 O 2 ) on the other hand does not suffer from this major drawback and is currently reconsidered as a promising green propellant for thruster applications. Hydrogen peroxide has a relatively low nominal propulsive performance compared to hydrazine [START_REF] Dolci | Platinum Catalysts Development for 98% Hydrogen Peroxide Decomposition in Pulsed Monopropellant Thrusters[END_REF] and highly concentrated hydrogen peroxide with high density must be used to improve the volume specific impulse. The most conventional catalyst for H 2 O 2 decomposition is metallic silver, which suffer from two major drawbacks, namely temperature limitations and poisoning from the stabilizers used to prevent the self-decomposition of hydrogen peroxide. Many research activities have been performed in the past few years about the catalytic decomposition of H 2 O 2 [START_REF] Torre | Performance of a Monopropellant Thruster Prototype Using Advanced Hydrogen Peroxide Catalytic Beds[END_REF][START_REF] Koopmans | A One-Dimensional Multicomponent Two-Fluid Model of a Reacting Packed Bed Including Mass, Momentum and Energy Interphase Transfer[END_REF][START_REF] Maia | Development and Optimization of a Catalytic Thruster for Hydrogen Peroxide Decomposition[END_REF][START_REF] Bonifacio | Novel Manufacturing Method for Hydrogen Peroxide Catalysts:APerformanceVerification[END_REF]. Precious metals as well as Mn oxides were extensively studied but no real consensus seems to emerge about the most active metal for this reaction [START_REF] Pirault-Roy | Hydrogen Peroxide Decomposition on Various Supported Catalysts Effect of Stabilizers[END_REF][START_REF] Rusek | New Decomposition Catalysts and Characterization Techniques for Rocket-Grade Hydrogen Peroxide[END_REF][START_REF] Romeo | Performance of Different Catalysts Supported on Alumina Spheres for Hydrogen Peroxide Decomposition[END_REF]. As a producer of the hydrazine decomposition catalyst HKC-12GA, Heraeus Deutschland GmbH & Co. KG. has gained a lot of experience in the past 15 years in the field of catalysis applied to thruster technology. The renewed interest for H 2 O 2 as a green propellant lead us to carry out some activity on the development of a precious metal-based catalyst for the decomposition of hydrogen peroxide. The present article illustrates the results of this development.

II. Pre-screening of catalysts prepared with various precious metals

A. Preparation of the catalysts

All the catalysts tested in the context of the present work were prepared by means of a conventional incipient wetness impregnation. The water uptake of the alumina carriers was measured, and the concentration of the various precious metal's impregnation solutions were then adjusted to reach the targeted precious metal loading of 5% on the catalysts. After impregnation, the obtained catalysts were dried, calcined and reduced. Table 1 below presents the catalysts prepared for the pre-screening. The various precious metals (Pt, Pd, Rh, Ir and Ag) were supported on the flight proven -alumina granules used to produce the hydrazine decomposition catalyst HKC-12GA (Al 2 O 3 -1). A grain size fraction between 18 and 20 mesh was selected for the preparation of the catalytic systems and kept constant for all the catalysts as this parameter can have a huge impact on the catalytic activity.

B. Laboratory apparatus and test procedure

The catalysts were tested in a simple laboratory apparatus composed of a flask, a syringe to introduce H 2 O 2 and a graduated cylinder to measure the volume of the released hot gas mixture (Fig. 1). Before the test, each catalyst was dried in an infra-red moisture analyzer and 200mg of solid are introduced in the reaction flask. Next, 1mL of H 2 O 2 (30% solution from Merck) solution is added to the catalyst with the syringe and the volume of the released hot gas mixture was measured as well as the decomposition time.

C. Results of the pre-screening

Each catalyst was tested three times and Fig. 2 below presents the average decomposition time measured for each catalyst. The Rh-based catalyst shows the lowest activity with a decomposition of 20,9s. Moreover, with this catalyst, H 2 O 2 is not fully decomposed and a volume of hot gases of 75mL was measured which is about half of the expected gas release based on the decomposition of 1mL of the 30% H 2 O 2 solution. On the other hand, the other catalysts tested for this pre-screening were all able to fully decompose H 2 O 2 and a volume of about 150mL of hot gases was measured. As can be seen from Fig. 2 below, Pt shows the shortest decomposition time followed by Ir, Ag, Pd and Rh. Pt and Ir showed similar activity with decomposition times of respectively 1,8s and 2,1s. Many different precursors of Pt are available by Heraeus which might have an big impact on the catalytic activity and Pt was therefore selected for the fine screening presented below.

III. Fine screening of Pt catalysts A. Preparation of the catalysts and testing

The aim of this fine screening was to investigate the influence of some preparation parameters on the catalytic activity of the Pt/Al 2 O 3 system. Three different Pt precursors (A, B and C) were used to prepare the catalysts and two different carriers were also studied, namely the -alumina granules used to produce the hydrazine decomposition catalyst HKC-12GA (Al 2 O 3 -1) and an alternative -alumina granules (Al 2 O 3 -2). The choice of -alumina is critical for this kind of catalytic bed as it was already shown in the past that -alumina supported catalyst tend to crack when brought in contact with highly concentrated H 2 O 2 solutions [START_REF] Surmacz | Influence of Various Types of Al 2 O 3 ∕Mn x O y Catalysts on Performance of a 100 MM Chamber for Decomposition of 98%+ Hydrogen Peroxide[END_REF][START_REF] Pasini | Testing and Characterization of a Hydrogen Peroxide Monopropellant Thruster[END_REF]. The size of the granules was again the fraction between 18 mesh and 20 mesh. The catalytic systems tested during this fine screening are listed in Table 2 below. These catalysts were prepared using the same incipient wetness impregnation method as the one used for the prescreening and the Pt loading was again kept constant and equal to 5%. The first tests for the fine screening were performed with the 30% H 2 O 2 solution, but with this concentration no difference could be observed between all the very active Pt systems and the concentration had to be decreased down to 10% to better investigate the impact of the studied parameters on the catalytic activity.

Table 2 Catalytic systems prepared for the fine screening

Catalyst

Pt content (%)

Pt (A)/ Al 2 O 3 -1-18-20 mesh 5 Pt (B)/ Al 2 O 3 -1-18-20 mesh 5 Pt (C)/ Al 2 O 3 -1-18-20 mesh 5 Pt (C)/ Al 2 O 3 -2-18-20 mesh 5

B. Characterization of the catalysts

Characterization of the catalysts consisted of determination of the precious metal surface by CO-adsorption, determination of the pore volume by Hg-porosimetry, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) and transmission electron microscopy (TEM).

The precious metal surface area tests were performed in a TPDRO 1100 device from the Thermo Finnigan Company. Before testing, the catalysts are reduced for 2h in a reductive gas mixture (95% Ar, 5% H 2 ).

The Hg-porosimetry analysis were conducted using two devices namely a Pacal P140 and a Pascal P440 from the company Thermo Fischer Scientific. Before testing, the samples are degassed at 150°C for 12h.

The SEM-EDX analyses were performed using the Scanning Electron Microscope Jeol JSM-IT100 equipped with integrated energy dispersive X-ray analysis system, using a 20 keVelectron beam. The depth penetration for 20 keV incident electrons is about 1 μm; therefore, SEM-EDX provides quantitative analysis of elemental composition of the specimen surface with a sampling depth of 1 μm. The methodology of investigation used is the following: one layer of pellets for each catalyst sample were embedded in resin, cross-sectioned in half and polished. In the SEM a BSE-picture (backscattered electrons) in minimum magnification was used to pick some characteristic/ representative pellets. TEM investigations were conducted using a FEI Talos 20-200 transmission microscope at 200 kV. The measurements were performed in TEM mode and in STEM using Bright Field (BF) imaging and HAADF detector. Energy-dispersive x-ray spectroscopy was used to detect differences in local chemical composition.

C. Results of the fine screening and discussion

The decomposition times for the four catalysts are presented in Fig. 3 below. As expected, the decomposition times measured with the diluted H 2 O 2 solution are much longer and allowed us a better discrimination of the catalytic systems. We observed full decomposition of the hydrogen peroxide and a volume of hot gases of 50mL was measured, which corresponds to the volume expected for the decomposition of 1mL of a 10% H 2 O 2 solution.

Fig. 3 Decomposition times measured during the fine screening

Among the three Pt precursors used for the preparation of the catalysts tested during this fine screening, Pt salt A shows the lowest activity with a decomposition time of 51s. This is about two times, respectively three times longer than the decomposition times observed for the catalyst prepared with precursor B and C. The SEM images of these three catalysts are presented in Fig. 4 below. As can be seen from these images, precursors A tends to travel much deeper in the alumina pores compared to precursors B and C and the alumina granules prepared with this salt are almost homogeneously impregnated. On the other side, in the catalysts prepared with precursors B and C, Pt tends to stay at the surface of the alumina granules and these catalysts present an egg-shell profile with much higher local concentrations of Pt at the surface of the granules. This higher concentration of Pt at the surface of the granules explains the higher activity for the decomposition of H 2 O 2 , which does not have to enter deeply the pores of the catalyst to react on the active sites. As can be seen from image b) and c), the egg-shell profile for the catalyst prepared with precursor B is sharper than the one observed for catalyst prepared with precursor C and a rough estimation of the shell thickness showed that the shell observed for precursor B is about two time thinner than for precursor C (respectively 50m and 100m). Nevertheless, catalyst prepared with precursor C shows the highest activity with a decomposition time about two times shorter than catalyst prepared with precursor B. The TEM images as well as the crystallite size distribution for the catalysts prepared with precursors B and C are presented in Fig. 5 and Fig. 6 below. As can be seen in the TEM images, the Pt nanoparticles obtained with precursor C are much smaller than the ones observed for the catalyst prepared with precursor B. This observation is confirmed by the crystallite size distribution presented in Fig. 6 This much smaller particle size, i.e. much better dispersion of the Pt observed with precursor C explains the higher catalytic activity observed for this catalyst compared to the catalyst impregnated with precursor B. This better dispersion was also confirmed by the metallic surface areas determined with the CO adsorption measurements presented in Table 3 below. The catalyst prepared with Pt salt C shows a metallic surface area about five time bigger than the one measured for precursor B. The Pt precursor C shows the highest activity among the three precursors tested during the fine screening. This precursor was therefore selected to prepare one additional catalyst supported on an alternative -alumina, namely Al 2 O 3 -2. This additional catalyst shows improved catalytic activity compared to the one prepared on the conventional alumina, with a decomposition time about two time shorter. The various characterizations performed on this additional catalyst are presented in Fig. 7 below. The results obtained for the catalyst supported on the alternative alumina are very similar to the one observed for the material impregnated on the standard alumina Al 2 O 3 -1. The SEM image presented in Fig. 7 below shows a relatively sharp egg-shell profile with a shell-thickness of about 130 m. The TEM investigations also shows very similar results as the ones observed for the standard 6 alumina. Some very small Pt nanoparticle can be observed on the TEM image below and the crystallite size distribution shows a very narrow distribution with most of the particles having a size comprised between 1 nm and 3 nm. These nanoparticles are slightly smaller than the one observed for the catalyst prepared on the conventional alumina, which might partially explain why this catalyst shows a better catalytic activity during the H 2 O 2 decomposition test performed in the laboratory apparatus. An additional explanation for this improved activity can be the differences observed in the pore volume and the pore diameter distribution presented in Table 4 and Fig. 8 below. The catalyst supported on the alternative alumina Al 2 O 3 -2 shows a total pore volume about two times bigger than the conventional alumina granules. Moreover, the pores size distribution presented in Fig. 8 below shows that the highest contribution to the total pore volume comes from the pores with a diameter between 3 nm and 5 nm for Al 2 O 3 -1, whereas the highest contribution to the total pore volume for Al 2 O 3 -2 comes from the pores having a diameter comprised between 5 nm and 10 nm. These differences in the micropores distribution might explain the better activity observed for the catalyst supported on the alternative alumina and further investigations are needed to better understand the influence of this parameter on the catalytic activity.

IV. Monopropellant test campaign

A. Catalysts selected for the campaign Four catalysts (A, B, C and D) were prepared for the test campaign performed by ONERA in their monopropellant test facility. The catalysts listed in Table 5 were prepared with the Pt-precursor C, which showed the most promising results during the laboratory screening. Catalyst A, B and C were prepared with three different granules sizes of the Al 2 O 3 -1 alumina. Catalyst D was prepared with the alternative -alumina Al 2 O -2 described before and with the same granules size as the one used to produce catalyst A. The Pt content was again kept constant and equal to 5%.

Table 5 Catalysts for the monopropellant test campaign

Catalyst

Pt content (%)

A -Pt (C)/ Al 2 O 3 -1 -10-14 mesh 5 B -Pt (C)/ Al 2 O 3 -1 -18-20 mesh 5 C -Pt (C)/ Al 2 O 3 -1 -20-25 mesh 5 D -Pt (C)/ Al 2 O 3 -2 -10-14 mesh 5

B. Description of the monopropellant test facility

The monopropellant test facility consists basically of the three following main components: the inlet manifold connected to the hydrogen peroxide feed line, an injector plate and the decomposition chamber containing the catalyst particles (Fig. 9). The injector plate was especially designed to spread the liquid hydrogen peroxide all over the cross section of the decomposition chamber. This chamber consists of an Inconel cylinder closed at both ends by refractory steel meshes to maintain the catalyst particles inside the decomposition chamber.

Fig. 9 Monopropellant test facility

This chamber is connected to a measurement module measuring the decomposition temperature at the outlet of the catalytic bed thanks to three thermocouples and the decomposition chamber pressure. The instrumentation of this facility also includes a Coriolis oxidizer mass flow measurement and temperature and pressure measurements of the liquid oxidizer upstream the manifold.

C. Test results

For this test campaign, each test was performed during 10 s and the oxidizer mass flow rate was kept constant and equal to 90 g/s. Indeed, it was important to keep this parameter constant to get a direct comparison between the four different catalysts since the transient phase duration depends on the oxidizer mass flow rate. A lower oxidizer mass flow rate leads to a higher transient phase duration and vice-versa.

Thanks to a constant tank pressure during the monopropellant test, the oxidizer mass flow rate is stable during the second part of the test due to steady temperature at the outside of the decomposition chamber (Fig. 10). During the first part of this test (up to 5 s), the decomposition temperature increases continuously which leads to a slight decrease of the oxidizer mass flow rate. Before going up to about 910 K, the decomposition temperature shows a short plateau at temperature of about 360 K which corresponds to the water decomposition temperature. With T norm the normalized temperature, T dec the measured decomposition temperature, T inj the injection temperature of the liquid oxidizer and T ad the adiabatic decomposition temperature, which was obtained thanks to a thermochemical equilibrium code. This equation was used to overcome the variations in the injection temperature as well as the variation observed in the hydrogen peroxide concentration test to test.

Before comparing the activity of the catalysts, each sample was submitted to four consecutives monopropellant tests to insure a good reproducibility of the results. The temperature evolution for the four tests performed on catalyst C are presented in Fig. 11 below. As can be seen from this figure, the temperature evolution for the last three monopropellant tests match well and an efficiency of 97.4% was reached based on the normalized temperature.

Fig. 11 Decomposition temperature evolution of catalyst sample C for the four tests

Finally, the normalized temperatures for the four catalyst samples are presented and compared in Fig. 12 below. First, the comparison between catalyst A, B and C shows that the smallest the particle size, the shorter the transient time and the higher the efficiency based on the normalized temperature. On the other hand, catalyst D, which was prepared with the same particle size as catalyst A but with the alternative alumina Al 2 O 3 -2, shows improved efficiency as well as a shorter transient phase duration. These results confirm the better catalytic activity of the catalyst supported on Al 2 O 3 -2, which was already observed during the laboratory screening.

Fig. 12 Comparison between the decomposition temperatures of the four catalyst samples

The final choice for the best catalytic system was finally made between catalyst C and D. On the one hand shows catalyst C a shorter transient time (2.5 s against 3.2 s). On the other hand, catalyst D supported on the alternative alumina granules shows improved efficiency compared to catalyst C (respectively 98.5% and 97.4%). Moreover, the post-test analysis of the catalytic bed prepared with sample C showed the tendency of these very fine particles to aggregation and some bigger particles were observed. This aggregation could have a negative impact on the performances of the catalyst bed during longer cumulative test durations and consequently catalyst sample D was selected for the hybrid firing test campaigns reported in Ref. [START_REF] Lestrade | Development of a Catalyst for High Concentrated Hydrogen peroxide[END_REF] and Ref. 14.

V. Conclusion

A reliable and effective catalytic bed is one of the key elements for the development of a highly concentrated hydrogen peroxide thruster. A pre-screening performed at Heraeus in a very simple laboratory apparatus on various precious metals supported on the flight proven -alumina granules used to produce the hydrazine decomposition catalyst HKC-12GA (Al 2 O 3 -1) showed that Pt is the most active precious metal for the catalytic decomposition of H 2 O 2 . A more detailled screening was then performed on several Pt catalysts prepared with three different Pt precursors as well as two different aluminas. These catalysts were fully characterized using various techniques like SEM-EDX, TEM and Hg-porosimetry. The most effective combination was found to be Pt (C)/Al 2 O 3 -2. This catalyst shows the highest Pt dispersion, with nanoparticles of about 2 nm as well as a good egg-shell profile. The monopropellant test campaign performed by ONERA on four Pt catalysts confirmed the better activity of this catalytic system under real conditions (87,5% H 2 O 2 ). Surprisingly, the several hot-firing tests performed on this catalytic system did not show a noticeable ageing of the catalyst and the decomposition activity of the catalyst remained almost unchanged after the exposition to several kilograms of highly concentrated hydrogen peroxide. A post-test characterization of the catalyst would be necessary to further understand the ageing behavior of this catalytic system during the hot-firing tests.

  O 3 -1 -18-20 mesh 5 Pd/Al 2 O 3 -1-18-20 mesh 5 Rh/Al 2 O 3 -1-18-20 mesh 5 Ir/Al 2 O 3 -1-18-20 mesh 5 Ag/Al 2 O 3 -1-18-20 mesh 5

Fig. 1

 1 Fig. 1 Laboratory apparatus for H 2 O 2 decomposition

Fig. 2

 2 Fig. 2 Decomposition times measured during the pre-screening

Fig. 4

 4 Fig. 4 SEM images of a) Pt (A)/ Al 2 O 3 -1, b) Pt (B)/ Al 2 O 3 -1, c) Pt (C)/ Al 2 O 3 -1

  . For the catalyst Pt (B)/ Al 2 O 3 -1, a very broad distribution is obtained and most of the particles comprised between 10 nm and 30 nm. On the other hand, a very narrow distribution is observed for catalyst Pt (C)/ Al 2 O 3 -1 and most of the nanoparticles of Pt have a diameter comprised between 1 nm and 4 nm.

Fig. 5 Fig. 6

 56 Fig. 5 TEM images of Pt (B)/ Al 2 O 3 -1 (left) and Pt (C)/ Al 2 O 3 -1 (right)

Fig. 7

 7 Fig. 7 SEM (left), TEM (middle) images and crystallite size distribution (left) for the catalyst Pt (C)/ Al 2 O 3 -2

Table 4 2 CatalystFig. 8

 428 Fig. 8 Pore size distribution for catalysts Pt (C)/ Al 2 O 3 -1 (left) and Pt (C)/ Al 2 O 3 -2 (right)

Fig. 10

 10 Fig. 10 Results of a test performed with the catalyst sample A Figure 11 below presents the decomposition temperature in the chamber, which was normalized according to Eq.1.

Table 1 Catalytic systems prepared for the pre-screening

 1

	Catalyst	Pt content (%)
	Pt/Al 2

Table 3 Metallic surface area measured for Pt (B)/ Al 2 O 3 -1 and Pt (C)/ Al 2 O 3 -1

 3

	Catalyst	Metallic surface area (m 2 /g)
	Pt (B)/ Al 2 O 3 -1-18-20 mesh	2,1
	Pt (C)/ Al 2 O 3 -1-18-20 mesh	11,0


Jean-François Girres

Guillaume Touya

Quality Assessment of the French OpenStreetMap dataset

Keywords: OpenStreetMap, spatial data quality, VGI, specifications

The concept of Volunteered Geographic Information (VGI) has recently emerged from the new Web 2.0 technologies. The OpenStreetMap project is currently the most significant example of a system based on VGI. It aims at producing free vector geographic databases using contributions from Internet users. Spatial data quality becomes a key consideration in this context of freely downloadable geographic databases. This article studies the quality of French OpenStreetMap data. It extends the work of Haklay to France, provides a larger set of spatial data quality element assessments (i.e. geometric, attribute, semantic and temporal accuracy, logical consistency, completeness, lineage, and usage), and uses different methods of quality control. The outcome of the study raises questions such as the heterogeneity of processes, scales of production, and the compliance to standardized and accepted specifications. In order to improve data quality, a balance has to be struck between the contributors' freedom and their respect of specifications. The development of appropriate solutions to provide this balance is an important research issue in the domain of user-generated content.

Introduction

With the advent of the current Web 2.0, contributors do not just look for content but are also creating it themselves, as showing the success of Facebook, MySpace, YouTube or forums. This new phenomenon generates new methods of production that [START_REF] Tapscott | Wikinomics: How Mass Collaboration Changes Everything[END_REF] describe as the crowdsourcing, which consists in using the expertise of a large number of users to perform tasks at a lower cost. At the same time, the success of open source software is extended by the emergence of open databases that can be used more or less freely depending on the type of license. Thus, some new spatial data producers as local administrations [START_REF] Touya | Elaboration d'un fond cartographique départemental[END_REF] prefer to make an entire community benefit from the data they produce rather than keep data for private use. Together with the democratisation of GPS, these two phenomena meet up today turning everyone into a sensor of geographic information, able to provide a contributively created geographic information or Volunteered Geographic Information (VGI) [START_REF] Goodchild | Citizens as Voluntary Sensors: Spatial Data Infrastructure in the World of Web 2.0[END_REF]. That is what [START_REF] Sui | The wikification of GIS and its consequences: Or Angelina Jolie's new tattoo and the future of GIS[END_REF] called the wikification of geographic information.

In this context, the rapid development of the OpenStreetMap project (OSM), born in 2004 in England, is not surprising (http://www.openstreetmap.org). Each project member is invited to submit data acquired mainly by GPS. It is then available mapped on the website like in cartography sites as GoogleMaps or the Geoportals. But above all OpenStreetMap data are freely downloadable in vector format. The project, which was originally limited to English roads, has spread around the world, and allows to capture topographic databases. Given the possible applications of spatial vector databases (mapping, geographic analysis, urban planning or risk prevention), the question of the quality of such data should be asked, including comparison with institutional geographic databases, as those of the French National Mapping Agency (Institut Geographique National, IGN). Thus, [START_REF] Haklay | How good is OpenStreetMap information? A comparative study of OpenStreetMap and Ordnance Survey datasets for London and the rest of England[END_REF] suggested comparing British OSM data with those of the Ordnance Survey, the English national mapping agency. The objective of the work presented in this paper is to extend this work to French data taking into account different aspects of quality of a geographic database, and to analyse the OpenSteetMap's production model compared to those of institutional databases.

The objective of this work is also to experiment quality control methods and tools for a geographic database.

The second part of this section describes more precisely the OSM project with its data, methods, objectives and its applications. The third part proposes a study of OSM data quality compared to the BD TOPO ® from IGN, regarding all aspects of spatial data quality. Then, the fourth section puts into perspective the question of databases production models. The fifth section discusses the definition of spatial data quality regarding the preceding analysis. Finally, the article ends with a conclusion and further research.

What is OpenStreetMap?

OpenStreetMap (OSM) is a collaborative project like Wikipedia, started in England in 2004 by Steve Coast. The aim of OSM is to create and provide free geographic data. The project aims to compensate the lack of free data because geographic data, even freely available, are provided with licenses restricting the use of information and the creativity according to project leaders. The data are distributed under the license "Creative Commons Attribution-ShareAlike 2.0 license". This license allows using the data completely freely, in condition to distribute any derived data under the same license. For instance, corrected OSM data can not be sold.

Data stored in OSM by contributors of the project are modelled and stored in tagged geometric primitives (see 3.2). For example, a road is a polyline with tags highway = "primary", oneway = "no" and name = "N10". Geometric primitives are of three types: points, paths (polylines) and relationships (linking points and paths with tags) that are not really geometric primitives. The surfaces are represented by closed paths. There are quite precise specifications listing the accepted tags and fields of values (Figure 1). A contributor can submit a new tag or a new value, that can be added to the specifications after a vote. Even if it is not advised, the contributor is free to use tags and values outside of these specifications. To facilitate the work of the contributor, there are also information files showing how to tag a given situation: in the Figure 2, the situation of the image is modelled by 3 items (2 roads and a tramway line). Data are available from any area specified for export in a specific XML based format. It has to be translated if anyone wishes to use the data in another application. Data is captured using a GIS software adapted to OSM data with editing functions to create OSM geometric primitives and tag them. Different software exist to edit and capture OSM data (Potlatch ®, JOSM ®, Merkaartor ®). In such software, data can be created directly but come generally from free sources like personal GPS tracks, satellite imagery or as cadastral data that have been made available for the project. OSM applications currently aim to foster mapping creativity of potential contributors of geographic data. Thus, there are various sites proposing OSM data as CloudMade (route calculation) or GeoFabrik (which provides OSM data in shapefile for example). [START_REF] Allan | Osm cycle maps[END_REF] offers maps suitable for cyclists.

Quality assessment of OpenStreetMap data in France

Quality of a geographic database

To assess the quality of spatial data provided by OpenStreetMap, we use quality control methods in a geographic database. For the geometric precision and some other aspects, we use as a reference dataset, the BD Topo ® Large Scale Referential (RGE) from IGN. [START_REF] Vauglin | Modèles statistiques des imprécisions géométriques des objets géographiques linéaires[END_REF] differentiates several components in the description of geographic database quality. We propose those retained by the ISO norm [START_REF] Kresse | Iso Standards for Geographic Information[END_REF]:

-Geometric accuracy: assesses the positioning and geometries resolution from the ground reality.

-Attribute accuracy: assesses the accuracy of attributes captured according to the specifications of the database.

-Completeness: evaluates if all data are present in the database.

-Logical consistency: assesses the degree of internal consistency as modelling rules and specifications (including respect of integrity constraints [START_REF] Servigne | A Methodology for Spatial Consistency Improvement of Geographic Databases[END_REF]).

-Semantic accuracy: assesses if the semantics carried by the objects correspond to the real world.

-Currentness: evaluates the actuality of the database relative to changes in the real world.

-Lineage: concerns the lineage of objects, their capture and their evolution.

-Usage: evaluates how well the database fits for the use that will be made.

Comparison of data models

Institutional geographic databases are structured in classes, attributes and relationships [START_REF] Goodchild | Geographical data modelling[END_REF]. An object of a geographic database belongs therefore to a class, for example road segment, and has values for different attributes of the class, such as "Highway" for the attribute nature and "20m" for the attribute width. The BD TOPO ® IGN is composed of thirty classes divided into different themes such as hydrography, buildings or transportation systems. As stated in section 2, OMS data follow a different model rather used in the management of Internet resources, the Resource Description Framework (RDF) defined in [START_REF] Manola | RDF Primer[END_REF]. Information is modelled as a triple (resource, property, value). In the case of OSM, the resource is a geometric primitive with coordinates, the property is a tag and the value is a value of the tag. The triple follow the form: (point(x, y); highway; primary). This RDF information is represented in XML as in (Figure 3). Compared to a conventional structure in classes and attributes, RDF allows the easy integration of not obvious objects to identify in classes but on which we know certain properties. [START_REF] Lemmens | Semantic Interoperability of Distributed Geo-Services[END_REF] explains that the RDF is particularly effective if it is coupled with a RDF Schema model that describes resources and properties with a vocabulary. In the case of OSM, specifications are not as structured as in a RDFS model. The RDF structure can pose several problems. It is quite complex to translate data into RDF classes and attributes. Several solutions are possible to switch from one to another, but the translation into classes inevitably generate losses on the tag information and the number of classes is not easy to decide. For example, tags that characterise highway roads may include points, lines, or surfaces and the separation into classes is not clear. The reverse transition is automatic [START_REF] Lemmens | Semantic Interoperability of Distributed Geo-Services[END_REF]. Another problem concerns identification of real world objects. The ideal way would be to carry a unique identifier on the real world entities, or on an object. In the case of OSM, identifiers refer to the geometric primitives which generates difficulties to manage and update databases.

Nevertheless, structuring geographic data in a RDF format can also prove interesting. [START_REF] Goodwin | Geographical Linked Data: The Administrative Geography of Great Britain on the Semantic Web[END_REF] translated Ordnance Survey Administrative database in RDF to facilitate interoperability but using an ontology to formalise properties contrary to OSM. [START_REF] Goodwin | Geographical Linked Data: The Administrative Geography of Great Britain on the Semantic Web[END_REF] also noticed that topology relations explicitly stored in RDF allowed good querying performance.

<node id="25213726" lat="43.3020693" lon="-0.3420626" version="3" changeset="52741" user="betsubetsu" uid="43565" visible="true" timestamp="2008-06-04T16:10:13Z"> <tag k="name" v="Station Shell"/> <tag k="postal_code" v="64000"/> <tag k="created_by" v="Potlatch 0.9c"/> <tag k="amenity" v="fuel"/> <tag k="fuel_lpg" v="yes"/> <tag k="is_in" v="PAU"/> <tag k="source" v="stations.gpl.online.fr"/> <tag k="address" v="65, Avenue Maréchal Leclerc"/> </node>

Geometric Accuracy

To estimate geometric accuracy, comparisons are realised between the OSM data (data sets to compare) and BD TOPO ® the reference datasets. BD TOPO® is a metric resolution database which means that positional accuracy is around 1 meter but varies among themes (some are more precise than others). "Homologous" objects are selected and matched manually to avoid errors related to an automatic process. Differences in position are then computed on each pair of homologous objects. A specific application is developed in Java, based on the GeOxygene library [START_REF] Bucher | GeOxygene: built on top of the expertness of the French NMA to host and share advanced GI Science research results[END_REF].

Points primitives

The comparison test of point objects is realised on road intersections from road layers of the two databases in the region of Hendaye. Pre-treatment using a topological map from GeOxygene allows generating nodes, corresponding to road intersections. A sample of 207 pairs of homologous point objects is achieved by manual selection, trying as best as possible to cover the entire study area (Figure 4). For each pair of selected point objects, Euclidean distance is used to generate a distance distribution drawn in Figure 5 and summarised in Table 1. The distribution of positional differences presented in Figure 5 shows a concentration between 2.5 and 10 meters. The average positional difference, computed from the sample, is about 6.65 meters. This distance is close to the average difference of about 6 meters observed between OSM and Ordnance Survey datasets according to [START_REF] Haklay | How good is OpenStreetMap information? A comparative study of OpenStreetMap and Ordnance Survey datasets for London and the rest of England[END_REF]. Important differences -the maximum distance recorded is 31.58 meters -are relatively marginal. They are certainly due to errors of capture by the operator. This first result already shows a key property of OSM namely the heterogeneity of data quality, highlighted by the important standard deviation that is three times more important that the reference precision (1.5 m).

Linear primitives

To characterise positional differences between linear objects, two distances are used: the Hausdorff distance and the average distance.

The Hausdorff distance measures the maximum deviation between two polylines, as shown in Figure 6a. The Hausdorff distance dH between polylines L1 and L2 is defined as follows: dH = max (d1, d2) where d1 represents the maximum of the shortest distances from L1 to L2 and d2 is the maximum of the shortest distances from L2 to L1. The average distance between two polylines, introduced by McMaster (1986), measures a distance defined by the ratio between the surface S separating the two polylines L1 and L2 with their average length, as shown in Figure 6b. The equation of the average distance dM is defined as follows:

dM = 2 2 1 L L S 
Two tests are performed to characterise the geometric accuracy of linear objects from OSM data: roads and coastline. For the road network, a selection of 50 pairs of homologous linear objects in the region of Hendaye is used. The distribution of Hausdorff distances shown in Figure 7 indicates a concentration between 5 and 15 meters with a peak between 5 and 10 meters. The average of maximum deviations (Table 2) reaches 13.57 meters. It is important to nuance the interpretation of extreme values (beyond 20 meters), which may be biased by the introduction of extremities of polylines (departure and arrival nodes) in the computation of Hausdorff distance. Once again, the standard deviation shows a significant heterogeneity in positional accuracy.

Table 2. Statistics calculated on distances between road linear features.

Distribution of average distances as shown in Figure 8 and Table 2 is fairly spread, but in very small distances (0 to 6 meters), knowing that in the reality, a road is around 6 meters wide. The distances computed between OSM and BD TOPO® road networks are close to reference accuracy but the standard deviation nearly as important as the mean shows heterogeneity.

Figure 8. Distribution of average distances from the sample of roads

A comparison of the theme coastline is also conducted in the region of Hendaye, after extraction of three pairs of homologous linear objects, as shown in Figure 9. Some portions of the coastline, with important differences, have not been taken into account. Indeed, the coastline specified by IGN on the BD TOPO® (level of highest tides with a 120 tide coefficient) can penetrate deeply inlands in some estuaries. This area however may not be represented in OSM data, because of the lack of specifications of capture, and the subjectivity of the operator in his perception of the coastline. The couple No. 1 (Figure 9), captured in urban areas, present small differences (Hausdorff distance of 25.93 meters and average distance of 0.82 meters). Couples No. 2 and No. 3 (Figure 9), captured in areas of bays and cliffs, present significant differences, respectively Hausdorff distances of 139.66 meters and 154.56 meters, and average distances of 32.21 and 26.58 meters. The curvilinear abscise difference between the two couple's reaches 11% of the total length of the reference dataset (BD TOPO®).

Such results are particularly interesting because they illustrate the importance of defining precise specifications to avoid subjectivity: subjectivity leads to big quality problems that could be easily avoided with specifications defining precisely what should be captured as a coastline and how it should be done.

Polygonal primitives

To characterise differences between polygonal objects, the distance used is the surface distance, proposed by Vauglin (1997) (Figure 10).

The distance is therefore surface dS: dS = 1 -    The distribution of surface distances (Figure 11) shows a concentration peak between 0.1 and 0.25, which corresponds to quite low differences. Indeed, [START_REF] Hadj | Geometrical Matching of Polygons in GISs and Assessment of Geometrical Quality of Polygons[END_REF] proposed a polygon object matching process partly based on surface distance and considered such distribution as showing little difference. They compared cadastral buildings with buildings from topographic data (considered as quite close in position and shape) and noticed that 25% were under 0.05, which is not the case here, and 70% were between 0.15 and 0.45. However, comparison of polygonal primitives requires both a comparison of positions but also of shapes. To cope with this requirement, granularity and compactness measures were carried out on the same dataset (Table 3). The granularity measure used simply returns the shortest segment of the polygon. It measures resolution differences and the standard deviation shows great heterogeneity compared to BD TOPO®. Compactness is computed using Miller's measure (McEachren, 1985):

2 2 perimeter area C   
Compactness differences allow the assessment of shape differences between OSM lakes and BD TOPO ones. Table 3 show that, here, the differences are small.

Semantic and attribute accuracy

Table 4 shows quantitative results on attribute accuracy while Figure 12 shows qualitative assessment on attribute accuracy. Quantitative results show that the main tag of each type of objects analysed is mostly informed (except for roads where the ratio is only 85%) while the secondary attributes are rarely informed. Qualitative results are based on the same method as for geometry but using here the Levenstein distance that compares strings. Qualitative results show that only 55% of the lake names are informed as in BD TOPO® but when they are, the names are nearly identical (a distance of between 1 and 3 generally correspond to simple spelling mistakes). Semantic accuracy assessment was also carried out on the nature or function of 585 roads represented in the tag highway in OSM data that was compared to the nature attribute of BD TOPO® roads. The main roads, corresponding to "Motorway" and "Primary" values have semantically correct classification as nearly 100% have their BD TOPO® homologous classified the same way. But only 49% of the secondary roads are semantically correct compared to BD TOPO®. The error made by the contributors is mainly the underestimation of road importance: roads considered as "secondary" in BD TOPO® are classified as "Residential" or "Tertiary" in OSM data. The number of local roads in the OSM sample was to low to do the same comparison. Our interpretation is that when semantic classification specifications are clear like for main roads, the semantic accuracy is good which pleads once again for better specifications. Other qualitative remarks can be made on the heterogeneity of attribute values allowed in OSM. The One Way tag, when filled, contains in the whole French road data the values 0, 1, -1, yes, no, true or false.

Semantic and attribute accuracy are not guaranteed by the specifications which explains the quantitative and qualitative results. Several issues explain the low accuracy:

-Specifications, by their global reach, are extremely detailed but don't cover a classification commonly accepted by all contributors, who according to their profiles will be concerned in a limited part of the semantic possibilities. This causes semantic incoherencies depending on contributors: for example, secondary, tertiary and residential roads will not be classified in the same way.

-Even if it is not recommended, you can enter tags and values that are not present in the specifications.

-There is not a commission for the management of names, no more than recommendations for formatting (capitalisation, prefixes, etc. ...). There is therefore a very high unaccuracy in the names that are captured in the tag "name": for example on the same point of interest, we can have "Eglise Saint Jacques" and "Ecole Saint-Jacques".

As a consequence, standardised specifications are highly required to improve semantic and attribute quality of OSM data. Goodchild (2009) claims that completeness and currentness are the most significant aspects of VGI quality. Some aspects of completeness are measured in Table 5. Regarding the number of objects, OSM is far from being complete for every theme considered. Regarding the total length/area of objects, the difference is less important. This result means that smaller objects are more likely to be absent, reflecting the fact that contributors are more focused on capturing attracting objects (the most useful for their interest). It appears that the data density does not depend only on the density of information on the territory but also on the density of OSM contributors, that clearly excludes a number of territories (Figure 13). Thus, territories are best represented in rich areas and / or with a young population as shown by the extracts of the 15th arrondissement of Paris (Figure 14a) or the University Paul Sabatier in Toulouse (Figure 14b). The middle-size towns remain captured exhaustively, but lacks are observed at street level in some areas (Figure 15a). Completeness becomes very problematic in more rural and non profitable areas as in the Creuse (Figure 15a) where only a public utility mission would require surveys. Even in tourist areas like the Côte d'Azur, there are empty areas. Our assessment is confirmed by a more systematic analysis performed on English OSM data from [START_REF] Haklay | How good is OpenStreetMap information? A comparative study of OpenStreetMap and Ordnance Survey datasets for London and the rest of England[END_REF], who has shown that disadvantaged areas were less well covered.

Logical consistency

The logical consistency measures the consistency of different database objects with other objects of the same theme (intra-theme consistency) or objects of other themes (inter-theme consistency). For example, linear roads must be captured in network and must share the same geometry as the administrative boundaries when it is the case in the reality. Respect for logical consistency is generally done by the introduction of integrity constraints [START_REF] Servigne | A Methodology for Spatial Consistency Improvement of Geographic Databases[END_REF]. However, OSM does not contain any integrity constraints. Respect for logical consistency depends on the carefulness of each contributor during the capture. This results in a very bad logical consistency. Different cases of logical inconsistency of OSM data are presented below. Regarding intra-theme consistency, connectivity of roads is relatively assured (approx. 95%) using the snapping capacity of the capture tools (Figure 16a) while in the BD TOPO ®, this

(a) (b) (a) (b)
error is guaranteed at less than 1% using auto-correction tools. Furthermore the structure of the network is not at all guaranteed (Figure 16b) while a good model implicates to finish each line at every intersection [START_REF] Egenhofer | What's Special about Spatial, Database Requirements for Vehicle Navigation in Geographic Space[END_REF]. Nevertheless, whatever the region tried, the use of a simple geoprocess in ArcGIS, that computes the administrative units from the linear limits, crashes because of bad topology. In this case, the bad intra-theme consistency limits the use of data like in the case of multiple captures of the same real world entity (Figure 17b). Regarding inter-theme consistency, Figure 17a and Figure 18 shows how the capture of the administrative boundary has been carried out independently of the roads or the coastline, which generates strong inconsistencies. Moreover some statistics have been computed on river/administrative limit consistency: 68% of the tested administrative unit are not consistent with rivers but with great heterogeneity. Some parts are completely consistent ("La Garonne" for example) but most are completely inconsistent ("La Seine").

Currentness

The process of publication and diffusion of a particular update is faster in OSM, where contributions are automatically integrated without moderation. But the management of updates is not systematic and depends on the interests of contributors. Table 6 shows the evolution of French OSM data in number of objects over three months: a 30% mean increase for all objects. This evolution illustrates the reactivity of OSM contributors although the evolutions are mainly completions than updates.

(a) (b)

Lineage

A data lineage is provided: a comment about the data storage is associated to the contributor. However, we don't know how the data were captured except the software used. We do not know which support was used, as GPS tracks or satellite imagery. This can be quite annoying in the case of data for which methods can be multiple or even illegal. Instead, in the BD TOPO ®, a source attribute informs the method used to capture data (photo restitution, map or field collection...).

Moreover, the historic of modifications is not kept, making the integration of institutional data difficult: if the geometry is changed, it becomes impossible to propagate the official updates. [START_REF] Goodchild | Spatial Accuracy 2.0[END_REF] claims that each contributor cannot be trusted the same between institutions qualified as authoritative and simple contributors that are at most asserted. [START_REF] Coleman | Volunteered Geographic Information: the nature of motivation of produsers[END_REF] confirms such confidence differences and proposes call upon moderators like in Wikipedia to control the different type of contributions. A proper lineage metadata would allow to grant trustful contributions with rights forbidding less trustful contributions to update them.

Usage

Following the precedent discussion, we agree with the conclusions of [START_REF] Haklay | How good is OpenStreetMap information? A comparative study of OpenStreetMap and Ordnance Survey datasets for London and the rest of England[END_REF] that typical applications of OSM vector data appear to be limited to mapping. Navigation applications are limited by the problems of logical consistency: it is necessary to restructure all data in a GIS to use them for navigation, but then the lack of completeness would cause errors hardly acceptable. Road and river topological inconsistencies would also prevent the network be automatically pruned [START_REF] Touya | River Network Generalisation based on Structure and Pattern Recognition[END_REF] which is necessary to display it at different scales. Even classical geo-processing process like generating administrative polygons from limits massively fails because of low quality logical consistency (see section 3.6). Urban planning applications would be limited in particular by the strong heterogeneity of geometric precision. Finally, any application based on OSM data would face major problems of updating management.

The strong heterogeneity of OSM data resolution also affects their mapping. [START_REF] Ruas | Le changement de niveau de détail dans la représentation de l'information géographique[END_REF] shows that resolution and scale represent key concepts in cartographic visualisation linked to generalisation purposes [START_REF] Mackaness | Generalisation of geographic information: cartographic modelling and applications[END_REF]. Differences in resolution between themes, or with other themes, severely limit the possibility of automatic generalisation, and therefore, a correct representation of data at different scales. This also blocks a real definition of the reference scale of these data, which corresponds in the optimal scale for mapping [START_REF] Mackaness | Generalisation of geographic information: cartographic modelling and applications[END_REF].

Specifications to Improve VGI Quality

The evaluation of different aspects of OSM data quality reveal the key role of specifications to ensure quality as several error types come from a lack or fuzzy specifications. The specifications of a geographic database gather the selection and modelling choices of the database (Figure 19). In OSM, the specifications are rich and complex but informal specifications instead of being recorded in written formalised and well accepted specifications. A contributor is advised to follow the specification but does not have to. For instance, the inaccuracy of the coastline (Figure 9) is due to a lack of capture specifications: expert contributors aware of the challenges of quality and consistent contributions will strictly meet the specifications while novice contributors will probably ignore them. [START_REF] Goodchild | Spatial Accuracy 2.0[END_REF] agrees with the idea that VGI data lack standardised and precise specifications to be quality geographic data. Indeed, he claims that although crowdsourcing has proved qualitative in different domains, geographic information has particularities that require modelling and capture agreements. Gesbert ( 2005) proposes a formal model for geographic database specifications to allow the specifications to be machine-interpretable. [START_REF] Abadie | Formal specifications to automatically identify heterogeneities[END_REF] extends this formalisation to propose an XML specification model that helps to detect automatically inconsistencies in data, which could be useful to improve OSM data.

The definition of a reference scale and of a resolution [START_REF] Ruas | Le changement de niveau de détail dans la représentation de l'information géographique[END_REF] is also related to the definition of specifications. The capture of a contribution should be based on a reference scale, a resolution described in the specifications to avoid resolution heterogeneity. OpenStreetMaps clearly lacks reference scale ranges: captured data should be specified to be used in a specific scale range where its quality and resolution is good enough like in the ScaleMaster [START_REF] Brewer | Framing Guidelines for Multi-Scale Map Design Using Databases at Multiple Resolutions[END_REF]. Actually, each OSM feature has its own reference scale and resolution (buildings and built-up areas do not have the same resolution but coexist in OSM) but the information is not available in metadata.

Real

However, [START_REF] Coleman | Volunteered Geographic Information: the nature of motivation of produsers[END_REF] show that the majority of VGI contributors are occasional contributors and mostly interested amateurs that could be afraid of strict specifications for contributions. The success of VGI lies in the simplicity of contributions and many debates in the OSM contributor community show that should not be too restrained, even to improve quality. We believe that the improvement of OSM data quality requires to find the ideal balance between specifications and contribution freedom. A convenient way to reach such balance is to use automatic consistency checking with strict specification. Brando and Bucher (2010) propose to let contributions be simple and free but to check contribution consistency with the specifications. The automatic process would provide corrected and consistent contribution to the contributor that would be free to accept the modifications or not.

Conclusion and further work

In conclusion, this article assessed the data quality of the reference site for free and contributively created data OpenStreetMap. We described its models and objectives and assessed its data quality by comparing different components with an IGN database as a reference. We have shown that a production model of topographic data such as OSM has the advantage of responsiveness, flexibility, enabling a wealth of contents in privileged places. But the main drawback is a very high heterogeneity in several domains, limiting highly the possible applications. The contribution of the paper is the demonstration that data quality should not be summarised to geometrical position. The heterogeneity is due to the lack of standardised and precise specifications to would allow contributors to provide good quality data.

In perspective, improvements and evolution of quality control processes represent an integrant part of ongoing researches in the conception of an evaluation model of geometric imprecision in under qualified vector databases using, or not, reference datasets. The aim of this work is also to produce a more relevant and useful information on spatial data quality to the final user, based on knowledge of processes of production. Those first experimentations performed on OSM databases can help to calibrate the future model and also propose a first reference in the evaluation of OSM data quality, with [START_REF] Haklay | How good is OpenStreetMap information? A comparative study of OpenStreetMap and Ordnance Survey datasets for London and the rest of England[END_REF]. A final issue that will be tackled is the use of formal specifications to allow good quality VGI: a PhD has just started in our lab on this specific topic.

Figure 1 .

 1 Figure 1. Preview of the specifications (in French) of the tag "highway".

Figure 2 .

 2 Figure 2. Preview of an information leaflet to tag situations. Here, a tramway line.

Figure 3 .

 3 Figure 3. Example of a geometric primitive (a point) tagged in RDF in the XML export format of OSM.

Figure 4 .

 4 Figure 4. Localization (in green) of the 207 pairs of homologous point objects

Figure 5 .

 5 Figure 5. Distribution of Euclidean distances from the sample of road intersections

Figure 6 .

 6 Figure 6. Hausdorff distance (a) and average distance (b) between two polylines

Figure 7 .

 7 Figure 7. Distribution of Hausdorff distances from the sample of roads

Figure 9 .

 9 Figure 9. Representation of the 3 sample couples of coastlines. The grey area shows the gap between each couple.

  Figure 10. Surface distance between two polygons.The surface distance is defined in the interval [0, 1]. If the distance is equal to 0, the two polygons are equal, if the distance is equal to 1, the two polygons are disjoint. The comparison of polygonal objects is carried on the theme lakes in the mountainous region of l'Alpe d'Huez. A sample of 68 pairs of homologous surfaces is used (Figure11).

Figure 11 .

 11 Figure 11. Selection of OSM polygonal objects (blue) and BD Topo (red) and distribution of surface distances.

  and compactness statistical analysis of lakes in OSM and BD TOPO®.

Figure 12 .

 12 Figure 12. Statistics of Levenstein distance between lake names in OSM and BD TOPO® and distribution of the distances on the right.

Figure 13 .

 13 Figure 13. Heterogeneity of "Administrative Limit" layer completeness

Figure 14 .

 14 Figure 14. Extracts of OSM in areas well covered where data is quite diverse and complete: (a) in the 15th arrondissement of Paris (b) on the University Paul Sabatier in Toulouse.

Figure 15 .

 15 Figure 15. Extracts of OSM: (a) the town-center of Pau : medium-sized cities are moderately complete: most buildings and some streets are missing. (b) in Creuse: The information is extremely poor (municipal limits and some main roads).

Figure 16 .

 16 Figure 16. Problems of internal consistency in the road network: (a) roads are not connected at an intersection, (b) unstructured road network.

Figure 17 .

 17 Figure 17. (a) The selected administrative limit (blue) is not at all consistent with the roads (b) Several lakes are captured in the same location.

Figure 18 .

 18 Figure 18. Example of both types of inconsistencies: between administrative borders, that are sometimes captured on the coastline and sometimes not, and between coastline and administrative borders that should share geometry.

Figure 19 .

 19 Figure 19. Production process of structured institutional databases (Balley et al, 2006).

Table 1 . Statistics computed from Euclidean distances between crossroads.

 1

Table 4 . Quantitative attribute quality assessment: attribute filling of the shapefile layers extracted from CloudMade (October 2009) covering completely France.

 4

Table 5 . Completeness analysis for different object layers with the BD TOPO® as reference.

 5

Table 6 . Comparison of "France_shapefile" themes (extracted from CloudMade) between June 2009 and October 2009.

 6

Post-print of the paper published in Transactions in GIS (http://dx.doi.org/10.1111/j.1467-9671.2010.01203.x)

Acknowledgments

We first thank Damien Boilley, an OpenStreetMap passionate contributor for his explanations on the project. We also thank Bénédicte Bucher, who initiated this study.