Eric Justes

Rémi Vezy

Sebastian Munz

Kirsten Paff

Laurent Bedoussac

Noémie Gaudio

Patrice Lecharpentier

Dominique Ripoche

Marie Launay

Recent advances in intercropping modelling: the new version of the STICS soil-crop model simulates consistently a wide

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Recent advances in intercropping modelling: the new version of the STICS soil-crop model simulates consistently a wide range of bi-specific annual intercrops.

JUSTES Eric 1,4 , VEZY Rémi 2,4 , MUNZ Sebastian 3 , PAFF Kirsten 4 , BEDOUSSAC Laurent 5 , GAUDIO Noémie 5 , LECHARPENTIER Patrice 6 , RIPOCHE Dominique 6 and LAUNAY Marie (Brisson et al., 2003). Intercropping increases the complexity of the system by adding inter-species competition. Crop models are useful tools for analyzing complex systems, as they allow the user far more control over individual variables than is possible in field experiments. An initial version of the STICS intercrop model was created by Brisson et al. (2004) from the standard version (Brisson et al., 2003). Recently [START_REF] Vezy | Second iCROP2020 symposium[END_REF] improved this initial version [STICS-Intercrop_v2020] by adding new formalisms and replacing some equations which were not found relevant during the first step of testing with our database. The aim of paper is to present the STICS-Intercrop_v2020 functions and to show the quality of simulations obtained for a wide range of winter and spring intercrop mixtures of durum wheat -winter pea and barley -spring pea. STICS-Intercrop_v2020 was tested for two sites in France using various plant densities and N-fertilizer rates in order to determine its relevance and validity domain for simulating intercropping. This research work was done in 3 main steps: 1) improving the existing formalisms and introducing new equations in the model algorithm for improving the light sharing and nitrogen competition between the two species, and then creating the new version STICS-Intercrop_v2020; 2) parameterising the model for sole crops only; and 3) independently evaluating the quality of predictions for intercrops without any re-parametrisation. The results of parametrisation were satisfactory with low Root Mean Square Errors and high Model Efficiencies, illustrating the robustness and accuracy for sole crops, as already shown for many crops, pedoclimatioc conditions and agronomic managements (Brisson et al., 2003). STICS-Intercrop_v2020 was reliably and efficient to simulate inter-specific interactions, development and growth variables, and provided coherent results for predicting yield and grain protein content for the two species, in winter and spring bi-specific cereal-legume intercrops, without any specific parameterization for intercropping. This illustrates the relevance of the formalisms to simulate dynamically both intra and inter-specific plant interactions. These results are very encouraging for using the STICS-intercrop model for future work where virtual experiments will help us to analyse the performance (LER, level of production in low inputs systems, etc.) and resilience of sole crops versus intercrops according to management practices, pedoclimatic variability and climate change scenarios. This work was part of the ReMIX project funded by EU H2020 program.


Ana Teresa Juarez-Facio

J Yon

Cécile Corbière

Laurence Chevalier

Jean-Marie Vaugeois

Christelle Monteil

Jean-Marie Vaugeois

Épithéliales Bronchiques

Humaines Beas-2b

A T Juarez Facio

J.-M Vaugeois

Toxicity of organic ultrafine particles in human bronchial epithelial BEAS-2B cells

Keywords: BEAS-2B, Interface air-liquide, miniCAST, particules ultrafines, composés organiques / BEAS-2B, Air-Liquid Interface, miniCAST, ultrafine particles, organic compounds

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INTRODUCTION

Air pollution by ambient air particulate matter is a serious issue due to its health impacts in many urban and industrialized zones. Due to its physical properties, airborne ultrafine particulate matter is having more attention. Ultrafine particles (UFPs) have an aerodynamic diameter of 100 nm or less. They enter the body through the lungs and can be translocated to extrapulmonary organs as liver, spleen, kidney, heart, and brain [START_REF] Oberdörster | Translocation of Inhaled Ultrafine Particles to the Brain[END_REF][START_REF] Kreyling | Size Dependence of the Translocation of Inhaled Iridium and Carbon Nanoparticle Aggregates from the Lung of Rats to the Blood and Secondary Target Organs[END_REF][START_REF] Schraufnagel | The Health Effects of Ultrafine Particles[END_REF]. Moreover, because of their higher specific area and surface reactivity, UFPs can absorb a significant amount of hazardous metals and organic compounds making them more threatening than larger PM [START_REF] Kwon | Ultrafine Particles: Unique Physicochemical Properties Relevant to Health and Disease[END_REF].

The principal source of UFPs emissions in urban environments is on-road vehicles, more specifically motor vehicles driven by diesel engines. Particles from diesel exhaust comprise of polyaromatics hydrocarbons which can diffuse easily through cell membranes because of their hydrophobic properties. Once in the cell, as free radicals, they cause oxidative damage to biological macromolecules including DNA [START_REF] Leikauf | Mechanisms of Ultrafine Particle-Induced Respiratory Health Effects[END_REF]. Studies show finer particle size increases the mutagenic and tumorigenic potential [START_REF] Pagano | Mutagenic Activity of Total and Particle-Sized Fractions of Urban Particulate Matter[END_REF]. In addition, UFPs induce inflammatory cytokines release such as IL-6, IL8. Therefore, organic compounds presented in UFPs represent a significant contribution to the toxicity and health effects.

Particle exposure of lung cells at the air-liquid interface (ALI) allows to study toxicity of particles in conditions close to the physiological ones. In this approach, cells are cultured on semi permeable membranes of a transwell insert in a culture well where medium is supplied only from the basal pole, then cells are exposed to an aerosol at the apical pole [START_REF] Bérubé | Human Primary Bronchial Lung Cell Constructs: The New Respiratory Models[END_REF][START_REF] Bhowmick | Cells and Culture Systems Used to Model the Small Airway Epithelium[END_REF]. Concerning the aerosol generation, several devices exist to carry controlled and repeatable cell exposures at relevant airborne concentrations. MiniCAST is a soot reference generator where soot particles, produced from a co-flow propane diffusion flame, are quenched, and quickly diluted to stabilize the soot particle stream and prevent water condensation. Depending on the operating conditions, particles characteristics as organic content, number and size distribution can be modified while maintaining a stable and repeatable source [START_REF] Moallemi | Characterization of Black Carbon Particles Generated by a Propane-Fueled Miniature Inverted Soot Generator[END_REF][START_REF] Moore | Mapping the Operation of the Miniature Combustion Aerosol Standard (Mini-CAST) Soot Generator[END_REF][START_REF] Jing | Standard Combustion Aerosol Generator (SCAG) for Calibration Purposes[END_REF]. Thus, MiniCAST soot generator depicts an adequate device for studying particles toxicity.

This work aims to show the toxicological effects of ultrafine particles with an important organic compounds content. Beas-2B model was exposed at the air-liquid interface and cell response was linked to the microphysical, chemical and morphological properties of miniCAST particles.

MATERIALS AND METHODS

Cell culture

BEAS-2B cell line (ATCC -CRL9609 TM ) derived from normal human bronchial epithelial cells were obtained from autopsy of non-cancerous individuals. BEAS-2B cell line was cultured in LHC-9 medium (GIBCO) supplemented with 100 U/ml penicillin and 100 mg/ml streptomycin in collagen pre-coated flasks to improve cell adherence until 75-80% of confluence. Then, they were seeded at a density of 90,000 cells per cm 2 on collagen-coated Transwell TM inserts. After 3 days, cells reached the confluence and medium is changed every 2 days. Culture is maintained in Liquid-Liquid interface for 7 days prior to exposure. Apical medium is removed, and Air-Liquid interface is carried during and after exposure. The cultures were maintained in 95% humidified air with 5% CO2 at 37°C.

Exposure system and cell exposure conditions

The exposure system is comprised of several main components: soot generator miniCAST (5201c), the Vitrocell® exposure system, and several devices for physical characterization (PPS, SMPS). The operating condition CAST3 was chosen for its high contents on organic carbon and for assuring a size distribution lower than 100 nm [START_REF] Bescond | Soot Optical Properties Determined by Analyzing Extinction Spectra in the Visible Near-UV: Toward an Optical Speciation According to Constituents and Structure[END_REF]). The Vitrocell® exposure system comprises two stainless steel modules (exposure module and air control module). Aerosols are delivered to the modules in parallel: the first module receiving the aerosol of study and the second module receiving filtered clean air as negative control. The modules are designed for simultaneous exposure of three independent replicates. Exposure module is equipped with electrodes for high voltage charging to increase particle deposition.

Prior to exposure, the exposure wells are filled with medium and heated at 37°C by a circulating water bath. During exposure, an electrical field (+1000 V) was used to increase the particle deposition efficiency. Aerosol is delivered to the system by the distribution/dilution column where is drawn by the trumpet pipe (isokinetic sampling) to each well. Here, aerosol passed over the apical pole of the cell layer cells at a flow rate of 10 ml/min/well covered by an external vacuum pump. Cells were exposed during 35 min and control samples were exposed for 35 min to ambient HEPA-filtered laboratory air through the Vitrocell System®. Each exposure condition was repeated on 2-3 separate days. We compared the response of soot-exposed cells with the response of control cells, 3 h and 24 h after exposure.

Characterization of PUFs

Number and particle size distributions were measured with Scanning Mobility Particle Sizer at a range of 12.2-552.3 nm immediately downstream of the Vitrocell® system after went through VKL dilutors. Mass emission was measured with the Tapered Element Oscillating Microbalance and controlled with the Pegasor Particle Sensor directly after VKL dilutors. Transmission electron microscopy (TEM) was used to examine the morphology of the miniCAST soot.

Toxicological endpoints

Cytotoxicity was measured by the MTT assay adapted to the exposures in transwell inserts. Gene expression. Total ARN was extracted from cells after exposure using TRI-REAGENT® (Sigma-Aldrich) and Direct-zol™ RNA MiniPrep (Ozyme). cDNA was prepared by reverse transcribing extracted RNA samples using Invitrogen™ Transcriptase inverse M-MLV. The expression of mRNA was determined by the quantitative real time polymerase chain reaction (qRT-PCR) using Brilliant III Ultra-fast SYBR Green QPCR Master Mix (Agilent Technologies). Expression profiles of Cyp1b1, Cyp1a1, NQO1, IL8 et IL6 were normalized to B2M.

RESULTS AND DISCUSSION

Aerosol characteristics

The size distribution by SMPS and transmission electron microscopy image are shown in Figure 1. The count median diameter (CMD) was 56.95 nm and total count concentration was 2.44E+08 N/cm 3 . Mass concentration during exposure was 57.85 mg/m 3 . Ratio of organic carbon (OC) and total carbon (TC) shows the organic content of particles and it was around 87±5 %. Estimated mass deposition during 35 min exposure was 1775 ng/cm 2 . These data are presented in Table 1. According to these data, CAST3 particles can be classified as ultrafine particles with a high content in organic compounds.

Cellular response to CAST3 particles

BEAS-2B were exposed to CAST3 particles during 35 min in Vitrocell® System. No cytotoxicity was observed at any condition (Figure 2.) nevertheless, gene expression, was modified. Expression of CYP1A1 and CYP1B1 were highly modified in both conditions and it was time dependent. Organic compounds as PAH's present in CAST3 can explain these results as they can induce cytochrome P450 (CYP) metabolic enzymes. Several studies have shown these enzymes play vital roles in oxidative stress and pro-inflammatory responses. IL8 and IL6 (inflammation markers) expression was reduced at 3 h and at 24 h was slightly increased. HO1, an antioxidant marker highly induced at 3 h but get back to the normal at 24 h. Finally, NQO1 another antioxidant marker was slightly induced in both conditions. The induction of NQO1 and IL8 after fine and ultrafine particles exposure has been shown in several studies [START_REF] Hawley | Time Course of Bronchial Cell Inflammation Following Exposure to Diesel Particulate Matter Using a Modified EAVES[END_REF][START_REF] Yuan | Ambient Fine Particulate Matter (PM2.5) Induces Oxidative Stress and pro-Inflammatory Response via up-Regulating the Expression of CYP1A1/1B1 in Human Bronchial Epithelial Cells in Vitro[END_REF][START_REF] Steerenberg | Diesel Exhaust Particles Induced Release of Interleukin 6 and 8 by (Primed) Human Bronchial Epithelial Cells (Beas 2b) In Vitro[END_REF][START_REF] Frias | Nrf2 Positively Regulates Autophagy Antioxidant Response in Human Bronchial Epithelial Cells Exposed to Diesel Exhaust Particles[END_REF]). Nrf2 has a protective role against atmospheric pollutants. Nrf2 induces the expression of NAD(P)H-quinone oxidoreductase (NQO1). This enzyme exerts cytoprotective, antioxidant and antiinflammatory effects in lungs [START_REF] Rubio | Essential Role of Nrf2 in Protection against Hydroquinone-and Benzoquinone-Induced Cytotoxicity[END_REF]. Table 2. Count Median Diameter (CMD), Count and Mass Concentration, ratio of organic carbon (OC) and total carbon (TC) and estimated mass deposition during exposure to CAST3 particles.

CONCLUSION

CAST3 particles induce several modifications in gene expression. Physicochemical properties as organic content and surface could explain these results. However, in this work, complete aerosol exhaust was used during cell exposures so the biological response cannot be attribute only to particles themselves but also to the gas phase. Further investigations are needed to better understand the contributions of particles alone in the toxic response. UFPs are presented in urban ambient air and physicochemical properties depend on emission source. These particles can reach many other organs than lungs causing several health effects. Study of UFPs in the last years had got more attention and become a new area of toxicology research.

Figure 2 .:

 2 Figure 2. : Cell viability determined by MTT test after 3 h and 24h exposure of CAST3 (+1000V) particles. The data was expressed as mean ± SD of 2-3 independent experiments.


published or not. The documents may come

Active flow control in an axial compressor for stability improvement: on the effect of flow control on stall inception

Gabriel Margalida, Pierric Joseph, Olivier Roussette, Antoine

Active flow control in an axial compressor for stability improvement: On the effect of flow control on stall inception Gabriel Margalida 1 , Pierric Joseph 1 , Olivier Roussette 1 and Antoine Dazin 1 * 1 Univ. Lille, CNRS, ONERA, Arts et Metiers Institute of Technology, Centrale Lille, UMR 9014 -LMFL Laboratoire de Mécanique des fluides de Lille -Kampé de Fériet, F-59000 Lille, France.

INTRODUCTION

One of the major issues that penalize modern aircraft engines, lies in the loss of stability when the compressor pressure ratio is increased.

To respond to the urgent need to reduce engines emissions, one solution is to increase the operating range of the compressors and to operate them at higher pressure ratios. To do so, delaying the rotating stall onset becomes more and more critical, especially with the future generation of engines which aims at smaller core and Boundary Layer Ingestion (BLI) where this issue is even more limiting.

To tackle this problem, lot of efforts have been invested into designing and implementing effective passive or active flow control systems to delay the onset of the stall phenomenon without affecting the performances at nominal conditions. In parallel to these works, the understanding of the inception of the first instability has been refined. Several routes to stall have been observed [START_REF] Camp | A Study of Spike and Modal Stall Phenomena in a Low-Speed Axial Compressor[END_REF], with the now well known "mode" [START_REF] Mcdougall | Stall Inception in Axial Compressors[END_REF]) and "spike" (Day 1993) type rotating stall, and detailed flow mechanisms for each process have been proposed thanks to a combination of experimental and numerical works.

While the definition of the "mode" type rotating stall inception is very clear in the literature, "spike" type rotating stall, and more importantly the "spike" itself, can correspond to different physical phenomena. In some cases, this denomination can thus be misleading, and a more precise analysis of the stall inception mechanisms is often necessary.

Historically, "spike" is the name given to the short-scale perturbation spotted in the velocity or pressure signals directly prior to the rotating stall, few revolutions earlier, and rotating at ~70% of the rotor rotation speed. A strong correlation has firstly been observed between the spike onset and the upstream displacement of the interface between the Tip Leakage Flow (TLF) and the incoming flow [START_REF] Hoying | Role of Blade Passage Flow Structurs in Axial Compressor Rotating Stall Inception[END_REF]. As the operating point moves closer to the stability limit, the mass flow decreases and thus the incoming flow weakens in respect to the TLF which then push the interface. More recently, experimental [START_REF] Deppe | Spike-Type Stall Inception in Axial-Flow Compressors Fujita H, Takata H (1984) A Study on Configurations of Casing Treatment for Axial Flow Compressors[END_REF]) and numerical works [START_REF] Vo | Criteria for Spike Initiated Rotating Stall[END_REF] have showed that the "spike" is directly linked to the tip leakage flow, and a criterion involving two simultaneous conditions has been proposed [START_REF] Vo | Criteria for Spike Initiated Rotating Stall[END_REF]) (see Fig. 1).

The first condition specifies that the interface between the tip leakage flow and the main flow should be parallel to the leading-edge plane. This condition leads to the disturbance of the leadingedge region by the Tip Leakage Vortex (TLV) and paves the way to the second condition. This second one relies on a simultaneous spillage of the tip leakage in front of the leading edge of a blade and a backflow at the trailing edge of the previous one. This backflow will then reinforce the spillage. However, recent studies have brought new elements on the table. [START_REF] Pullan | Origins and Structure of Spike-Type Rotating Stall[END_REF] have observed typical pressure spikes on the pressure signals extracted from numerical simulations undergone on configurations without any tip gap, and thus without tip leakage nor backward spillage. Moreover, they spotted the formation of radial vortices, extending from the suction side of blades to the casing, and they linked these structures to the "spike" they observed. These radial vortices had been already observed, in fact, by [START_REF] Inoue | Comparative Studies on Short and Long Length-Scale Stall Cell Propagating in an Axial Compressor Rotor[END_REF], 2004) and Yamada et al. [START_REF] Yamada | Effects of Tip Clearance on the Stall Inception Process in an Axial Compressor Rotor[END_REF], in a large tip gap configuration in parallel to tip leakage vortex breakdown, just prior stall formation. Even more recently, these results have been completed by a numerical analysis of the effect of the tip gap size on the flow mechanism involved during stall inception and a strong link between both has been reported [START_REF] Hewkin-Smith | The Role of Tip Leakage Flow in Spike-Type Rotating Stall Inception[END_REF]. It has been especially shown that for small tip clearance (under a critical value of τ=0.5%.C), no spillage happens and that the rotating stall is caused by an increase of the incidence due to blockages at the exit of the blade passage. On the contrary, for bigger tip clearance, the leakage momentum prevents the formation of these blockages, but backflows can occur. These two modes of "spike" type rotating stall are respectively referred as "casing corner separation dominated" and "tip leakage jet dominated".

In parallel Active control technics, and more precisely discrete tip injection, have shown their effectiveness on the improvement of the compressor stability by moving away the surge line [START_REF] Suder | Compressor Stability Enhancement Using Discrete Tip Injection[END_REF][START_REF] Nie | Micro Air Injection and Its Unsteady Response in a Low-Speed Axial Compressor[END_REF][START_REF] Kefalakis | Detailed Measurements on an Axial Compressor Stage with Application of Discrete Tip Injection for Increasing the Surge Margin[END_REF][START_REF] Li | The Dual Mechanisms and Implementations of Stability Enhancement With Discrete Tip Injection in Axial Flow Compressors[END_REF]. Some successful applications have also been demonstrated on real aircraft engines [START_REF] Stößel | Rotating Stall Inception Inside the Low Pressure Compressor of a Twin-Spool Turbofan Engine[END_REF][START_REF] Stößel | Ejector Tip Injection for Active Compressor Stabilization[END_REF]. Understanding their effects is thus of first interest but surprisingly, few works have investigated the mechanisms involved when active flow control is applied and how it affects the flow patterns in the tip gap while extensive work in this domain has been done with casing treatments [START_REF] Takata | Stall Margin Improvement by Casing Treatment-Its Mechanism and Effectiveness[END_REF][START_REF] Smith | Flow Phenomena in Compressor Casing Treatment[END_REF]Fujita and Takata 1984;[START_REF] Cumpsty | Part-Circumference Casing Treatment and the Effect on Compressor Stall[END_REF].

The main goal of this paper is to analyze the effect of the active flow control system on a compressor stage: how the flow patterns are affected by the blowing and how it affects the performances of the stage. To do so, the paper will first present the flow mechanisms associated with the stall inception in the compressor, in a base state, without control. For these tasks, well refined unsteady pressure measurements are used to monitor the development of the first instants of the rotating stall and to produce detailed pressure fields.

EXPERIMENTAL SETUP

Test compressor and control system

The experimental study of the effect of the pulsed air injection on stability improvement has been conducted on the single stage low pressure axial compressor CME2 operated at 3200RPM. The main characteristics of the compressor are reminded in Table 1and more details are available in a previous paper [START_REF] Veglio | Pressure Measurements in an Axial Compressor: from Design Operating Conditions to Rotating Stall Inception[END_REF]. A general view of the test rig is given in Fig. 2 and the performance curve is plotted in Fig. 3, where the total-to-static pressure rise coefficient Ψ t-s = ΔPt-s /0.5ρUtip 2 is represented as a function of the flow coefficient Φ =Vx/Umid . More information about the compressor can be found in a previous paper [START_REF] Margalida | Detailed Pressure Measurements During the Transition to Rotating Stall in an Axial Compressor: Influence of the Throttling Process[END_REF]. The air injection system is composed of 20 injection blocks divided into two injectors (this configuration of 20 blocks of two injectors has been preferred to 20 bigger injectors mainly because of the space constraints and to limit their protrusion on the inner surface of the casing). Each injector has its own solenoid valve (Matrix, model MX821) whose frequency ranges from 0 to 500Hz with an inlet pressure between 0 and 8 bar relative generating jet speed up to 200m.s -1 . Each injection block can be operated independently from the others and they are all linked to the same compressed air supply through 10 distributors equally spaced around the compressor.

Fig. 3 Compressor performances without and with active flow control (continuous injection, 30 injectors, qi=1.7%.q, vi=137m.s -1 ; pulsed injection, 20 injectors, qi=0.7%.q, vi=108m.s -1 )

To benefit as much as possible of the injection at the tip leading edge (the performances of the injection increases when the jet is closer to this region [START_REF] Nie | Micro Air Injection and Its Unsteady Response in a Low-Speed Axial Compressor[END_REF])) the injectors are located 10mm upstream of the leading edge of the rotor (x = -20%.Cx) and they have been designed to take profit of Coandă effect, already used by [START_REF] Strazisar | Compressor Stall Control Through Endwall Recirculation[END_REF], allowing a tangential blowing, attached to the casing. The injected mass flow rate can be changed from 0% to 2% of the compressor mass flow rate either by changing the supply pressure or the duty cycle of the solenoid valves when pulsed injection is used. For this study, the injectors blow axially through 10mm wide slots.

The control system is presented on Fig. 4.

Measurement setup and experimental procedure

To evaluate the performances of the compressor, three static pressures (Ps02, Ps10, Ps20) and one total pressure (Pt01) are measured at different axial locations of the test bench. Their location is displayed on Fig. 5.(a). In controlled operation, the total pressure at inlet is corrected by the added kinetic pressure coming from the jets, which was determined by hot-wire measurements. The mass flow rate and static-to-static pressure rise measurement uncertainties are respectively equal to ±0.012kg.s -1 and ±1.3Pa.

The instantaneous pressure measurements were realized with unsteady pressure transducers (Endevco model 8507C-1 and 8510B-1), flush mounted on the casing. To monitor the development of the rotating stall, 20 sensors distributed all around the compressor just upstream the leading edge of the rotor (x/Cx = -0.06), as represented on The sampling rate for each test was set to 100 kHz, corresponding to more than 60 measurement points on each blade channel at 3200RPM, allowing to catch perturbations of small temporal and spatial extent. The pressure transducers uncertainty ranges from 0.5% to 1.5% of the full-scale output. To trigger the rotating stall, the mass flow rate is slowly decreased using the throttling valve visible on Fig. 2. For all instantaneous measurements (i.e. pressure fields and perturbation monitoring), the throttling process is continuous and similar to the procedure described in a previous paper [START_REF] Margalida | Comparison and Sensibility Analysis of Warning Parameters for Rotating Stall Detection in an Axial Compressor[END_REF]. For performance measurements however, the process is realized point by point, interspersed with stabilization phase, by carefully decreasing the mass flow from one operating point to the next one. For the last stability point (last point of the performance curve before stall), it is worth mentioning that the stall onset is spontaneous.

RESULTS AND DISCUSSION

Stall onset in the axial compressor CME2

Before looking at the effect of the air injection on the flow pattern at stall onset, it is necessary to clearly determine the mechanism involved without control. The first thing that can be noticed is the slope of the total-to-static performance curve at the last stability point. When the stability limit is reached before the maximum of the performance curve, the compressor is most likely to exhibit "spike" type rotating stall [START_REF] Camp | A Study of Spike and Modal Stall Phenomena in a Low-Speed Axial Compressor[END_REF]. A look at the filtered instantaneous pressure traces (Butterworth, 3 rd order, 250Hz), represented on Fig. 6, confirms this first observation. Indeed, no modal activities can be spotted long before rotating stall while a sudden pressure spike is visible at the beginning of the phenomenon. The absence of modal perturbations has also been confirmed through spectral analysis.

In the literature, tip leakage spillage and spike type rotating stall has been observed experimentally on compressors with relatively large tip clearances, between τ = 1 % to τ = 2% (see To determine the mechanism involved at stall onset, flow features at the scale of the blade passage are investigated in order to check for the clues of tip leakage spillage, associated to the "spike", observed by other authors. One of them is associated to the backflow at the exit of a blade passage where the first spike occurs. We can see on The observation is also true when comparing with other compressors where spillage has been observed. On Fig. 7, two instantaneous pressure contours of the first half of the passage and distant of one blade pitch, are displayed near stall. Each blade passage will be referred by the number of the blades surrounding it and the same reference is kept from one plot to the other. On these plots, one can easily notice the presence of numerous low pressure spots that can be the signature of radial vortices (noted from "A" to "F"), equivalent to the ones highlighted by [START_REF] Pullan | Origins and Structure of Spike-Type Rotating Stall[END_REF]. It is also possible to see that those spots have a very short lifespan, of the order of one blade pitch.

Indeed, spot "C" and "D" or "A" and "E" are not visible respectively one blade pitch later and one blade pitch earlier on 2012). However, the disturbances observed here differ from them on some points. Rotating instabilities, which evolve in compressor with larger tip clearance, tend to have longer, but irregular, lifespan (several revolutions, [START_REF] Young | Stall Warning by Blade Pressure Signature Analysis[END_REF][START_REF] Yamada | Effects of Tip Clearance on the Stall Inception Process in an Axial Compressor Rotor[END_REF]). This characteristic, coupled with their unsteady nature, leads to the presence of a "hump" [START_REF] Young | Stall Warning by Blade Pressure Signature Analysis[END_REF], visible on the frequency spectrum, at a fraction of the blade passing frequency. As explained earlier, the perturbations observed here have a shorter lifespan which explain the absence of the typical "hump" associated to rotating instabilities. These disturbances appear downstream the leading edge of the rotor and they are quickly convected from the suction side of one blade to the pressure side of the following one. a blocked channel between blade 3 and 4, noted "B" (greater static pressure at the inlet and pressure drop "D" at the exit, circumference wise, and zero pressure gradient blade to blade and leading edge to trailing edge) and a large low-pressure region between blade 4 and 5, noted "C", can be seen. When the filtered pressure trace is observed at the leading edge, one can see that the perturbation is characterized by a rapid pressure oscillation, of the same kind of typical "spike". However, 1.5 revolutions earlier, on Fig. 8.(b), it can be seen that the channel 2-3 exhibits a pressure drop, referred "A", from one blade to the other. This event is well highlighted by an iso-pressure line at the rotor inlet, represented by the black line, which connects the suction side of blade 4 to the leading edge of blade 5. Finally, it is possible to see that the well-defined TLV on Fig. 8.(a) disappears close to the stability limit, on pressure contour (b) and (c), suggesting a highly unsteady TLV (this feature had already been seen in past works, on phase averaged pressure maps with the TLV trace splitting in two close to the stability limit, see [START_REF] Veglio | Pressure Measurements in an Axial Compressor: from Design Operating Conditions to Rotating Stall Inception[END_REF])). The last characteristic associated to the "spike" is its rotational speed. It is commonly admitted in the literature that it rotates at 70% of the rotor speed during several revolutions before triggering the rotating stall which rotates at 40% of the rotor speed. In the present case, it has been impossible to find a consistent mean rotational speed close to this value. Thereby, the rotational speed evolution of the first perturbation (visible on Fig. 8.(c) in our case) has been monitored in order to better describe the transition into stall. The method used to determine the speed of the perturbation, already presented in details in a previous paper [START_REF] Margalida | Detailed Pressure Measurements During the Transition to Rotating Stall in an Axial Compressor: Influence of the Throttling Process[END_REF], consists in determining the position of the wave front of the perturbation and to derive it to obtain its speed. This method has been applied on 45 tests and to the pressure traces found in the work of [START_REF] Weichert | Detailed Measurements of Spike Formation in an Axial Compressor[END_REF] where the measurement configuration is equivalent to the one used here and where tip leakage flow spillage has been observed. The average speed law has then been compared to the one computed from the literature. They are both displayed on Fig. 9. In the Deverson compressor, used in the study of [START_REF] Weichert | Detailed Measurements of Spike Formation in an Axial Compressor[END_REF], a constant speed perturbation is indeed present during several revolutions before stall onset. However, in the present case, the speed evolution exhibits a continuous decrease from around 85%.ω, when the perturbation is first detected, to 40%.ω, when the rotating stall is established, in less than 5 rotor revolutions. The main difference between both experiments is the size of the tip gap which support its importance and its influence in the mechanism involved. Two explanations can be considered to explain this difference in behavior. On one hand, the life span of the "spike" (i.e. the time during which its speed is constant around 70%.ω) can be a function of geometrical features of the compressor, such as the tip gap size or the blade tip stagger angle, and that some configurations can promote it during several revolutions. On the other hand, the "spike" exhibited herein may be the signature of a phenomenon different from other cases where the spike is linked to tip leakage spillage, TLV breakdown or radial vortices, such as an embryonic stall cell for example that directly slows down because of its large impact on the upstream incidence, If we sum up everything, the rotating stall onset in the compressor used in the present study is directly preceded by a "spike" type perturbation, as historically defined, that appears suddenly and quickly slows down to the typical speed of the rotating stall. This perturbation, that occupies more than one blade pitch, at the time of its detection, is characterized by a decrease of the net work in the affected blade passage, leading to a decrease of the static pressure directly behind it. Moreover, no sign of tip leakage spillage is spotted but several revolutions before the rotating stall onset, lowpressure zones appear between the blades. These spots are most likely the trace of radial vortices, as predicted by [START_REF] Pullan | Origins and Structure of Spike-Type Rotating Stall[END_REF] in small tip gap configurations (zero tip gap in theirs). They are also the evidence of leading-edge separations at the tip. As mentioned earlier, a criteria of the spike initiation has been proposed in previous papers [START_REF] Deppe | Spike-Type Stall Inception in Axial-Flow Compressors Fujita H, Takata H (1984) A Study on Configurations of Casing Treatment for Axial Flow Compressors[END_REF][START_REF] Vo | Criteria for Spike Initiated Rotating Stall[END_REF]) but other observations doesn't fit quite well in this definition with one or more discrepancies (tip leakage vortex breakdown in addition to backflow [START_REF] Yamada | An Explanation for Flow Features of Spike-Type Stall Inception in an Axial Compressor Rotor[END_REF], "spike" and radial vortices in a zero tip gap geometry [START_REF] Pullan | Origins and Structure of Spike-Type Rotating Stall[END_REF], different mechanisms depending on the size of the tip gap leading to a "spike" (Hewkin- [START_REF] Hewkin-Smith | The Role of Tip Leakage Flow in Spike-Type Rotating Stall Inception[END_REF]).

The evidences highlighted in this work has led to two main hypothesis when it comes to the mechanism involved in the stall formation. A schematic representation of some of these events is proposed in Fig. 10 to support the explanation. At their root, these mechanisms are similar to the one proposed by [START_REF] Pullan | Origins and Structure of Spike-Type Rotating Stall[END_REF]. An increase in incidence (Fig. 10.(a.1)) at the tip will provoke a leading edge separation and then trigger the rotating stall without spillage. However, the main difference lies in the definition of the "spike". In our case, the radial vortices (Fig. 10.(a.2)) cannot be considered as "spike", because they are not strong enough yet to induce a visible 'spike' in a single pressure signals. On the other hand, the first perturbation that could be defined as so, depicted on Fig. 10.(b.2), do not exhibit a constant speed (as seen in Fig. 9) and already provoke performance degradation, as seen in the pressure field of Fig. 8.(c) at mark "D". We can therefore wonder whether, in the present case, the first perturbation is not already an embryonic stall cell.

Active flow control effect

The effect of the tip injection has been tested for two injection modes, continuous and pulsed. An example of the performance curves with continuous and pulsed injection compared to the baseline is given in Fig. 3. As it has already been observed, active flow control is improving the performance curve in two ways: (i) the stable operating range of the compressor is increased (decrease of the flow coefficient at stall) and, (ii) at a given flow coefficient, the pressure rise is increased. The following paragraphs aim at giving the physical explanations to this dual improvement.

To begin with, similarly to the analysis realized in the previous section, several instantaneous pressure contour maps have been compared at different operating points. The first feature that should be affected by the air injection is the tip leading edge separation and the formation of radial vortices, previously exhibited in pressure contour. To evaluate this aspect, operating points at the same flow coefficient (Φ=0.37, point A and B on Fig. 11), have been compared with and without control. A sufficiently high injected mass flow rate has been chosen to ensure noticeable effects on the flow pattern. The effectiveness of the device is also highlighted by the comparison of the stall mechanism with and without control. For this work, instantaneous pressure contour maps near stall have been analyzed for both configurations (i.e. Φ = 0.37 without control and Φ = 0.31 with control. Points A and C on Fig. 11). These maps are displayed on Fig. 13, for 30 revolutions. For the sake of visibility, the blade passage frequency has been filtered. One can observe two very different behaviors from one configuration to the other. Without control we can recognize the sudden pressure rise just before stall around t = 532 revs on Fig. 13.(a). However, several differences are visible when control is activated. High pressure spots can be seen between x = 50%.Cx and x = 60%.Cx from t = 575 revs to t = 590 revs when rotating stall starts. These high-pressure spots are accompanied by strong axial fluctuations of the pressure field, clearly visible through the wave pattern in the first half of the passage, and on the iso-pressure line at rotor inlet, on Fig. 13.(b). These long wavelength and small amplitude fluctuations which appear several dozens of revolutions before the onset of stall and slowly increase in strength, clearly recall mode-like perturbations (should it be in amplitude or in frequency). It should also be noted, that, in the controlled configuration, (see Fig. 3) the slope of the total-to-static performance curve, at the last stable point, is no more negative, which is a typical favorable configuration for the development of mode-like stall inception. To evaluate how the air injection affects the stability of the compressor and thus the position of the stability limit, an extensive parametric study of the injection has been realized. Several parameters have been studied and two main cases are particularly interesting for this matter: the effect of the arrangement and the effect of the pulse width, or more precisely, the duration without blowing, called t off, which is changed through the duty cycle of the solenoid valve. To study specifically the effect of the toff, the duty cycle and the frequency of the injection have been setup to ensure that (i) the frequency has no effect on the performances and (ii) the toff is significantly smaller than the total time of one injection cycle. This type of injection, schematized in Fig. 14, is referred as pseudo-continuous blowing. The investigated ranges of toff and their corresponding frequency are summarized in Fig. 14. The first improvements of the stalling mass flow rate are obtained when the duration without blowing becomes smaller than the order of magnitude of few rotor revolutions. Indeed, as depicted on Fig. 15.(b), under 3-4 revolutions, the mass flow rate at stall starts decreasing quickly and continuously. A plateau is then reached when this duration gets close to the order of magnitude of few blade passages. This behavior can be linked to the mechanism presented earlier, involving the formation of radial vortices. When the toff duration is small enough, these structures have less time to develop1 F 2 . For very small toff, radial vortices, and leading-edge separations, are totally suppressed by the blowing and decreasing it does not bring more benefits, thus explaining the plateau. That is also why reducing the off-blowing duration from one revolution to one blade pitch only affect the mass flow rate at stall. The same results are obtained when the angular space between each injector is changed. Decreasing this distance, allows to decrease the mass flow rate at stall.

The second effect visible concerns the pressure rise improvement. While the improvement on the mass flow rate suddenly disappears for larger toff values, a noticeable gain in pressure rise remains, even for extended period without blowing. In the present case, for duration ranging from 10 to 50 rotor revolutions, this gain slowly disappears but it is still visible for the longer periods... As showed by [START_REF] Suder | Compressor Stability Enhancement Using Discrete Tip Injection[END_REF], and as pointed out above, this improvement is obtained through the decrease of the blade loading at the tip of the blades and its redistribution along the span. It appears, 2 It can be noted here that for long toff, the improvement on the mass flow rate becomes negative. In those situations, rotating stall onset most likely appears during the period without blowing because of the radial vortices. During this period, the apparent flow coefficient is the same than without blowing. However, each points of the curve are averaged over one injection cycle. The displayed flow coefficient is thus a combination of the main and injected mass flow rate.

The flow coefficient at stall thus appears greater and that is why the SMIq is negative.

through our results, that this improvement, unlike the effect on the mass flow rate, is persistent and can be obtained with small and sparse impulses.

Thus, in summary, increasing the axial velocity at tip through air blowing (i) prevents the occurrence of the separations on the suction side, (ii) decreases the blade loading at tip. This leads to a stabilization of the TLV and probably also to a decrease of its magnitude.

These two effects prevent completely the development of the flow mechanism at the origin of the spike-type instability described in the first part of the paper. The operating range of the compressor is thus extended until it reaches an area where the slope of the characteristic curve is becoming slightly positive, which leads to a mode-like stall inception. The mode perturbation become possible when the critical incidence at the tip isn't reached before the maximum of the performance curve [START_REF] Camp | A Study of Spike and Modal Stall Phenomena in a Low-Speed Axial Compressor[END_REF]. Thus, using active flow control at the tip allows to avoid spike type rotating stall but does not immunize from mode perturbation that can therefor still grow. Secondly, the secondary flows at tip, and the losses associated with them, are strongly reduced which explains the increase of the pressure rise.

CONCLUSION

In this paper, the flow mechanisms involved during the stall inception with and without active flow control have been analyzed.

Without control, the axial compressor used in this study exhibits identical characteristics than "spike" type rotating stall, such as the negative slope of the total-to-static performance curve at stall or the absence of modal activities.

However, despite apparent similarities with "spike" mechanisms, some differences emerge from the present results compared to previous experimental studies but seem to confirm observations from numerical works for small tip clearance geometries.

While no sign of backflow or tip leakage spillage has been observed, several low-pressure spots are present several dozens of revolutions before stall onset. These spots are most likely the signature of radial vortices formed by leading edge separations at the tip of the blade, as described by [START_REF] Pullan | Origins and Structure of Spike-Type Rotating Stall[END_REF], with one end attached to the suction side and the other end to the casing.

Moreover, it has been shown that the first perturbation detected, classically called "spike", doesn't rotate at a fixed rotational speed of 70%.ω but instead quickly and continuously slows down from 85% to 40%.ω. This perturbation occupies more than one blade pitch and is followed, downstream, by a drop of the static pressure suggesting that this perturbation is already an embryonic stall cell.

This paper has then investigated the effect of active flow control on the mechanisms involved at stall. The control system has successfully increased the operating range of the compressor by counteracting "spike" mechanisms and by moving the last stability point close to the maximum of the performance curve, where "mode" type rotating stalls can occur. Indeed, radial vortices totally disappear when blowing is used and modal oscillations are spotted before the onset of rotating stall.

An emphasize can be made on this last observation because it also suggests that the SMI granted by any active flow control system, designed to act on the spike mechanisms, will always be limited by the maximum of the performance curve.

The active control system enables a dual improvement by decreasing the stalling mass flow rate and increasing the pressure rise for a given operating point. The first effect is obtained by the suppression of the leading-edge separations at the tip, highlighted by the disappearance of radial vortices, and the second by a reduction of the losses associated to the secondary flows and the instability of the Tip Leakage Vortex.

A particular emphasize has also been carried out on the different characteristic times of these two effects. On one hand, to suppress correctly and permanently leading-edge separations, the duration between two encounters of a jet by a given blade need to be small enough (1-2 blade passages in our case). This will constraint the minimum frequency for pulsed injection and the number of injectors to be used which will also depend on the rotational speed of the rotor. On the other hand, the pressure rise improvement, obtained by redistribution of the blade loading, appears to last longer (around 50 revolutions in our case).

Fig. 1

 1 Fig.1Schematic representation of the flow conditions required for spike onset,[START_REF] Vo | Criteria for Spike Initiated Rotating Stall[END_REF]

Fig

  Fig. 2 CME2 axial compressor Rotational speed 3200 rpm Design mass flow rate 5.3 kg.s -1 Design axial velocity, LE 43 m.s -1 Rotor blade number 30 Stator blade number 40 Casing diameter 550 mm Hub-tip ratio, LE 0.75 Rotor tip chord 84 mm Rotor tip stagger angle 54 ° Rotor tip gap 0.5 mm

  Fig. 5.(b), have been simultaneously used. The pressure contours of the tip leakage flow are obtained through 14 pressure probes aligned axially every 4mm, at the same angular position, from the leading edge (x/Cx = -0.04) to the trailing edge (x/Cx = +1) (see Fig. 5.(c)).

Fig. 4

 4 Fig. 4 Drawings of the control system. Axial (a) and radial (b) cutaway of the control system implemented on the compressor, isometric view of one injection bloc (c) and bottom view (d).

Fig. 5

 5 Fig. 5 Measurement setup

  Fig. 6.(a) that no overpressure is visible, unlike Deppe et al. (2005) measurements, shown on Fig. 6.(b), where tip leakage spillage has been observed. Both experimental setups are operated at an equivalent rotational speed and exhibit relatively close nominal conditions, but they have very different tip clearance sizes relative to their chord length.

Fig. 6

 6 Fig. 6 Filtered instantaneous pressure trace for three axial locations in the CME2 compressor (a) and from the literature exhibiting backflows (Deppe et al. 2005) (b)

  Fig. 7.(a) and Fig. 7.(b). On the contrary spots "B" and "F", firstly seen on Fig. 7.(b) close to the suction side of blade 2 and 6, are closer to the leading edge of blade 3 and 7 on Fig. 7.(a)0 F 1 . These radial vortices, also called tornado like vortex in the literature, are often associated to rotating instabilities such as in the work of Young et al. (

Fig. 7

 7 Fig. 7 Pressure contour at two angular positions, one blade pitch apart, near stall On Fig. 8 three pressure contours are represented for three operating points, near nominal conditions, marked t0, and two at stall onset, 1.5 revolutions apart, marked t1 and t2. A filtered pressure trace at the beginning of the rotating stall is also presented on Fig. 8.(d) for reference. On Fig. 8.(c), a blocked channel between blade 3 and 4, noted "B" (greater static pressure at the inlet and pressure drop "D" at the exit, circumference wise, and zero pressure gradient blade to blade and leading edge to trailing edge) and a large low-pressure region between blade 4 and 5, noted "C", can be seen. When the filtered pressure trace is observed at the leading edge, one can see that the perturbation is characterized by a rapid pressure oscillation, of the same kind of typical "spike". However,

Fig. 8

 8 Fig. 8 Pressure contour for three operating points at θ=311°. Near nominal conditions (a), near stall conditions (bc) and associated filtered static pressure trace at leading edge few revolutions before stall (d). 0Pa isoline in plain black.

Fig. 9

 9 Fig. 9 First perturbation speed law in the present case (small tip gap, τ=0.006) and in the literature (Weichert and Day 2013) (medium tip gap, τ=0.012)

  From the previous observations, this stall cell could be initiated by one of the leading edge separations, scenario S1 (a.1 → b.1 → b.2), or by one of the resulting radial vortices, scenario S2 (a.1 → a.2 → b.2).

Fig. 10

 10 Fig. 10 Schematic representation of the proposed stall onset mechanism. (a) radial vortices formation through leading edge separation; (b) full channel blockage (large separation or bigger radial vortex) leading to embryonic stall cell formation

Fig. 11

 11 Fig. 11 Operating points compared The instantaneous pressure contour maps for these two conditions are displayed on Fig. 12. On this figure, the signature of radial vortices is well visible on several blade passages close to the leading edge (Fig. 12.(a)) but they are totally absent when active control is activated (Fig. 12.(b)). It is also possible to see that in presence of flow control, the trace of the Tip Leakage Vortex is visible again (one of them is represented by a dotted line on Fig. 12.(b)) when it was impossible to clearly detect it without control. This observation thus confirms the stabilizing effect of tip blowing on the tip of the blade by suppressing the leading-edge separation which is at the origin of the radial vortices generation and tip leakage vortex unsteadiness.

Fig. 12

 12 Fig.12Instantaneous pressure contour at the same operating point, Φ=0.37, without control (a) and with control (continuous blowing, 40 injectors, qi=1.7%.q, vi=110m.s -1 ) (b)

Fig. 13

 13 Fig. 13 Instantaneous filtered pressure contour, near stall, without control, Φ=0.37, (a), and with control, Φ=0.31 (continuous blowing, 40 injectors, qi=1.7%.q, vi=110m.s -1 ) (b). 0 Pa isoline in plain black

Fig. 14

 14 Fig. 14 Schematic representation of a blowing cycle The effect of the toff is firstly examined through Fig. 15.(b) where the Stall Margin Improvement (SMI) is plotted as a function of the duration without blowing. Four performance curves, with and without control, are also displayed on Fig. 15.(a). When the duration off-injection is changed, the compressor experiences two decoupled effects. To account that, the SMI has been decomposed

Fig. 15

 15 Fig.15Effect of the duration without control on the stability and performances of the compressor (pseudo-continuous blowing, 40 injectors, qi=0.75%.q, vi=60m.s -1 )

Table 1

 1 Compressor parameters

Table 2

 2

	Compressor used	τ=s/C
	Pfleiderer-Inst. stage 2, (Deppe et	0.015
	al. 2005)
	Deverson compressor, (Day 1993;	0.012
	Weichert and Day 2013)
	E 3 Rotor B, (Vo et al. 2008; Pullan	0
	et al. 2015; Hewkin-Smith et al. 2019)	0.018
	CME2	0.006

), compared to the critical value exhibited by

[START_REF] Hewkin-Smith | The Role of Tip Leakage Flow in Spike-Type Rotating Stall Inception[END_REF]

. The axial compressor CME2 however, features a tip clearance of τ = 0.6 %, making it more representative of the configuration used by

[START_REF] Pullan | Origins and Structure of Spike-Type Rotating Stall[END_REF]

and

[START_REF] Hewkin-Smith | The Role of Tip Leakage Flow in Spike-Type Rotating Stall Inception[END_REF]

for small tip clearances.

Table 2

 2 Normalized tip clearance size, τ=s/C, for different compressors

It is hard, though, to assert with total certainty that spot "B" and "F" of Fig.7.(a) are the same than their counterparts of Fig.7.(b) and not new ones. However, even if the second case apply, the conclusion of the short lifespan of the perturbation remains valid.

LIST OF SYMBOLS

Xt-s

Total-to-static quantity ΔP Pressure rise (Pa) Ψ Pressure rise coefficient, Ψt-s = ΔPt-s /0.5ρUtip 2 . q Mass flow rate (kg.