W Abou Saoud

Aymen Amine Assadi
email: aymen.assadi@ensc-rennes.fr

Monia Guiza

Abdelkrim Bouzaza

Wael Aboussaoud

Isabelle Soutrel

Abdelmottaleb Ouederni

Dominique Wolbert

Sami Rtimi
email: sami.rtimi@epfl.ch

Wala Abou Saoud

Abatement of ammonia and butyraldehyde under non-thermal plasma and photocatalysis: Oxidation processes for the removal of mixture pollutants at pilot scale

Keywords: odorous pollutants, photocatalysis, DBD-plasma, mixture effect, synergetic effect

published or not. The documents may come

Abatement of ammonia and butyraldehyde under non-thermal plasma and photocatalysis Oxidation processes for the removal of mixture pollutants at pilot scale

Introduction

Inorganic compounds such as ammonia and hydrogen sulfide, and a large variety of Volatile Organic Compounds VOCs like aldehydes, ketones, mercaptans, aromatics etc., which are frequently extruded by various industrial and agricultural processes as well as animal husbandry and fertilizer applications [START_REF]Pollutions olfactives: origine, législation, analyse, traitement[END_REF][START_REF] Schiavon | Non-thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: a Review[END_REF][START_REF] Behera | Ammonia in the atmosphere: a review on emission sources, atmospheric chemistry and deposition on terrestrial bodies[END_REF], constitute a significant origin of malodorous air pollutants. These harmful pollutants cause a serious problem which may present a threat to the human health and the environment [START_REF]Pollutions olfactives: origine, législation, analyse, traitement[END_REF][START_REF] Schiavon | Non-thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: a Review[END_REF][START_REF] Sultana | Abatement of VOCs with Alternate Adsorption and Plasma-Assisted Regeneration: A Review[END_REF]. Therefore, it is indispensable to reduce its concentration discharge into the atmosphere. Various methods have been largely applied for the remediation of atmospheric pollution such as adsorption [START_REF] Rodrigues | Ammonia adsorption in fixed bed of activated carbon[END_REF], thermal and catalytic incineration [START_REF] Doggali | Combustion of volatile organic compounds over Cu-Mn based mixed oxide type catalysts supported on mesoporous Al 2 O 3 , TiO 2 and ZrO 2[END_REF] and bio-filtration [START_REF] Hu | Biofiltration perfermonce and characteristics of high-temperature gaseous benzene, hexane and toluene[END_REF]. Among these traditional methods, bio-filtration based techniques showed higher efficiency for odor control [START_REF] Kaosol | Ammonia Gas Removal from Gas Stream by Biofiltration using Agricultural Residue Biofilter Medias in Laboratory-scale Biofilter[END_REF][START_REF] Parmar | Emerging Control Technologies for Volatile Organic Compounds[END_REF][START_REF] Schiavon | Non-thermal plasma assisting the biofiltration of volatile organic compounds[END_REF], however does have some limitations. The requirement of a large floor surface constitutes one of the major inconveniences which limit its use [START_REF] Kaosol | Ammonia Gas Removal from Gas Stream by Biofiltration using Agricultural Residue Biofilter Medias in Laboratory-scale Biofilter[END_REF][START_REF] Parmar | Emerging Control Technologies for Volatile Organic Compounds[END_REF]. Alternatively, there is an extensive attention in developing innovative technologies as an efficient and least-cost treatment method [START_REF] Schiavon | Non-thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: a Review[END_REF]. For this purpose, advanced oxidation processes (AOP) such as non-thermal plasma (NTP) [START_REF] Schiavon | Non-thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: a Review[END_REF][START_REF] Sultana | Abatement of VOCs with Alternate Adsorption and Plasma-Assisted Regeneration: A Review[END_REF][START_REF] Karatum | A comparative study of dilute VOCs treatment in non-thermal plasma reactor[END_REF][START_REF] Bahria | Impact of design parameters on the performance of non-thermal plasma air purification system[END_REF] and photocatalysis (TiO 2 /external UV-lamp) [START_REF] Boyjoo | A review on photocatalysis for air treatment: From catalyst development to reactor design[END_REF][START_REF] Jiang | Catalysis Removal of Indoor Volatile Organic Compounds in Room Temperature: From Photocatalysis to Active Species Assistance Catalysis[END_REF][START_REF] Rochetto | Degradation of volatile organic compounds in the gas phase by heterogenous photocatalysis with titanium dioxide/ultraviolet light[END_REF][START_REF] Huang | Removal of Indoor Volatile Organic Compounds via Photocatalytic Oxidation: A short Review and Porspect[END_REF] have been proved to be efficient oxidation techniques to purify air pollution. Several studies on TiO 2 /UV oxidation have been investigated to be a promising process for VOCs degradation into nonhazardous mineral species (carbon dioxide and water vapor) [START_REF] Boyjoo | A review on photocatalysis for air treatment: From catalyst development to reactor design[END_REF][START_REF] Rochetto | Degradation of volatile organic compounds in the gas phase by heterogenous photocatalysis with titanium dioxide/ultraviolet light[END_REF][START_REF] Schneider | Understanding TiO 2 Photocatalysis: Mechanisms and Materials[END_REF]. Dielectric barrier discharge DBD-plasma as one of the NTP methods generated by electrical discharge has been extensively applied for removing pollutants including aromatics [START_REF] Karuppiah | Catalytic nonthermal plasma reactor for the abatement of low concentrations of benzene[END_REF], aldehydes [START_REF] Gharib-Abou Ghaida | Association of surface dielectric barrier discharge and photocatalysis in continuous reactor at pilot scale: Butyraldehyde oxidation, by-products identification and ozone valorization[END_REF], inorganic compounds [START_REF] Liang | Performance of non thermal DBD plasma reactor during the removal of hydrogen sulfide[END_REF] and mercaptans [START_REF] Abou Saoud | Study of synergetic effect, catalytic poisoning and regeneration using dielectric barrier discharge and photocatalysis in a continuous reactor: Abatement of pollutants in air mixture system[END_REF], through of very reactive species created (e.g., O • , HO • and O 3 ) under ambient conditions (atmospheric pressure and room temperature) [START_REF] Karatum | A comparative study of dilute VOCs treatment in non-thermal plasma reactor[END_REF][START_REF] Bahria | Impact of design parameters on the performance of non-thermal plasma air purification system[END_REF][START_REF] Zhu | Influence of energy efficiency on VOCs decomposition in non-thermal plasma reactor[END_REF][START_REF] Huu | Methane, Propene and Toluene Oxidation by Plasma-Pd/c-Al 2 O 3 Hybrid Reactor: Investigation of a Synergetic Effect[END_REF]. Therefore, there is a growing interest in DBD-plasma's activity enhancing when combined with photocatalytic process, could acquire synergetic effect and higher efficiency either at pilot or at industrial scale [START_REF] Dou | Enhanced removal of toluene by dielectrical barrier discharge coupling with CU-Ce-Zr supported ZSM-5/TiO 2 /Al 2 O 3[END_REF][START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF][START_REF] Mei | Plasma-photocatalytic conversion of CO 2 at low temperatures: Understanding the synergistic effect of plasmacatalysis[END_REF][START_REF] Assadi | Use of DBD plasma, photocatalysis, and combined DBD plasma/photocatalysis in a continuous annular reactor for isovaleraldehyde elimination-Synergetic effect and byproducts identification[END_REF]. Moreover, a number of studies on dielectric barrier discharge (DBD) plasma-photocatalysis coupling [START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF][START_REF] Maciuca | Synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air[END_REF][START_REF] Aouadi | MnOx/TiO 2 Catalysts for VOCs Abatement by Coupling Non-thermal Plasma and Photocatalysis[END_REF] reveal very interesting prospects on single VOCs oxidation, other than few are being further developed on VOCs mixture treatment.

The Novelty of this work is to follow the degradation behavior of a mixture of contaminants from different nature like organic and inorganic compounds with a high flow. In this study, butyraldehyde (C 4 H 8 O) and ammonia (NH 3 ) were selected as model molecules. Photocatalysis (TiO 2 /UV-lamp) and dielectric barrier discharge DBD-plasma are applied simultaneously in a continuous reactor to properly appraise the performance of the system on pollutants removal and mineralization. Effect of experimental conditions on ozone formation inside the DBD-plasma reactor wasalso studied and discussed. In addition, at pilot scale, the synergistic effect resulting from the combination of DBD-plasma and photocatalysis (TiO 2 /UV-lamp) was determined under equimolar mixture condition. Finally, butyraldehyde alone and in equimolar mixture with ammonia (NH 3 /C 4 H 8 O) intermediate by-products are identified.

Experiment and measurement

Experimental setup

The experimental setup designed to study the coupling between DBD-plasma and (TiO 2 /external UV-lamp) photocatalysisis shown in Figure 1(a). The processis equipped with a polluted air generation system, cylindricalreactor, gas sampling equipment and analysis systems.The (TiO 2 /UV)-DBD reactor consists of two concentric cylinders one outer cylinder of 76 mm and an inner cylinder of 58 mm (Figure 1(b)) with wall thickness of 4 mm, both 100 cm long. Glass Fiber Tissue (GFT) was coated withTiO 2 as performed by Ahlstrom Research and Services.It is maintained on the inner reactor wall by the stainless steel grid electrode. It can be simultaneously submitted to surface DBD-plasma and UV irradiation. Thus, UV lamp is placed in the inner concentric cylinder in order to have a homogeneous irradiation. To generate DBD-plasma, high voltage power is applied to the reactor which is covered by a copper grid forming the outer electrode and the inner is on aluminum. The applied voltage is delivered by a generator as a sinusoidal waveform up to 10 V and then amplified by an amplifier to achieve 30 kV. More details about the electrical system of the DBD-plasma reactor has been described in detail in a previous work [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF]. The polluted air generation was performed according to the same methodologies as applied in our previous study [START_REF] Abou Saoud | Study of synergetic effect, catalytic poisoning and regeneration using dielectric barrier discharge and photocatalysis in a continuous reactor: Abatement of pollutants in air mixture system[END_REF]. The pollutants, butyraldehyde (C 4 H 8 O) and ammoniac (NH 3 ), are injected continuously in liquid state (from a pure liquid 99.9% and 20% aqueous solution, respectively) by means of a syringe-syringe driver system (Kd Scientific Model 100) (Figure 1(a)).

Pollutants and by-products identification

Initial concentration of butyraldehyde and ammonia, gas flowrate, and relative humidity were monitored. A concentration ranging from 0.5 to 4 mmol m -3 was achieved by regulating the mass flow meter (Bronkhorst In-Flow). For experiments under humid condition, a portion of the air flow is derived through a packed column where water flows in counter current and the relative humidity value ranging from 5 to 90%. Temperature and relative humidity (RH) are determined using a specific sensor (Testo 445).

Butyraldehyde concentration during each test was measured manually with a syringe of 500 µL using a gas chromatography (Fisons GC9000) equipped with a flame ionization detector (GC-FID), whereas to identify the by-products formed during the experiments, a Gas Chromatograph-Mass Spectrometer (GC-MS) coupled to an infrared (IR) detector was used. NH 3 concentration was determined via spectrophotometric method using obtain colored solution. Ammoniac concentration was then determined by measuring light absorbance at 420 nm with a spectrophotometer. The plot of absorbance with the amount of NH 3 in solution obeyed to a linear relationship with a correlation coefficient of 0.997. The ozone concentration was monitored by iodometric titration method. Thus, outlet air flow is bubbled in a potassium iodide (KI) solution at 10 -2 M, until a yellow color emerges.The sample is then titrated with sodium thiosulfate solution (Na 2 S 2 O 3 ) at 10 -2 M until a colorless solution is obtained [START_REF] Costa | Study of butyraldehyde degradation and by-products formation by using a surface plasma discharge in pilot scale: Process modeling and simulation of relative humidity effect[END_REF]. All experiments were operated under ambient conditions (atmospheric pressure and ambient temperature).

Analytic methods

Process performance was evaluated by measuring:

• Inlet [C in ] and outlet [C out ] concentrations of pollutants • Removal efficiency of pollutants: RE ሺ%ሻ ൌ ሺሾେ ሿିሾେ ౫౪ ሿሻൈ ଵ ሾେ ౫౪ ሿ (1)
• Specific input energy: SIE ሺJ L െ1 ሻ ൌ 3600 * P ሺWሻ Q൫m 3 h െ1 ൯ൈ 1000 [START_REF] Schiavon | Non-thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: a Review[END_REF] where P is input power adjusted by changing the applied voltage (U a ) and Q is the flowrate.

• Selectivity of CO:

SCO ሺ%ሻ ൌ ሾେ ౫౪ ሿିሾେ ሿ ସ ൈ ୖሺ%ሻൈ ሾେ ሿ * 10000 (3) • Selectivity of CO 2 : SCO 2 ሺ%ሻ ൌ ሾCO 2 out ሿെሾCO 2 in ሿ 4 ൈ REሺ%ሻൈ ሾC in ሿ * 10000 (4)

Surface atomic concentration as by X-ray Photoelectron Spectroscopy (XPS)

The surface atomic concentration of the species existing in the top most layers of the tissue were evaluated using XPS as previously described [START_REF] Rtimi | TiON and TiON-Ag sputtered surfaces leading to bacterial inactivation under indoor actinic light[END_REF]. An AXIS NOVA photoelectron spectrometer (Kratos Analytical, Manchester, UK) equipped with monochromatic Al Ka (hm = 1486.6 eV) anode was used during the study. The carbon C1s line with position at 284.6 eV was used as a reference to correct the charging effects. The surface atomic concentration of elements was determined from peak areas using known sensitivity factors [33]. Spectrum background was subtracted according to Shirley [START_REF] Shirley | Corrections of electrostatic charged species in SP-spectroscopy[END_REF]. The applied subtraction was Gaussian-Lorentzian 30 (GL [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF]). The XPS spectra of the species were analyzed by mean of spectra deconvolution Casa XPS software. The samples were kept in flasks previously flushed with Argon to reduce the risk of contamination and reactivity after the experiment.

Results and discussion

Degradation of ammonia and butyraldehyde was studied as single compounds as well as equimolar mixture with ammoniac. Photocatalytic experiments with external UV source (TiO 2 /UV-lamp) were done first, then the DBD-plasma was studied without external UV lamp, and finally the combination of photocatalysis and plasma (TiO 2 /UVlamp/DBD-plasma) in the same reactor was studied.

Study of monocompound system

Effects of inlet concentration and flowrate on photocatalytic oxidation

To determine the performance of the photocatalytic oxidation, the influence of some process parameters should be clearly defined. For this purpose, the effects of inlet pollutants concentration and gas flowrate were investigated. Figure 3 shows the removal efficiency (RE) of butyraldehyde and ammonia using a photocatalytic reactor.

The experiments were performed ranging from 0.6 to 4 mmol m -3 for two different flowrates i.e. 1 and 2 m 3 h -1 . It was observed, that by increasing the flow rate, the removal efficiency of two pollutants decreased as it is readily seen in Figure 3. This can be attributed to the decrease in the contact time between the pollutant and the catalytic active sites. This leads to a reduced amount of pollutants able to react with active species attached to the surface. Photocatalytic degradation of butyraldehyde and ammonia clearly decreased as inlet concentration increased. However, higher concentration increases the quantity of pollutants in the reactor which potentially limits the degradation under the same experimental condition (amount of catalysts and UV intensity). Hence, efficiencies decrease at higher concentrations as reported in literature [START_REF] Abou Saoud | Study of synergetic effect, catalytic poisoning and regeneration using dielectric barrier discharge and photocatalysis in a continuous reactor: Abatement of pollutants in air mixture system[END_REF][START_REF] Zhong | Experimental and modeling study of visible light responsive photocatalytic oxidation (PCO) materials for toluene degradation[END_REF]. As for ammonia, results indicate that the RE remains low (~ 38%) compared to butyraldehyde and that category of inorganic compound is very hard to degrade by photocatalytic oxidation by itself. Moreover, this behavior can be attributed to the molar mass of butyraldehyde hampering the adsorption and thus the photocatalytic oxidation.

Effect of humidity on the photocatalytic oxidation

Humidity constitutes also an important parameter that can directly affect the trend of gas-phase photocatalytic reactions. Thus, the influence of relative humidity on butyraldehyde removal using photocatalysis was studied and to understand its effect, three various relative humidity conditions were controlled (dry air, medium humidity 50%, and high humidity 90%) at different butyraldehyde inlet concentrations. The results

BUTY: Q= 1m3/h BUTY: Q= 2m3/h NH3: Q=1 m3/h NH3: Q=2 m3/h C 4 H 8 O: Q= 1 m 3 h -1 C 4 H 8 O: Q= 2 m 3 h -1 NH 3 : Q= 1 m 3 h -1 NH 3 : Q= 2 m 3 h -1
are shown in Figure 4. The presence of water molecules in the air stream improves butyraldehyde oxidation for either values of inlet concentration. Under dry condition (HR=50%), the (C 4 H 8 O) removal efficiency was 44% and reached an optimum of 56% at 1.4 mmol m -3 of (C 4 H 8 O). However, the slight increase of humidity helps the photogeneration of hydroxyl radical HO • as very reactive specie, leading to an enhancement in the VOCs degradation. When moisture increases, the competitive effect between water molecules and pollutants on active sites becomes more important which causes a decrease of removal efficiency. At initial concentration of 4mmol m -3 , it can be noted that in the case of high humidity (RH=90%), the (C 4 H 8 O) degradation decreased of 50% compared to 45% RH. This trend can be due to the competition between water vapor and butyraldehyde to adsorb on the active sites of the photocatalytic surface. We note also that, whatever the inlet pollutant concentration increases, the removal rate increases at low RH. Hence, based on these results, the optimal water vapor value was found to be 50%. It was shown by Martinez et al. that the increase of humidity beyond an optimal level decreases BTEX photocatalytic degradation [START_REF] Martinez | BTEX abatement by photocatalytic TiO 2 -bearing coatings applied to cement mortars[END_REF]. Assadi et al., obtained similar results when they studied the isovaleric acid degradation [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF]. removal efficiencies. This can be explained by the high-energy values and the electric discharges producing more reactive species, which may contribute to the improvement of pollutant degradation. In Figure 5, the bisector-line was set to compared the removal efficiency obtained by coupling plasma and photocatalysis (vertical axis) to the results obtained by the sum of removal efficiencies by plasma process alone or photocatalytic process alone (horizontal axis). If there is no synergistic effect when coupling both plasma and photocatalysis, or if there is a negative effect, the points have to be confused with the bisector (Y axis = X axis ) or below it. The points obtained are above the bisector, wish means that there is a positive effect when coupling both processes.

Comparing the combined processes (TiO 2 /UV-lamp/DBD-plasma) with photocatalysis and DBD-plasma separately, results show that the coupling exhibits higher performance, which is greater than the sum of DBD-plasma alone and photocatalysis alone (Figure 5). For instance, at SIE = 15 JL -1 , the rate of butyraldehyde removal efficiency obtained by combined processes reached 57.71%, whereas the sum of RE by DBD-plasma and photocatalysis is 47.88%. As for ammonia, the degradation rate was 59.26% when applying TiO 2 /UV-lamp/DBD-plasma with respect to 52.5% of the sum of both processes. In this case, we note an enhancement of 7-10% on removal efficiency.

Results indicate that, whatever the pollutant used, there is a strong synergy between both processes. Hence, plasma can greatly contribute to the desorption of the degraded byproducts adsorbed on TiO 2 surface through the active species which leads to increase catalytic activity of TiO 2 and consequently, improves its photocatalytic degradation.

Study of the removal of gas mixture system

Effect of initial concentration increase on photocatalytic oxidation

The aim here was to compare the behavior of butyraldehyde and ammonia degradations when treated separately, to the behavior when both gazes are mixed. In this experiment, competition adsorption of pollutants on the photocatalysts active sites. This behavior is similar to what it was reported by Debono et al. [START_REF] Debono | VOC ternary mixture effect on ppb level photocatalytic oxidation: Removal kinetic, reaction intermediates and mineralization[END_REF]. The latter evidenced the inhibitive effect along the photocatalytic degradation of three VOCs mixture. Indeed, Figure 6 indicates efficiencies of NH 3 and H 2 S strongly depend on the applied voltage. It was observed that boosting energy improves the level of electrons, which enhances the production of reactive radicals and consequently leads to greater degradation efficiency. These observations are in agreement with other studies [START_REF] Xia | Removal of ammonia from gas streams with dielectric barrier discharge plasmas[END_REF][START_REF] Maxime | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF]. Figure 7 shows that compared to the photocatalytic oxidation, the removal efficiency of ammonia by DBD-plasma is clearly more effective than to photocatalysis. It should be noted from Figure 7 that coupling (TiO 2 /UV-lamp/DBD-plasma) significantly enhances the removal efficiencies of C 4 H 8 O and NH 3 . In the case of (C 4 H 8 O/NH 3 ) mixture, results show that the synergetic effect observed for the system "TiO 2 /UV-lamp/DBD-plasma" was significantly better for ammonia than that for butyraldehyde. Thus, when photocatalysis was used, just 7% of the C 4 H 8 O was decomposed. During DBD-plasma process, at SIE= 18 JL -1 and without external UV-lamp, BUTY removal reached to 28% and when coupling photocatalysis with DBD-plasma, the BUTY removal increased to 40%. Regarding ammonia, at the same applied energy (18 JL -1 ), it can be noted that the removal efficiency of ammonia was around 20% for photocatalysis, reaches 51% for DBD-plasma and increased to 83% for coupling photocatalysis and DBD-plasma. Removal Efficiency RE (%)

Effect of SIE increase on mixture oxidation

Plasma

Photocatalysis

Combination Plasma/Photocatalysis (SIE= 9.9 J L -1 ) (SIE= 9.9 J L -1 ) (SIE= 18 J L -1 ) (SIE= 18 J L -1 )

NH 3 NH 3 C 4 H 8 O C 4 H 8 O

Ozone formation

Ozone is considered a primary by-product generated under DBD-plasma reactor. To investigate the effect of relative humidity and UV on ozone formation, the concentration of ozone at the outlet of the reactor monitored at various energy densities. 8 shows the ozone concentration measured either under DBD-plasma alone or when coupling photocatalysis (TiO 2 /UV lamp) with plasma under dry or humid conditions. The results show that humidity strongly affected ozone production in the reactor. At high humidity (HR=90%), ozone concentration is strongly decreased whether for DBD-plasma or when coupling DBD-plasma with photocatalysis. Ozone concentration drop from 30ppm (DBD-plasma) and 23ppm (TiO 2 /UV-lamp/DBDplasma), under dry condition, to 6ppm (DBD-plasma) and 5ppm (TiO 2 /UV-lamp/DBDplasma) under high humidity for SIE= 18 JL -1 . This lower ozone concentration can be attributed to the fact that ozone can be decomposed by the photo-generated radicals (H • , HO • ) created at the photocatalyst interface according to equation 5-7 below [START_REF] Maxime | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF][START_REF] Shi | Abatement of Gaseous Xylene Using Double Dielectric Barrier Discharge Plasma with In Situ UV Light: Operating Parameters and Byproduct Analysis[END_REF], leading to its very. In the same way, adding photocatalysis (TiO 2 /UV-lamp) to DBDplasma led to ozone removal. Thus, for its decomposition into very reactive species (O 2

•-and HO 2 • ), ozone requires highly energetics photons, mainly UV [START_REF] Abou Saoud | Study of synergetic effect, catalytic poisoning and regeneration using dielectric barrier discharge and photocatalysis in a continuous reactor: Abatement of pollutants in air mixture system[END_REF]. reached the highest selectivity for photocatalysis with a rate of 72% against 45% for plasma and 56% for the combination DBD-plasma and photocatalysis. Regarding carbon monoxide CO, Figure 9 shows that CO selectivity measured at the outlet of the reactor under combined processes became lower with a mineralization rate of less than 8%. In order to better understand the degradation pathways of (C 4 H 8 O/NH 3 ) mixture, all other by-products generated under the combined processes (TiO 2 /UV-lamp/DBDplasma) were identified by Gas Chromatograph-Mass Spectrometer (GC-MS). All [Mixture] = 2 mmol m -3 , SIE= 9.9 J L -1 , RH = 5%).

H ଶ O

Catalyst poisoning and process sustainability

The surface atomic concentrations of elements before and after the mixture decomposition were investigated by XPS. Table 1 shows the catalyst poisoning before the treatment and after the effect of the combined processes [START_REF] Abou Saoud | Study of synergetic effect, catalytic poisoning and regeneration using dielectric barrier discharge and photocatalysis in a continuous reactor: Abatement of pollutants in air mixture system[END_REF]. It is readily shown that the combined processes (TiO 2 /UV-lamp/DBD-plasma) degraded the mixture pollutants and reduced the surface poisoning. This is seen after the poisoning of the catalyst especially for the surface atomic concentration of carbon and nitrogen species, which decrease after the pollutants degradation and mineralization as seen in Table 1. It has been reported that during the photocatalytic oxidation, some carboxylic acid intermediated can be formed and rapidly mineralized to CO 2 and H 2 O by photo-Kolbe reaction [START_REF] Rtimi | Insight on the photocatalytic bacterial inactivation by co-sputtered TiO 2 -Cu in aerobic and anaerobic conditions[END_REF].

Conclusion

In this study, the performance of three oxidation processes e.g. photocatalysis (TiO 2 /UV-lamp), DBD-plasma and their combination (TiO 2 /UV-lamp/DBD-plasma) was described. On the one hand, this work investigates the decomposition of butyraldehyde and ammonia separately. On the other hand, it compares their mixture using three methods selected i.e. (i) DBD-plasma, (ii) photocatalysis, (iii) their combination (TiO 2 /UV-lamp/DBD-plasma). The combination of plasma and photocatalysis was shown to improve significantly the process efficiency. These improvements were related to the degradation and the mineralization: (i) the removal efficiency of butyraldehyde and ammonia was remarkably enhanced and a synergetic effect was shown for all experimental conditions; (ii) a better performance in mineralization rate was observed. Indeed, CO 2 selectivity attained 11% higher than DBD-plasma alone, and (iii) low ozone concentration was detected. The process validation for the mixture (C 4 8 O/NH 3 ) was also studied in detail. The amounts of ozone were lower than that of single butyraldehyde and ammonia treatment. Ammonia could be decomposed easily via (TiO 2 /UV-lamp/DBD-plasma) process compared to photocatalysis. At high humidity, the competitive behavior between water molecules and butyraldehyde towards the catalytic active sites became higher. This competitive behavior led to the decrease in the butyraldehyde removal and to the decrease of ozone formation during the process.

Moreover, butyraldehyde alone and in equimolar mixture (NH 3 /C 4 H 8 O) intermediate byproducts are identified. Surface poisoning was observed before applying the combined process. After applying the combination (TiO 2 /UV-lamp/DBD-plasma), the catalytic active sites were liberated and the pollutants were mineralized. The coupling of the two processes shown in this study is seen to be a promising solution for indoor/outdoor air treatment and needs more investigation in the near future.

Figure 1 .

 1 Figure 1. Schematic (a) of the experimental setup and sectional drawing (b) of the annular reactor

Figure 3 .

 3 Figure 3. Variation of removal efficiency (RE) of butyraldehyde and ammonia with inlet concentration at different flow rates (T = 20 °C, UV intensity= 20 W m -2 , RH = 55-60 %)

  Removal efficiency RE (%)[Pollutants] in (mmol m -3 )

Figure 4 . 3 Figure 5 5 .

 4355 Figure 4. Variation of butyraldehyde removal efficiency with the inlet concentration at different values of relative humidity (T = 20°C, Q = 2 m 3 h -1 )3.1.3. Effect of the specific input energy increase on pollutants removal

Figure 5 .

 5 Figure 5. Variation of butyraldehyde and ammonia removal efficiency with SIE using three processes (Q= 2 m 3 h -1 , [BUTY] = [NH 3 ] = 2 mmol m -3 , RH = 5%)

C 4 H 8 OFigure 6

 486 Figure 6 shows that photocatalytic removal efficiency of C 4 H 8 O and NH 3 decreases in the case of (C 4 H 8 O/NH 3 ) mixture treatment. It is likely that this is related to the

Figure 6 .

 6 Figure 6. Effect of mixture on removal efficiency of butyraldehyde and ammonia

Figure 7

 7 Figure 7 shows the C 4 H 8 O/NH 3 mixture degradation at a flow rate of 2 m 3 h -1 and a concentration of 2 mmol m -3 under photocatalysis, DBD-plasma and their combination.It is readily seen that an increase of the specific energy from 9 to 18 JL -1 enhances the

Figure 7 . 3 . 3 .

 733 Figure 7. Variation of the removal efficiency of BUTY and ammonia in mixture with SIE using the three processes (Q= 2 m 3 h -1 , [Mixture] = 2 mmol m -3 , RH = 5%)

  samples are concentrated immediately on a Carbotrap (25 ml) and then removed by thermal desorption unit coupled with GC-MS. The chromatograms are shown in Figure10. The detected by-products were essentially: butyric acid (C 4 H 8 O 2 ), propionic acid (C 3 H 6 O 2 ), butyramide (C 4 H 9 NO), propanamide (C 3 H 7 NO) and acetic acid (C 2 H 4 O 2 ).Consequently, according to these results, the mechanism for (C 4 H 8 O/NH 3 ) mixture removal when using the combined processes (TiO 2 /UV-lamp/DBD-plasma) is suggested in Figure11. Abou Ghaida et al. have studied the decomposition of butyraldehyde as pure compound by coupling surface dielectric barrier discharge and photocatalysis in continuous reactor[START_REF] Zhu | Influence of energy efficiency on VOCs decomposition in non-thermal plasma reactor[END_REF].

Figure 9 .

 9 Figure 9. Variation of CO x selectivity (%) when applying the three processes (Q=2 m 3 h -1 ,

Figure 10 .

 10 Figure 10. By-products identified by GC-MS for mixture degradation when applying photocatalysis and coupling DBD-plasma/photocatalysis: (a) Process tested: photocatalysis, (b) Process tested: DBD-plasma/photocatalysis (Q=2 m 3 h -1 , [Mixture] = 2 mmol m -3 , SIE= 9.9 J L -1 , RH = 5%).

Figure 11 .

 11 Figure 11. Suggested mechanism for (C 4 H 8 O/NH 3 ) mixture removal under TiO 2 /UV-lamp/DBDplasma combined processes.

Table 1 .

 1 Surface percentage atomic concentration on the catalyst surface at time zero, after the catalyst poisoning and after the plasma treatment: evidence for surface regeneration.

	Ti2p	Si2p	O1s	C1s	N1s


Jan Bolting
email: jan.bolting@isae.fr

Martin Stolle
email: martin.stolle@onera.fr

Soheib Fergani
email: soheib.fergani@isae.fr

Jean-Marc Biannic
email: jean-marc.biannic@onera.fr

François Defaÿ
email: francois.defay@isae.fr

An open benchmark for distributed formation flight control of Fixed-Wing Unmanned Aircraft Systems

Keywords: UAS, Formation flight, benchmark UAS, Formation flight, benchmark UAS, Formation flight, benchmark UAS, Formation flight, benchmark UAS, Formation flight, benchmark

The capability of autonomous formation flight has the potential to significantly enhance the utility and efficiency of small low-cost Unmanned Aerial Systems (UAS). Formations of small, inexpensive fixed-wing UAS allow for the sharing of remote sensing functionality, mission-level redundancy and range enhancements due to aerodynamic interactions widely exploited by migratory birds. This article presents a benchmark problem for scalable distributed flight control of formations of UAS with only local relative state information, one of the open problems in this field as of today. The benchmark is openly available and comprises a nonlinear six degrees of freedom dynamics model of an electric glider UAS. In this article we furthermore introduce a nominal guidance frame that does not require state information of other UAS and point out a fundamental issue related to wake vortex tracking during formation maneuvers. A set of LQ baseline controllers that are part of the benchmark is presented along with simulation results.

INTRODUCTION

Migratory birds routinely perform flights of several thousand kilometers. They do so using efficient formation patterns that allow them to benefit from the induced airflow generated by the birds upstream of them. Achievable energy savings of the order of 10 % are reported in the literature, [START_REF] Weimerskirch | Energy saving in flight formation[END_REF]. For some time there have been efforts to imitate this behavior to enhance the range of aircraft. The NASA AFF program has demonstrated the feasibility of this approach for manned fighter aircraft in a 2-aircraft configuration. Maximum fuel flow reductions of 18 % for the follower are reported, [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF]. In terms of implementation effort, this strategy is a very attractive one, since no structural changes have to be made to the aircraft to gain considerable fuel savings (otherwise only achievable by considerable advancements in e.g. the fuel efficiency of the engines), only by adapting the flight control software and possibly adding sensors for relative localization. In order to bring this capability to small unmanned aircraft, significant advances need to be made regarding relative guidance performance. Reported experimental results for 2-formations, [START_REF] Park | Avionics and Control System Development for Mid-Air Rendezvous of Two Unmanned Aerial Vehicles[END_REF], and 3-formations, [START_REF] Gu | Design and flight testing evaluation of formation control laws[END_REF], feature relative guidance errors of the order of meters. However, both theoretical considerations [START_REF] Hummel | Aerodynamic aspects of formation flight in birds[END_REF] and flight experiments [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF] suggest that the UAS needs to stay in a window of wingtip-wingtip separations (∆x ∆y ∆z) T roughly specified by ∆y ∈ [-0.2b, -0.1b], ∆z ∈ [-0.1b, 0] to benefit from significant energy savings. The longitudinal separation ∆x is less critical and can be several wingspans [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF]. For small UAS, the spanwidth b is of the order of 1 m, resulting in guidance error requirements of the order of 0.1 m laterally and vertically. Testing autonomous formation flight control algorithms using inexpensive UAS also constitutes an interesting way for the commercial aviation industry to build confidence in and evaluate the safety implications new formation flight control systems before implementing them on manned aircraft. In control, binary properties of a system such as stability or stability robustness -the system can either shown to be closed-loop stable over a given set of uncertainties or not -can usually be evaluated unambiguously for a given system. In contrast to that, control performance is tied to the problem that is used to evaluate a certain control approach. Benchmark problems play an essential role in every performance-driven field of research, as they allow to quantitatively compare different approaches. As of today in the field of aircraft formation flight control there is no lack of contributions and promising approaches. However there appears to be a lack of reference problems to compare them and to make an informed judgment about their performance and implementability. This gap is it we are attempting to address with the presented benchmark. Inspired by existing benchmarks (Biannic and Roos

INTRODUCTION

Migratory birds routinely perform flights of several thousand kilometers. They do so using efficient formation patterns that allow them to benefit from the induced airflow generated by the birds upstream of them. Achievable energy savings of the order of 10 % are reported in the literature, [START_REF] Weimerskirch | Energy saving in flight formation[END_REF]. For some time there have been efforts to imitate this behavior to enhance the range of aircraft. The NASA AFF program has demonstrated the feasibility of this approach for manned fighter aircraft in a 2-aircraft configuration. Maximum fuel flow reductions of 18 % for the follower are reported, [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF]. In terms of implementation effort, this strategy is a very attractive one, since no structural changes have to be made to the aircraft to gain considerable fuel savings (otherwise only achievable by considerable advancements in e.g. the fuel efficiency of the engines), only by adapting the flight control software and possibly adding sensors for relative localization. In order to bring this capability to small unmanned aircraft, significant advances need to be made regarding relative guidance performance. Reported experimental results for 2-formations, [START_REF] Park | Avionics and Control System Development for Mid-Air Rendezvous of Two Unmanned Aerial Vehicles[END_REF], and 3-formations, [START_REF] Gu | Design and flight testing evaluation of formation control laws[END_REF], feature relative guidance errors of the order of meters. However, both theoretical considerations [START_REF] Hummel | Aerodynamic aspects of formation flight in birds[END_REF] and flight experiments [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF] suggest that the UAS needs to stay in a window of wingtip-wingtip separations (∆x ∆y ∆z) T roughly specified by ∆y ∈ [-0.2b, -0.1b], ∆z ∈ [-0.1b, 0] to benefit from significant energy savings. The longitudinal separation ∆x is less critical and can be several wingspans [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF]. For small UAS, the spanwidth b is of the order of 1 m, resulting in guidance error requirements of the order of 0.1 m laterally and vertically. Testing autonomous formation flight control algorithms using inexpensive UAS also constitutes an interesting way for the commercial aviation industry to build confidence in and evaluate the safety implications new formation flight control systems before implementing them on manned aircraft. In control, binary properties of a system such as stability or stability robustness -the system can either shown to be closed-loop stable over a given set of uncertainties or not -can usually be evaluated unambiguously for a given system. In contrast to that, control performance is tied to the problem that is used to evaluate a certain control approach. Benchmark problems play an essential role in every performance-driven field of research, as they allow to quantitatively compare different approaches. As of today in the field of aircraft formation flight control there is no lack of contributions and promising approaches. However there appears to be a lack of reference problems to compare them and to make an informed judgment about their performance and implementability. This gap is it we are attempting to address with the presented benchmark. Inspired by existing benchmarks (Biannic and Roos

INTRODUCTION

Migratory birds routinely perform flights of several thousand kilometers. They do so using efficient formation patterns that allow them to benefit from the induced airflow generated by the birds upstream of them. Achievable energy savings of the order of 10 % are reported in the literature, [START_REF] Weimerskirch | Energy saving in flight formation[END_REF]. For some time there have been efforts to imitate this behavior to enhance the range of aircraft. The NASA AFF program has demonstrated the feasibility of this approach for manned fighter aircraft in a 2-aircraft configuration. Maximum fuel flow reductions of 18 % for the follower are reported, [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF]. In terms of implementation effort, this strategy is a very attractive one, since no structural changes have to be made to the aircraft to gain considerable fuel savings (otherwise only achievable by considerable advancements in e.g. the fuel efficiency of the engines), only by adapting the flight control software and possibly adding sensors for relative localization. In order to bring this capability to small unmanned aircraft, significant advances need to be made regarding relative guidance performance. Reported experimental results for 2-formations, [START_REF] Park | Avionics and Control System Development for Mid-Air Rendezvous of Two Unmanned Aerial Vehicles[END_REF], and 3-formations, [START_REF] Gu | Design and flight testing evaluation of formation control laws[END_REF], feature relative guidance errors of the order of meters. However, both theoretical considerations [START_REF] Hummel | Aerodynamic aspects of formation flight in birds[END_REF] and flight experiments [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF] suggest that the UAS needs to stay in a window of wingtip-wingtip separations (∆x ∆y ∆z) T roughly specified by ∆y ∈ [-0.2b, -0.1b], ∆z ∈ [-0.1b, 0] to benefit from significant energy savings. The longitudinal separation ∆x is less critical and can be several wingspans [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF]. For small UAS, the spanwidth b is of the order of 1 m, resulting in guidance error requirements of the order of 0.1 m laterally and vertically. Testing autonomous formation flight control algorithms using inexpensive UAS also constitutes an interesting way for the commercial aviation industry to build confidence in and evaluate the safety implications new formation flight control systems before implementing them on manned aircraft. In control, binary properties of a system such as stability or stability robustness -the system can either shown to be closed-loop stable over a given set of uncertainties or not -can usually be evaluated unambiguously for a given system. In contrast to that, control performance is tied to the problem that is used to evaluate a certain control approach. Benchmark problems play an essential role in every performance-driven field of research, as they allow to quantitatively compare different approaches. As of today in the field of aircraft formation flight control there is no lack of contributions and promising approaches. However there appears to be a lack of reference problems to compare them and to make an informed judgment about their performance and implementability. This gap is it we are attempting to address with the presented benchmark. Inspired by existing benchmarks (Biannic and Roos

INTRODUCTION

Migratory birds routinely perform flights of several thousand kilometers. They do so using efficient formation patterns that allow them to benefit from the induced airflow generated by the birds upstream of them. Achievable energy savings of the order of 10 % are reported in the literature, [START_REF] Weimerskirch | Energy saving in flight formation[END_REF]. For some time there have been efforts to imitate this behavior to enhance the range of aircraft. The NASA AFF program has demonstrated the feasibility of this approach for manned fighter aircraft in a 2-aircraft configuration. Maximum fuel flow reductions of 18 % for the follower are reported, [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF]. In terms of implementation effort, this strategy is a very attractive one, since no structural changes have to be made to the aircraft to gain considerable fuel savings (otherwise only achievable by considerable advancements in e.g. the fuel efficiency of the engines), only by adapting the flight control software and possibly adding sensors for relative localization. In order to bring this capability to small unmanned aircraft, significant advances need to be made regarding relative guidance performance. Reported experimental results for 2-formations, [START_REF] Park | Avionics and Control System Development for Mid-Air Rendezvous of Two Unmanned Aerial Vehicles[END_REF], and 3-formations, [START_REF] Gu | Design and flight testing evaluation of formation control laws[END_REF], feature relative guidance errors of the order of meters. However, both theoretical considerations [START_REF] Hummel | Aerodynamic aspects of formation flight in birds[END_REF] and flight experiments [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF] suggest that the UAS needs to stay in a window of wingtip-wingtip separations (∆x ∆y ∆z) T roughly specified by ∆y ∈ [-0.2b, -0.1b], ∆z ∈ [-0.1b, 0] to benefit from significant energy savings. The longitudinal separation ∆x is less critical and can be several wingspans [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF]. For small UAS, the spanwidth b is of the order of 1 m, resulting in guidance error requirements of the order of 0.1 m laterally and vertically. Testing autonomous formation flight control algorithms using inexpensive UAS also constitutes an interesting way for the commercial aviation industry to build confidence in and evaluate the safety implications new formation flight control systems before implementing them on manned aircraft. In control, binary properties of a system such as stability or stability robustness -the system can either shown to be closed-loop stable over a given set of uncertainties or not -can usually be evaluated unambiguously for a given system. In contrast to that, control performance is tied to the problem that is used to evaluate a certain control approach. Benchmark problems play an essential role in every performance-driven field of research, as they allow to quantitatively compare different approaches. As of today in the field of aircraft formation flight control there is no lack of contributions and promising approaches. However there appears to be a lack of reference problems to compare them and to make an informed judgment about their performance and implementability. This gap is it we are attempting to address with the presented benchmark. Inspired by existing benchmarks (Biannic and Roos

INTRODUCTION

Migratory birds routinely perform flights of several thousand kilometers. They do so using efficient formation patterns that allow them to benefit from the induced airflow generated by the birds upstream of them. Achievable energy savings of the order of 10 % are reported in the literature, [START_REF] Weimerskirch | Energy saving in flight formation[END_REF]. For some time there have been efforts to imitate this behavior to enhance the range of aircraft. The NASA AFF program has demonstrated the feasibility of this approach for manned fighter aircraft in a 2-aircraft configuration. Maximum fuel flow reductions of 18 % for the follower are reported, [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF]. In terms of implementation effort, this strategy is a very attractive one, since no structural changes have to be made to the aircraft to gain considerable fuel savings (otherwise only achievable by considerable advancements in e.g. the fuel efficiency of the engines), only by adapting the flight control software and possibly adding sensors for relative localization. In order to bring this capability to small unmanned aircraft, significant advances need to be made regarding relative guidance performance. Reported experimental results for 2-formations, [START_REF] Park | Avionics and Control System Development for Mid-Air Rendezvous of Two Unmanned Aerial Vehicles[END_REF], and 3-formations, [START_REF] Gu | Design and flight testing evaluation of formation control laws[END_REF], feature relative guidance errors of the order of meters. However, both theoretical considerations [START_REF] Hummel | Aerodynamic aspects of formation flight in birds[END_REF] and flight experiments [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF] suggest that the UAS needs to stay in a window of wingtip-wingtip separations (∆x ∆y ∆z) T roughly specified by ∆y ∈ [-0.2b, -0.1b], ∆z ∈ [-0.1b, 0] to benefit from significant energy savings. The longitudinal separation ∆x is less critical and can be several wingspans [START_REF] Jake | F/A-18 Performance Benefits Measured During the Autonomous Formation Flight Project[END_REF]. For small UAS, the spanwidth b is of the order of 1 m, resulting in guidance error requirements of the order of 0.1 m laterally and vertically. Testing autonomous formation flight control algorithms using inexpensive UAS also constitutes an interesting way for the commercial aviation industry to build confidence in and evaluate the safety implications new formation flight control systems before implementing them on manned aircraft. In control, binary properties of a system such as stability or stability robustness -the system can either shown to be closed-loop stable over a given set of uncertainties or not -can usually be evaluated unambiguously for a given system. In contrast to that, control performance is tied to the problem that is used to evaluate a certain control approach. Benchmark problems play an essential role in every performance-driven field of research, as they allow to quantitatively compare different approaches. As of today in the field of aircraft formation flight control there is no lack of contributions and promising approaches. However there appears to be a lack of reference problems to compare them and to make an informed judgment about their performance and implementability. This gap is it we are attempting to address with the presented benchmark. Inspired by existing benchmarks [START_REF] Biannic | Flare control law design via multi-channel H ∞ synthesis: Illustration on a freely available nonlinear aircraft benchmark[END_REF]) tailored to a specific challenging problem, e.g. Our modeling approach is focused on the current main issues of distributed formation control -mesh stability with minimum shared information and control performance that is compatible with the challenging demands of wake energy harvesting -and intentionally does not take into account secondary aspects such as sensor modeling and state estimation. To be useful, a benchmark must be accessible to other researchers. Therefore, the proposed benchmark is complemented by an implementation as an openly accessible Matlab ® /Simulink ® library. A set of LQ baseline controllers comes with the benchmark to provide a basic closed loop solution more advanced approaches can be compared to. Furthermore, a cross-platform 3D visualization environment is part of the benchmark. Numerous aircraft models [START_REF] Beard | Small Unmanned Aircraft: Theory and Practice[END_REF], vortex simulation [START_REF] Dogan | Wakevortex induced wind with turbulence in aerial refuelingpart b: Model and simulation validation[END_REF]; [START_REF] Saban | Simulation of wake vortex effects for uavs in close formation flight[END_REF] and trajectory planning techniques are available in the literature. Combining a specific set of these options into a simulation environment to evaluate a control technique constitutes a considerable, error-prone effort, and closedloop results are not necessarily comparable due to the specificities of different modeling approaches or other subtleties such as simulation sampling times or assumed actuator model time constants. This work pulls together a number of contributions to provide a standard test case that addresses important challenges of distributed formation flight control as of today without requiring researchers to implement the simulation environment themselves. The article is structured as follows: after elaborating on the role of information constraints on formation control in section 2, the aircraft model and the simulation environment are presented in section 3. Sections 4 and 5 deal with the benchmark trajectory and baseline control laws, respectively. In section 6 first simulation results are presented and section 7 provides some concluding remarks.

IMPACT OF INFORMATION CONSTRAINTS

A variety of approaches for formation flight control have been proposed in the past. A focus has been formation scalability, i.e. the capability to add members to a formation without adapting control laws and without violating performance bounds. This is expressed by the requirement of mesh stability. In mesh stable formations, position errors stay locally confined and are not amplified while propagating through the formation. As a general trend, the control problem becomes more challenging in inverse proportion to the available feedback information. Reduced feedback information is an attractive property, since less sensors and less inter-vehicle communications are necessary. Centralized approaches, [START_REF] Nageli | Environment-independent formation flight for micro aerial vehicles[END_REF], pose no particular challenge, since standard control techniques can readily be applied, but require large amounts of inter-vehicle communication to obtain full state information. For mesh stable control with leader information linear techniques exist [START_REF] Shaw | Controller design for string stable heterogeneous vehicle strings[END_REF]. The proposed benchmark addresses a more challenging scenario, distributed control with local state information, i.e. each member has only access to its own relative position and velocity w.r.t. to the preceding aircraft. There are fundamental results that linear interconnected systems with at least two integrators such as aircraft formations are always mesh unstable under local linear feedback [START_REF] Pant | Mesh stability of unmanned aerial vehicle clusters[END_REF]), ruling out linear control techniques. Very few solutions based on nonlinear techniques such as Port-Hamiltonian systems, [START_REF] Knorn | Passivity-based control for multi-vehicle systems subject to string constraints[END_REF] and Sliding Mode control, [START_REF] Galzi | UAV formations control using high order sliding modes[END_REF]; [START_REF] Bolting | Discrete sliding mode control of small uas in tight formation flight under information constraints[END_REF], have been proposed so far to tackle this problem and no experimental results are known to the authors. Furthermore, the existing approaches have been applied to very simplified models, basically point-mass approximations, neglecting a large part of the complex dynamics and constraints of a fixed-wing UAS.

AIRCRAFT MODEL 3.1 Aerodynamics

There is an abundance of possibilities to model the aerodynamic coefficients of an aircraft, reaching from look-up tables to neural networks. Following the principle of minimum necessary complexity, here a classic, mostly linear in the states model of the aerodynamic force and moment coefficients has been adopted, [START_REF] Beard | Small Unmanned Aircraft: Theory and Practice[END_REF]

C D = C D0 + C L0 + 1 πeΛ C Lα α 2 + C D δe + c 2V a C Dq q C Y = C Y β β + b 2V a C Yp p + C Yr r + C Y δa δ a + C Y δr δ r C L = C L0 + C Lα α + c 2V a C Lq q + C L δe δ e C l = C l β β + b 2V a C lp p + C lr r + C l δa δ a + C l δr δ r C m = C m0 + C mα α + c 2V a C mq q + C m δe δ e C n = C n β β + b 2V a C np p + C nr r + C n δa δ a + C n δr δ r
(1) with the Oswald factor e and the wing aspect ratio Λ. Estimates of the coefficients have been obtained with the software AVL from a 3D model of a small electric glider airplane, see [START_REF] Stolle | Towards vision-based autonomous crosscountry soaring[END_REF].

Kinematics

Using 1, the aerodynamic forces and moments in the body frame are computed as

F a = R bs qS C D C Y C L + T 0 0 (2) M a = R bs qSb 0 0 0 qSc 0 0 0 qSb C l C m C n (3)
with R bs the rotation matrix between stability frame and body frame, the vector of forces F a and the vector of moments M a , assuming that the thrust vector is aligned with the body frame x axis and coincides with the center of gravity. Translational accelerations in the body frame result to

vb = 1 m F a -(ω b × v b ) + R T eb g e (4)
Proceedings of the 20th IFAC World Congress Toulouse, France, July 9-14, 2017

where v b is the translational velocity of the body frame w.r.t the inertial frame expressed in body frame axes, ω b is the angular velocity of the body frame w.r.t the inertial frame expressed in body frame axes and g e is the gravity vector. The rotation rate of the earth can be neglected in this particular case due to the short duration of the considered UAS flight maneuvers. Angular accelerations about body frame axes follow as

ωb = -J -1 (ω b × (J ω b )) + J -1 M a (5)
with the inertia tensor J. With these accelerations, standard integration techniques and transformations can be used to propagate angular and translational velocities and positions over time to compute the aircraft's full state, which is given by

x = p e T v e T q T eb ω b T T (6)
with NED position p e , NED velocity v e , the attitude quaternion q eb and the vector of rotation rate about body axes ω b .

Exogenous perturbations

The aircraft dynamics are subject to exogenous disturbances due to atmospheric free air turbulence. These are taken into account as additional system inputs, extending the input vector to

u = δ e δ a δ r δ en v T w ω T w T (7)

Actuators and Engine

The control surface servo actuators have been identified in previous work, Ameho (2013), as second order systems

δi δi = 0 1 -ω 2 n -2ζω n δ i δi + 0 ω n δ i,c (8)
where ω n = 62.8 s -1 and ζ = 0.8. This model is augmented by adding realistic saturations on deflection and deflection rate

|δ i | ∈ [- π 4 , π 4 ] rad, | δi | ∈ [-5.8, 5.8] rad s (9)
The engine is modeled as first order system δen = τ -1 en (δ en,c -δ en )

where τ en = 0.27 -1 s -1 . The thrust T is then computed using a simple model based on momentum theory [START_REF] Beard | Small Unmanned Aircraft: Theory and Practice[END_REF]) as

T = ρ 2 S en C en ((k en δ en ) 2 -V 2 a ) (11)
with the propeller disc surface S en , the thrust constant C en , the engine constant k en and the airspeed V a .

Wind

The wind is assumed to be composed of a constant ambient part and a time-varying stochastic part, inducing velocities

v w (t) = v w,a + v w,s (t) (12)
and induced angular rates ω w (t). Wind time series are generated according to the Dryden turbulence spectrum (MIL-STD-1797A ( 1990)) as implemented by the Matlab ® / Simulink ® Aerospace Blockset ™ .

-100 0 100 200 300 400 500

x[m] The benchmark includes a 3D trajectory composed of C 2 -continuous cubic Bézier splines, based on the work presented by [START_REF] Yang | 3D smooth path planning for a UAV in cluttered natural environments[END_REF]. The trajectory features a linear part to evaluate cruise flight performance, followed by an approximately helical part to evaluate climb and turn maneuvers. See [START_REF] Looye | Helical flight path trajectories for autopilot evaluation[END_REF] for the compelling properties of helical trajectories for the evaluation of automatic flight control laws.

LQ BASELINE CONTROL LAWS

To provide a standard to compare new control approaches, a set of baseline LQ control laws is provided as part of the benchmark. For controller synthesis, the respective subsystems are extracted from the full linear model that results from linearizing the nonlinear aircraft model at cruise flight trim conditions. The structure of the control laws is based on the usual assumptions of weak coupling between lateral and longitudinal dynamics and timescale separation of attitude dynamics and translational dynamics.

The synthesis procedure is the same for all four subsystems. It is shortly recalled here, and the specific design systems for every controller are given in the respective subsections.

The time-invariant linear quadratic regulator (LQR) minimizes a quadratic integral performance index

J = ∞ t0 (x(t) T Qx(t) + u(t) T Ru(t))dt (13)
for a given linear system ẋ(t) = Ax(t) + Bu(t) (14) where x ∈ R n and u ∈ R m are the state and input vectors respectively and Q ∈ R n×n and R ∈ R m×m are positive definite design matrices. To track a given reference state x c , the tracking error dynamics are derived (dropping in the following the explicit dependence on time) as ẋe = ẋẋc (15) = Ax + Buẋc (16) The reference state is either constant, as in the static relative position tracking case, or unknown, since the reference state (e.g. the commanded bank angle) is generated by the outer loop controller. Its derivative is thus treated as unknown disturbance and set to zero for controller design, leading to the state error dynamics that are identical with the state dynamics.

Proceedings of the 20th IFAC World Congress Toulouse, France, July 9-14, 2017 ẋe = Ax + Bu (17) Tracking is thus approached by a simple change of state coordinates ẋe = xx c (18) To cope with steady state tracking errors, integral action is added by augmenting the system (18) with integral states

ẋe ẋi = A 0 G 0 x e x i + B 0 u ( 19
)
where G is a sparse matrix defining the states that are selected for integral action. As is common practice, wind disturbances and actuator and engine dynamics are neglected for the LQR design systems.

Coordinate frames

The system to be controlled is a formation of n UAS flying in an arbitrary pattern. In the literature, a variety of guidance frames has been proposed, ranging from a planar frame aligned with the follower's velocity [START_REF] Gu | Design and flight testing evaluation of formation control laws[END_REF]) to the predecessor's body frame [START_REF] Schumacher | Adaptive control of UAVs in close-coupled formation flight[END_REF]). The primary objective of tight formation flight is to keep each UAS at the position of maximum efficiency in the wake of its predecessor. Being induced by the aerodynamic flow, the wake vortices are approximately aligned with the predecessor's wind frame, neglecting trajectory curvature. For maximum energy savings, the follower thus needs to keep its relative position constant in this frame. Since small UAS typically are not equipped with sensors for angle of attack and side slip angle, a local guidance frame (index g) is introduced as an approximation. Its x axis unit vector u x,g is aligned with each predecessor's NED velocity vector. Its y axis should be aligned with the y axis of the predecessor's body frame, since the central axis of the vortices lies somewhere on the predecessor's wing. To avoid having to communicate the NED speed and attitude of each UAS to its follower, the nominal speed and acceleration vector of each UAS on the current formation trajectory is used to derive a nominal guidance frame. (24) The orientation of the z axis unit vector u z,g is derived from the simplifying assumption that gravitational acceleration and centrifugal forces due to trajectory curvature are compensated for by the thrust and drag along u x,g and by the aerodynamic lift in a plane normal to u x,g . Further assuming that the aerodynamic lift Z lies in the x-z plane of the body frames, first the total centrifugal and gravitational acceleration acting on p is computed as a t = |ω e p | 2 r + g e (25) The z unit vector u z is then computed by projecting and normalizing the total acceleration on the z-y plane of the g frame

a t,zy = a t - a t • u x,g u x,g • u x,g • u x,g (26)
u z,g = |a t,zy | -1 a t,zy (27) and u y,g completes a right-handed Cartesian coordinate frame u y,g = u z,g × u x,g (28) leading to the corresponding rotation matrix R eg = [u x,g u y,g u z,g ] (29)

Predecessor tracking

Longitudinal

The control inputs of the longitudinal position control loop are the commanded pitch angle Θ c and the engine command δ en,c . In the absence of external disturbances it regulates the relative position and velocity errors asymptotically to zero. Using Θ c as control input instead of the angle of attack α, which is more directly related to aerodynamic lift, is imposed by the usual absence of AoA sensors on board of small UAS. To obtain a good approximation of α, the pitch angle w.r.t to the local guidance frame is used. The design system is given by 

      ∆ ṗx ∆ ṗz ∆ vx ∆ vz ∆ ṗx,int ∆ ṗz,int        = A lon,pos 0 [I 2 0 2 ] 0        ∆p x ∆p z ∆v x ∆v z ∆p x,int ∆p z,int        + B lon,pos δ en Θ c (30)
Lateral The lateral control strategy is derived from the simplified system (34) and the commanded bank angle in the local guidance frame is computed using (33).

n y = Z mg sin(Φ) (31) Z ≈ mg (32) ⇒n y ≈ sin -1 (Φ) (33

Attitude tracking

The inner loops consist of separate pitch angle and bank angle tracking laws. Both angles are not the usual Euler Proceedings of the 20th IFAC World Congress Toulouse, France, July 9-14, 2017 angles w.r.t. to the NED frame but are defined as the rotation angles needed to rotate the body frame into the local guidance frame. That being said, the resulting commanded attitude rotation matrix w.r.t. to the NED frame is composed as

R eb,c = R eg R gb ,c (Φ c , Θ c ) (35)
where

R gb ,c (Φ c , Θ c ) = cos Φ c sin Φ c cos Θ c sin Φ c cos Θ c -sin Φ c cos Φ c cos Θ c cos Θ c sin Θ c 0 -sin Θ c cos Φ c ( 36
) Note that the local heading angle is not actively controlled and as such is set to zero.

Longitudinal The pitch attitude design system is given by 

 ∆ Θ q ∆ Θint   = A lon,att 0 [1 0] 0 ∆Θ q ∆Θ int + B lon,att (δ e ) (37)

Lateral

The lateral attitude law is driven by two requirements: tracking a commanded local bank angle and adding damping to the weakly damped dutch roll mode. The second one is achieved by adding the yaw rate r to the state vector of the design system and choosing an appropriate weight in the LQR synthesis matrix Q. The design system is thus:

    ∆ Φ ṗ ṙ ∆ Φint     = A lat,att 0 [1 0 0] 0    ∆Φ p r ∆Φ int    + B lat,att δ a δ r (38)

SIMULATION RESULTS

Flight on the benchmark trajectory has been simulated for a formation of three UAS both in calm air and under turbulent wind with v w,a = (-3 -3 0) T m s i.e. about 28 % of the nominal airspeed of 15 m s . The separation vectors have been selected as ∆p i (-b b 0) T for i = 1...3.

Visualization

A lightweight visualization environment based on the Java jmonkey game engine (see jmonkeyengine.org ( 2016)) has been developed to display the attitude and positions of multiple UAS in a synthetic 3D environment. It has proven to enhance productivity while developing and debugging guidance laws. It is interfaced with the Simulink dynamics simulation via a UDP (User Datagram Protocol) network link. Visuals have to be updated only at a moderate rate of about 30 Hz for human perception, thus sending UDP packets adds a small simulation time overhead. The tool is open software and currently provides several generic UAS 3D models as well as the possibility to display wake vortices.

Calm atmosphere

Position errors in the local guidance frames along with the longitudinal control inputs under calm air conditions The first is their expected mesh instability. The second one is related to the selection of guidance frames, see section 5. Since the local guidance frames have been selected to track the predecessor's wake vortices, changes of trajectory curvature create perturbations w.r.t. the NED frame that are dependent on the separation vectors and vehicle index. This leads -with linear controllers -to position errors that grow with the vehicle index. Tracking the vortex during maneuvers is therefore not a scalable guidance strategy. This constitutes an open problem.

Turbulent atmosphere

Position errors in the local guidance frames along with the longitudinal control inputs under wind and turbulence are shown in fig. 2. Position errors are considerably larger during the initial cruise flight phase. On the helical part of the trajectory, perturbations due to reference frame perturbations dominate. The helical trajectory clearly reveals another shortcoming of the baseline control strategy.

As the formation descends and the changing trajectory heading leads to tailwind, the engine inputs saturate as the longitudinal position controller attempts to maintain constant ground speed. It seems therefore advisable to add some kind of wind estimation scheme into the guidance strategy to maintain constant airspeed.

CONCLUSION

A new benchmark for distributed formation flight control for small UAS has been presented, along with simulation results for a set of baseline controllers. The results confirm their expected mesh instability as well as the ability of the chosen reference trajectory to reveal this and various other deficiencies of the baseline guidance strategy. These deficiencies provide a starting point for future improvements. It is demonstrated that tracking the predecessor's wake vortex during maneuvers is not a scalable guidance strategy.

We invite other researchers working in this domain to adopt this benchmark to evaluate new promising control

Proceedings of the 20th IFAC World Congress Toulouse, France, July 9-14, 2017

Fig. 3. Position errors in g frame and longitudinal control inputs for turbulent atmosphere and guidance approaches such as [START_REF] Padhi | Formation flying with nonlinear partial integrated guidance and control[END_REF] and to contribute to it e.g. by proposing and implementing new benchmark trajectories or by adding to the documentation. The benchmark and the tools it is built on are version controlled for guaranteed comparability and openly available, Jan [START_REF] Bolting | Formation Flying Benchmark[END_REF]; [START_REF] Bolting | Formation Flying Benchmark[END_REF]. The implementation depends on a number of Matlab ® /Simulink ® toolboxes and blocksets. To reduce this threshold, we are currently evaluating the possibility of a hosted benchmark that is accessible via the Internet.

  Fig. 1. Top view of benchmark trajectory 4. BENCHMARK TRAJECTORY

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  ) thus lateral load factors in the local guidance frame are generated by inclining the lift vector via the local bank angle Φ. The resulting LQR design system is given by ∆ B lat,pos (n y,c )

Fig. 2 .

 2 Fig. 2. Position errors in g frame and longitudinal control inputs for calm atmosphere are shown in fig. 2. Simulating the benchmark formation in calm air clearly reveals two deficiencies of the baseline controllers.The first is their expected mesh instability. The second one is related to the selection of guidance frames, see section 5. Since the local guidance frames have been selected to track the predecessor's wake vortices, changes of trajectory curvature create perturbations w.r.t. the NED frame that are dependent on the separation vectors and vehicle index. This leads -with linear controllers -to position errors that grow with the vehicle index. Tracking the vortex during maneuvers is therefore not a scalable guidance strategy. This constitutes an open problem.