W Abou Saoud

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

Abdoulaye Kane

A.-V Jung

Pierre Le Cann

Anne Gerard

Frederic Bazantay

Abdelkrim Bouzaza

Dominque Wolbert

Wala Abou Saoud

Aude-Valerie Jung

Le Cann Pierre

Integrated process for the removal of indoor VOCs from food industry manufacturing: elimination of Butane-2,3-dione and Heptan-2-one by cold plasma-photocatalysis combination

Keywords: indoor air of food industry, Butane-2, 3-dione/Heptan-2-one mixture, (photocatalysis-DBD plasma) association, synergetic effect

HAL is

Introduction

Volatile Organic Compounds (VOCs) e.g. ketones, aldehydes and BTEX (Benzene, Toluene, Ethylbenzene and Xylenes), are one of the most significant groups of indoor air pollutants [START_REF] Cao | Occurrence of 13 volatile organic compounds in foods from the Canadian total diet study[END_REF][START_REF] Holzer | Non-thermal plasma treatment for the elimination of odorous compounds from exhaust air from cooking processes[END_REF]. Some odorous compounds such as Butane-2,3-dione and Heptan-2-one, emitted during food processing and preparation can be responsible for olfactory pollution [START_REF] Scotto | Olfactory pollution in urban environment: special case of odors in restaurants[END_REF], and are hazardous to health which can present a damage for overall human exposure to polluted environments. Therefore, removing/destroying VOCs emitted from dietary sectors is a major issue. Promising technologies, namely Advanced Oxidation Processes (AOPs), more efficient to treat organic compounds resistant to conventional processes, are under development and can provide interesting solutions in air control. AOPs such as photocatalysis [START_REF] Huang | Removal of Indoor Volatile Organic Compounds via Photocatalytic Oxidation: A short Review and Porspect[END_REF][START_REF] Rochetto | Degradation of volatile organic compounds in the gas phase by heterogenous photocatalysis with titanium dioxide/ultraviolet light[END_REF], cold plasma [START_REF] Holzer | Non-thermal plasma treatment for the elimination of odorous compounds from exhaust air from cooking processes[END_REF][START_REF] Bahria | Impact of design parameters on the performance of non-thermal plasma air purification system[END_REF][START_REF] Karatum | A comparative study of dilute VOCs treatment in non-thermal plasma reactor[END_REF][START_REF] Thirumdas | Cold Plasma: A novel Non-Thermal Technology for Food Processing[END_REF] and catalytic ozonation [START_REF] Rezaei | Toluene Removal from waste air stream by the catalytic ozonation process with MgO/GAC composite as catalyst[END_REF][START_REF] Zaitan | Application of high silica zeolite ZSM-5 in a hybrid treatment process based on sequential adsorption and ozonation for VOCs elimination[END_REF], based on ozone and/or free reactive species (atomic oxygen, peroxide ions and hydroxyl radicals), appear as innovative J o u r n a l P r e -p r o o f technologies to overcome limitations encountered by traditional processes. However, each method used alone can present some limitations: (i) accumulation of residues on the catalyst surface leads to a gradual decrease in photocatalytic activity, which regularly requires a regeneration step [START_REF] Guillard | Photocatalytic degradation and mineralization of a malodorous compound (dimethyl disulfide) using a continuous flow reactor[END_REF][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] Alonso-Tellez | A parametric study of the UV-A photocatalytic oxidation of H2S over TiO2[END_REF] and (ii) a low CO2 selectivity was observed via plasma treatment and also (iii) undesirable by-product formation (carbon monoxide CO and ozone O3) [START_REF] Schiavon | Non-thermal Plasma as an Innovative Option for the Abatement of Volatile Organic Compounds: a Review[END_REF][START_REF] Jiang | Post plasma-catalysis of low concentration VOC over alumina-supported silver catalysts in a surface/packedbed hybrid discharge reactor[END_REF], each of them constitutes significant limitations which limit single mode usage. Therefore, it is better to associate photocatalysis and plasma together to ensure (i) high treatment efficiency and (ii) overcome existing disadvantages [START_REF] Dou | Enhanced removal of toluene by dielectrical barrier discharge coupling with CU-Ce-Zr supported ZSM-5/TiO2/Al2O3[END_REF][START_REF] Huu | Methane, Propene and Toluene Oxidation by Plasma-Pd/c-Al2O3 Hybrid Reactor: Investigation of a Synergetic Effect[END_REF][START_REF] Abou Saoud | Abatement of ammonia and butyraldehyde under non-thermal plasma and photocatalysis: Oxidation processes for the removal of mixture pollutants at pilot scale[END_REF][START_REF] Mei | Plasmaphotocatalytic conversion of CO2 at low temperatures: Understanding the synergistic effect of plasma-catalysis[END_REF][START_REF] Zadi | Treatment of hospital indoor air by a hybrid system of combined plasma with photocatalysis: Case of trichloromethane[END_REF][START_REF] Assadi | Pilot scale degradation of mono and multi volatile organic compounds by surface discharge plasma/TiO2 reactor: investigation of competition and synergism[END_REF][START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining J o u r n a l P r e -p r o o f photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF]. In this context, several studies have focused on the oxidation of VOCs via photocatalysis-plasma technologies and reported that an enhancement of more than 10% was observed on removal efficiency of malodorous pollutant exhaust from animal quartering centers like ammonia (NH3), dimethyl disulfide (CH3SSCH3) and butyraldehyde (C4H8O) [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] Abou Saoud | Abatement of ammonia and butyraldehyde under non-thermal plasma and photocatalysis: Oxidation processes for the removal of mixture pollutants at pilot scale[END_REF]. In this work, a commercial Glass Fiber Tissue (GFT coated with TiO2), suitable for photocatalysisplasma processes, was investigated for Butane-2,3-dione and Heptan-2-one degradation under photocatalysis, Dielectric Barrier Discharge (DBD) plasma and by coupling both. Photocatalytic degradation performance was monitored in continuous reactor, for various operating conditions like flowrates, VOCs concentration and complex air flow (mixture of Butane-2,3-dione and Heptan-2-one). Humidity tests were performed to evaluate the efficiency of Butane-2,3-dione/Heptan-2-one removals under photocatalytic oxidation. The integration of DBD plasma treatment in photocatalytic reactor was investigated in order to improve indoor air quality. Good mineralization and an improvement on VOCs destruction were observed resulting in synergies between two oxidation techniques, as well as the formation of oxidized byproducts was controlled for mixture treatment.

J o u r n a l P r e -p r o o f

Materials and methods

Experimental setup

VOCs oxidation runs using a photocatalytic reactor and/or DBD plasma reactor were conducted using the experimental setup shown in Figure 1. It consists of a glass annular reactor (2 concentric Pyrex cylinders, 100 cm length), one outer cylinder of 76 mm and an inner cylinder of 58 mm with wall thickness of 4 mm. The reactor can be used in a photocatalytic mode via UV lamp (Philips TL 40W/05) and/or in a plasma mode (Dielectric Barrier Discharge DBD system) or in a simultaneous mode.

Experiments were realized at ambient temperature (25°C) and atmospheric pressure.

In order to have a homogeneous irradiation on the total catalyst surface, a UV lamp (100 cm length) was placed in the inner concentric cylinder. A commercial Glass Fiber Tissue TiO2/GFT (BET surface: 300 m 2 g -1 , diameter: 5-10 nm, Anatase 100%), produced by Ahlstrom Research and Services, was used as catalyst. As for DBD plasma equipment, Figure 1. shows the different experimental parts used to create the plasma. DBD plasma was created by applying high voltage power via a signal generator (BFi OPTILAS (SRS) reference DS 335/1). The applied voltage, as a sinusoidal waveform of 10V, was then amplified to 30 kV by an amplifier (TREK 30A/40) [START_REF] Zadi | Treatment of hospital indoor air by a hybrid system of combined plasma with photocatalysis: Case of trichloromethane[END_REF][START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining J o u r n a l P r e -p r o o f photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF][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][START_REF] Guillerm | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, non thermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF][START_REF] Abou Saoud | Synergism between non-thermal plasma and photocatalysis: Implications in the post discharge of ozone at a pilot scale in a catalytic fixed-bed reactor[END_REF]. This amplifier is connected to the reactor outer/inner electrodes, in a copper and Al grid form for the outer and inner electrodes, respectively. The voltages applied in the plasma reactor are measured by high-voltage probes recorded with a digital oscilloscope (Lecroy wave surfer 24Xs, 200 MHz).

J o u r n a l P r e -p r o o f

VOC injection

Two organic compounds: Butane-2,3-dione (diketone, two C=O groups) and Heptan-2-one (ketone groups) were used. The Butane-2,3-dione (C4H6O2) 99% purity was purchased from Janssen Chimica. As for Heptan-2-one, (C7H14O, 98% purity) was supplied from Acros Organics. The polluted air flow was generated continuously by means of a syringe pump system (Kd Scientific Model 100). In order to create a complex air flow, two similar injection systems are used for each compound (Figure 1). A static mixer upstream ensures a good mixture of the complex Butane-2,3-dione/Heptan-2-one. A set of valves and a flow controller (Bronkhorst In-Flow) ensured a variable input flow (from 1 to 10 m 3 h -1 ) into the reactor. Relative Humidity (RH) can also be modified. A variable fraction of gas stream came from a humidifier packed column to obtain humidity ranges from 40 to 90%. Humidity and temperature J o u r n a l P r e -p r o o f in the inlet of photocatalytic/plasma reactor were monitored via a specific sensor (Testo 445).

Analysis system

VOC analysis

Gas Chromatography (GC), equipped with a flame ionization detector (FID), was used to monitor the evolution of VOCs over time. The analysis was carried out using a Clarus GC-500 chromatograph quipped with a 60 m 0.25 mm polar DB-MS capillary column (film thickness, 0.25 µm). The detector (FID) is powered by a mixture of air and hydrogen (H2). Helium (He) was used as a carrier gas at a flow rate of 1 ml min -1 . The samples (500 μL) are injected manually via a gas syringe. Analysis conditions, as injection detection temperatures (250°C for both), were used to quantify INLET/OUTLET VOC concentrations. Oven temperature, initially kept at 50°C for 3 min, was programmed to rise from 50 to 100°C at 2°C/min, remaining at the maximum temperature for 10 min thereafter.

CO and CO2 analysis

The measurement of the amount of CO2 produced during C4H6O2/C7H14O treatment was carried out on an online analytical system via a Fourier Transform Infrared Spectrophotometer (FTIR) of Environment SA (MIR 9000H). To monitor the mineralization step continuously during the oxidation test, a pump system of the gas stream was connected to the outlet of the reactor. CO measurement was made at the outlet of the reactor, a gas analyzer NO/CO ZRE marketed by Fuji Electric France S.A.S was connected online.

Ozone analysis

Ozone was generated via DBD plasma step during VOC degradation. The amount of Ozone produced was determined by sodium thiosulfate titration method. Exhaust gas J o u r n a l P r e -p r o o f flow, equal to 285 L h -1 , was delivered by a membrane pump (KNF lab N86k18) to a bubbler (500 ml), containing a solution of iodide of potassium (KI, at 10 -2 M).

A chemical reaction between potassium iodide (KI) and Ozone occurs, giving yellow color emerges, as first step (Equation 1) and then it move towards a titration by sodium thiosulphate solution (Na2S2O3, at 10 -3 M), until a colorless solution is obtained, as second step (Equation 2) [START_REF] Guillerm | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, non thermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF]. The titration is carried out in an acid medium by dripping concentrated sulfuric acid (H2SO4) in the final solution.

(1)

(2)

Analytic methods

In order to understand the performance of the plasma-photocatalytic reactor, some parameters were monitored as:

 C4H6O2 Removal Efficiency:

(3)

 C7H14O Removal Efficiency:

(4)

 Specific Input Energy:  Elimination Capacity:

 Selectivity of CO:

 Selectivity of CO2:

(8)

Results and discussion

J o u r n a l P r e -p r o o f

VOC removal under photocatalysis alone

Single compounds

Effects of flow rate & VOC concentration

Firstly, the performance of the photocatalytic oxidation process was studied on two separate VOCs: Butane-2,3-dione/Heptan-2-one, treated alone. Figure 2 shows the variation of the removal efficiency at variable concentrations (ranging from 5 to 27 mg m -3 ) and for two flow rates (2 and 4 m 3 h -1 ), all experiments on C4H6O2 and C7H14O removal were realized under a dry condition (RH ~ 5%). As shown in Figure 2, increasing the inlet concentration of VOCs can negatively influence the performance of degradation. At a constant air flow (2 m 3 h -1 ), the increase of the C4H6O2 amount from 5 to 20 mg m -3 , results in a decrease of the degradation rate from 74.75 to 44.93%. Our previous studies confirm this behavior and relate this decrease to the saturation/deactivation of the active sites of catalyst [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] Abou Saoud | Abatement of ammonia and butyraldehyde under non-thermal plasma and photocatalysis: Oxidation processes for the removal of mixture pollutants at pilot scale[END_REF][START_REF] Zadi | Treatment of hospital indoor air by a hybrid system of combined plasma with photocatalysis: Case of trichloromethane[END_REF]. In addition, it was found (Fig. 2), that the oxidation performance was clearly decreased by increasing the flow of air/VOCs. The decrease is largely due to the short contact time between VOCs/catalyst, which leads to influence the removal capacity.

Regarding Heptan-2-one behavior, the same trend was observed on the degradation.

However, it is important to note that the degradation rate for C7H14O remains low (~ 32.41% at 2 m 3 h -1 ) compared to Butane-2,3-dione (~ 74.75% at 2 m 3 h -1 ). Heptan-2-one presents a long molecular chain (C7H14O), harder to degrade using photocatalytic treatment, which explains the decreasing trend of the efficiency.

J o u r n a l P r e -p r o o f (T= 25 °C, UV intensity= 20 W m -2 , RH= 5%)

Effect of humidity on VOC removal

Indoor air in the food industry is usually humid (around 75-95% relative humidity (RH)) [START_REF] Li | Measurement of temperature, relative humidity, concentration distribution and flow field in four typical Chinese commercial kitchens[END_REF]; therefore, to simulate real operating conditions in the industrial sector, the photocatalytic reactor was fed with a continuous humid air flow at a rate of 2 m 3 h -1 and at various humidity values. Figure 3 shows the variation of the removal efficiency, at four values of relative humidity (~5%, ~45%, ~75% and ~90% RH), of shows that the removal efficiency was significantly affected by increasing relative humidity from 5% to 90%, for both 5 and 10 mg m -3 of Butane-2,3-dione. For Butane-2,3-dione removal was 68.79% and gradually decreased to become 56.9%, 48.56% and 30.32% with ~ 45%, ~ 75% and ~ 90% of the humidity levels, respectively. Thus, the effect of the competitive adsorption on the catalyst active sites between the water vapor molecules (H2O), the butane-2,3-dione (C4H6O2) and its oxidation by-products is present whatever the relative humidity value is. In addition to that, the higher the humidity, the more there is deactivation of catalyst active sites. As a result, a permanent decrease in the butane-2,3-dione removal efficiency is observed due to the water vapor amount increase. As for Heptan-2-one, the same previous methodology was used and the results are shown in Figure 3 hydroxyl radicals OH° (according to Eq. 9-10), known as highly reactive chemical species, thus helping to ensure a better performance.

ii. On the other hand, a high level of moisture (RH~90%) leads to increase the competition between water vapor/Heptan-2-one on the catalyst surface and differently affect the degradation efficiency. For the same concentration ([C7H14O]= 6.6 mg m -3 ), it should be noted that at RH~90%, the C7H14O [START_REF] Zadi | Treatment of hospital indoor air by a hybrid system of combined plasma with photocatalysis: Case of trichloromethane[END_REF], aromatics [START_REF] Martinez | BTEX abatement by photocatalytic TiO2-bearing coatings applied to cement mortars[END_REF] and aldehydes [START_REF] Abou Saoud | Synergism between non-thermal plasma and photocatalysis: Implications in the post discharge of ozone at a pilot scale in a catalytic fixed-bed reactor[END_REF], respectively. The effect of humidity on mixtures was also studied and the results are discussed below (see part 3.1.3., Figure 5).

Mixture of compounds

Effect of composition of gas

A complex air flow of C4H6O2 and C7H14O, at an inlet concentration of 5.2 ppm, was applied using a photocatalytic process for variety molar compositions as (50% Butane-2,3-dione & 50% Heptan-2-one), (25% Butane-2,3-dione & 75% Heptan-2one) and (75% Butane-2,3-dione & 25% Heptan-2-one). Indeed, the objective of the target molecules mixture study was first to validate the coupled process efficiency in a case more closer to the real conditions where several molecules must be eliminated simultaneously and to see if there is always improvement of pollutants removal in the case of mixing. The objective was also to check the toxicity of the byproducts resulting from the oxidation in the case of mixture and to check if new byproducts will appear, and the results shows that we obtained the same conventional (Q=2 m 3 h -1 , [Mixture]= 5.2 ppm, RH= 5%)

VOC removal under DBD plasma alone

VOC removal experiments are detailed in our previous studies, [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] Abou Saoud | Abatement of ammonia and butyraldehyde under non-thermal plasma and photocatalysis: Oxidation processes for the removal of mixture pollutants at pilot scale[END_REF][START_REF] Abou Saoud | Synergism between non-thermal plasma and photocatalysis: Implications in the post discharge of ozone at a pilot scale in a catalytic fixed-bed reactor[END_REF][START_REF] Assadi | Comparative study between laboratory and large pilot scales for VOC's removal from gas streams in continuous flow surface discharge plasma[END_REF][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][START_REF] Assadi | Pilot scale degradation of mono and multi volatile organic compounds by surface discharge plasma/TiO2 reactor: investigation of competition and synergism[END_REF] either via plasma technology alone or via coupling plasma/photocatalysis indicate that a significant amount of exhaust ozone was produced by increasing energy supplied to plasma. In this context and to avoid very high ozone level generation, experiments have been carried out on a range of specific input energies ranging from 2 to 10 J L -1 . DBD plasma tests were realized in the same continuous reactor under a dry condition. The gas stream was fixed at 2 m 3 h -1 and the C4H6O2 concentration was 10 mg m -3 . The C4H6O2 removal efficiency via DBD reactor was studied and the results are reported in Figure 6. A positive effect of the Specific Input Energy (SIE) increase on C4H6O2 oxidation was observed. At higher energy values, a larger quantity of reactive species was formed in the discharge phase like atoms, ions, radicals and metastables, can contribute to enhance VOC degradation by different Butane-2,3-dione and Heptan-2-one tested separately via DBD plasma system. The initial concentration of C4H6O2/C7H14O gazes was 2.6 ppm and at a constant flow rate of 2 m 3 h -1 with 5% of relative humidity. It is readily seen that, by using DBD plasma alone (Fig. 7), the same behavior on removal efficiency was observed as that resulting from photocatalysis alone (Fig. 2). Comparing both ketones, Heptan-2-one is less degradable compared to Butane-2,3-dione. The Heptan-2-one degradation was around 10.48% (at SIE= 2.25 JL -1 ) and 21.95% (at SIE= 4.5 JL -1 ), contrast to Butane-2,3-dione when 17.32% and 30.11% was removed for 2.25 and 4.5 JL -1 , respectively. These results confirm that the group of molecules, irrespective of the system used, can play an important role in removal efficiency. (Q= 2 m 3 h -1 , [C4H6O2]= 10 mg m -3 , RH= 5%).

[Ozone] C4H6O2 removal

J o u r n a l P r e -p r o o f were studied via photocatalysis, DBD plasma and photocatalytic/DBD plasma processes. Figure 8 compares the evolution of the removal efficiency for single and mixture compounds condition. Coupling plasma-photocatalysis in the same system is more effective for the oxidation of both VOCs (Fig. 8). Either for C4H6O2 or C7H14O oxidation tests, the performance of the degradation was significantly improved via photocatalytic/DBD plasma system. The results for C4H6O2 removal by coupling plasma-photocatalysis compared to the results obtained by the sum of removal and 62.11%, respectively. As for C7H14O degradation study, 5.09% was obtained by plasma, 24.39% by photocatalysis and 35.15% by their association. This good synergy observed between cold plasma and photocatalysis is in agreement with other studies for butyraldehyde removal [START_REF] Abou Saoud | Synergism between non-thermal plasma and photocatalysis: Implications in the post discharge of ozone at a pilot scale in a catalytic fixed-bed reactor[END_REF][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] and for methane, propene and toluene oxidation [START_REF] Huu | Methane, Propene and Toluene Oxidation by Plasma-Pd/c-Al2O3 Hybrid Reactor: Investigation of a Synergetic Effect[END_REF]. Synergetic effects mainly occur thanks to the important role of generated plasma species which enhance photocatalytic reaction by desorption of oxidation byproducts. The best improvement in this study is 5.7%, but this is simply because the energy applied to the plasma is low (only 4.5 JL -1 ). We have shown in our previous studies [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] Abou Saoud | Abatement of ammonia and butyraldehyde under non-thermal plasma and photocatalysis: Oxidation processes for the removal of mixture pollutants at pilot scale[END_REF][START_REF] Zadi | Treatment of hospital indoor air by a hybrid system of combined plasma with photocatalysis: Case of trichloromethane[END_REF][START_REF] Assadi | Pilot scale degradation of mono and multi volatile organic compounds by surface discharge plasma/TiO2 reactor: investigation of competition and synergism[END_REF][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][START_REF] Assadi | Pilot scale degradation of mono and multi volatile organic compounds by surface discharge plasma/TiO2 reactor: investigation of competition and synergism[END_REF] that the COV removal efficiency is clearly improved by coupling plasma and photocatalysis when using higher energies, but the use of high energy generates a significant amount of ozone [START_REF] Abou Saoud | Synergism between non-thermal plasma and photocatalysis: Implications in the post discharge of ozone at a pilot scale in a catalytic fixed-bed reactor[END_REF], which is not desired in our current case of indoor air treatment.

Mineralization, Ozone and by-product formation

Ozone can be seen as a by-product either in plasma treatment or in photocatalytic/plasma treatment; zero ozone was observed during photocatalytic treatment alone (Figure 9). Results show that the quantity of ozone (13.52 mg m -3 at the reactor outlet) under photocatalytic/plasma oxidation was decreased compared to ozone produced under the plasma technique (33.80 mg m-3). This amount of ozone reacted on the surface of the catalyst and served to improve the oxidation as shown on the mechanism figure (Fig. 11 below). This reduction presents the positive effect of UV irradiation on ozone decomposition into active species [START_REF] Huang | Combination of photocatalysis downstream the non-thermal plasma reactor for oxidation of gas-phase toluene[END_REF].

O3 + e -→ O •-+ O2 ( 11)

O3 + h -(254 < λ < 310 nm) → O • + O2 (12)
As for the mineralization step, CO and CO2 selectivity were monitored for photocatalysis, plasma and coupling of both. According to the Figure 9, it is notable that the selectivity of CO2 was improved (CO2 ~ 70.79%), when using plasmaphotocatalysis as oxidation process to degrade C4H6O2 and C7H14O in a mixture system. Oxidation by-products created during C4H6O2/C7H14O mixture degradation by photocatalysis alone, plasma alone and the association of both was identified by Gas Chromatography coupled to Mass Spectrometry (GC-MS). Outlet oxidation reaction samples were concentrated on a Carbotrap (25ml) and then removed by a thermal desorption unit coupled with GC-MS. Specific oxidation by-products are identified in

Conclusion

Efficiency of photocatalysis, cold plasma and combined both oxidation technologies for the degradation of Butane-2,3-dione and Heptan-2-one as a targets compounds of indoor air pollution were investigated. Photocatalytic performance of a single pollutant was firstly monitored under a variety of experimental conditions: (i) higher flowrate (2 to 4 m 3 h -1 ), (ii) pollutants inlet concentration ranging from 5 to 20 mg m -3 and (iii) humidity rate of reaction medium (5 to 90%). Comparing the photocatalytic oxidation of Butane-2,3-dione to that Heptan-2-one, opposite behavior was observed for binary mixture oxidation under humid conditions. Highly efficient synergetic combination was observed in terms of (i) VOC removal efficiency, (ii) CO2 selectivity and (iii) ozone generation. This system would be useful to limit or eliminate exposure of food industry workers to these harmful VOCs.

J o u r n a l P r e -p r o o f

Figure 1 .

 1 Figure 1. Schematic of the experimental setup: abatement of VOCs in air mixture

( 5 )

 5 With: P (W) is input power adjusted by changing the applied voltage (Ua) at a constant frequency (50 Hz) and Q (m 3 h -1 ) is the flowrate tested. [VOCs]INLET, [VOCs]OUTLET are the INLET/OUTLET gas mass concentrations, respectively.

Figure 2 .

 2 Figure 2. Variation of the removal efficiency of Butane-2,3-dione and Heptan-2-one

  (a) Butane-2,3-dione and (b) Heptan-2-one with two different inlet concentrations like 5-10 mg m -3 and 6.6-13.25 mg m -3 , respectively. Different behaviors were observed during the photocatalytic degradation of both VOCs treated separately. Regarding the influence of humidity on the Butane-2,3-dione degradation, a result (Figure 3 (a))

Butane- 2 , 3 -

 23 dione : Q= 2 m 3 h -1 Butane-2,3-dione : Q= 4 m 3 h -1 Heptan-2-one: Q= 2 m 3 h -1 Heptan-2-one: Q= 4 m 3 h -1 J o u r n a l P r e -p r o o f example, the experiment realized at low humidity (RH ~ 5%) at [C4H6O2]= 10 mg m -3 ,

  (b). A different behavior was observed in comparison with that previously described on Butane-2,3dione alone: when the water vapor amount is low, the created active intermediate species help to increase the oxidation of the heptan-2-one (C7H14O) and the effect of the competitive adsorption is neglected. Moreover, this behavior can be attributed to the high molecular weight of C7H14O compared to C4H6O2 which hampers the competitive adsorption and thus improves photocatalytic degradation of heptan-2one. But when the water vapor amount is more and more higher, the effect of the competitive adsorption becomes more important which causes the heptan-2-one removal decrease. Therefore, the presence of moisture in a gaseous stream can present a double effect on C7H14O oxidation: i. On the one hand, under humid oxidation conditions (from 45 to 75% humidity), the removal efficiency of Heptan-2-one was enhanced. Under dry flow gas (HR ~ 5%) and for an inlet concentration of 6.6 mg m -3 , 32.41% of Heptan-2one was removed and for a humidity level of 75% the degradation rate reached 45.62%. A slight increase in water molecules contributes to generate J o u r n a l P r e -p r o o f

[Butane- 2 , 3 - 3 [

 233 dione]= 5 mg m -Butane-2,3-dione]= 10 mg m -3 (a) J o u r n a l P r e -p r o o f

Figure 3 .

 3 Figure 3. Effect of humidity on (a) Butane-2,3-dione and (b) Heptan-2-one removal

[Heptan- 2 - 3 [

 23 one]= 6.6 mg m -Heptan-2-one]= 13.25 mg m -3 (b) J o u r n a l P r e -p r o o f by-products (section 3.3.2. below). The flow rate of the mixture stream was set to 2 m 3 h -1 and the relative humidity was around 5%. Results of the VOC's elimination capacity are presented in Figure 4. As seen in Figure 3 (a) and 3 (b), relative humidity constitutes an important experimental condition that can differently affect the trend of the photocatalytic VOC degradation. As a consequence, the influence of humidity on C4H6O2/C7H14O mixture degradation was monitored at 5%, 45% and 90% of relative humidity. These humidity tests are presented in Figure 5. Compared to the monocompound oxidation study (Fig.3 b), a different behavior of C7H14O degradation was obtained in the case of C4H6O2/C7H14O mixture.

Figure 4 .Figure 5 .

 45 Figure 4. Variation of the elimination rates of C4H6O2 alone, C7H14O alone and binary

Butane- 2

 2 ,3-dione Heptan-2-one J o u r n a l P r e -p r o o f ways (atomic oxygen O°, OH° hydroxyl radicals, HO2° hydroperoxyl radicals, ozone O3, N° radicals…) [2,8,31-34]. Figure 7 compares the degradation behavior of

Figure 6 .

 6 Figure 6. Variation of the removal efficiency of C4H6O2 alone and ozone

Figure 7 . 3 . 3 . 3 . 3 . 1 .

 733331 Figure 7. Effect of specific energy on removal efficiency of C4H6O2 and C7H14O

Butane- 2

 2 ,3-dione Heptan-2-one SIE= 2.25 J L -1 SIE= 4.5 J L -1 J o u r n a l P r e -p r o o f efficiencies via DBD plasma test alone and photocatalytic test alone were 63.39%

Figure 8 .

 8 Figure 8. Variation of the removal efficiency of C4H6O2, C7H14O, and their mixture

  PhotocatalysisDBD-plasmaPhototcatalysis/DBD-plasma

Figure 10 i

 10 Figure 10 i and ii as: (1) acetone (C3H5O) and acetaldehyde (C2H4O), (2) Butane-2,3-

Figure 9 .

 9 Figure 9. Mineralization rate (%) and ozone concentration (mg m -3 ) via the three

  r n a l P r e -p r o o f

Figure 10 .

 10 Figure 10. Oxidation by-products identified by GC-MS for C4H6O2/C7H14O

  r n a l P r e -p r o o f

Figure 11 .

 11 Figure 11. Suggested mechanism for Butane-2,3-dione & Heptan-2-one removal

  degradation rate dropped to 38.73% versus 45.62% at RH~75%. These results are in agreement with those reported by Zadi et al., Martinez et al. and Assadi et al. concerning the oxidation of chlorinated VOCs

31-Oct-2018 + 16:55:25 couplage

	31102018_walaa_tubePE15504										1: Scan EI+
	100												TIC 3.44e9
					9.59
	%							14.07
			4.83
	1.70									18.06
	0
	31102018_walaa_tubePE9194										1: Scan EI+
	100												TIC 3.44e9
					9.61
	%							14.11
			4.83
	0
	31102018_walaa_tubeA016307										1: Scan EI+
	100				9.61								3.44e9 TIC
			4.83
	%
	1.70
								14.21
	0												Time
	0.50	2.50	4.50	6.50	8.50	10.50	12.50	14.50	16.50	18.50	20.50	22.50	24.50
	13.24 31102018_walaa_tubePE15504 0 100 % , 12.24 14.24 15.24 16.24 17.24 18.24 14.07 18.06 16.17	19.24	20.24 Time 1: Scan EI+ TIC 2.20e9


Alexandre Leclercq

Gwenola Hardouin

Bertrand Lombard

reference methods in the microbiology of the food chain

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

Editorial

European and International validation of 15 main reference methods in the microbiology of the food chain A large amount of qualitative and quantitative analytical results are produced internationally on a daily basis by laboratories involved in the microbiological testing of the food chain. These results are used by food business operators and risk managers to ensure food safety for consumers and fair trade of goods. To avoid the use of multiple methods for the same target, standardization bodies are developing and validating harmonised reference methods. Moreover, with quality management and accreditation of testing laboratories, it is widely accepted that only validated methods should be used to meet the customers' need of reliable results.

At international level, it is the task of the Sub-Committee 9 Microbiology of Technical Committee 34 Food products of ISO, the International Organization for Standardization (ISO/TC 34/SC 9). At regional level in Europe, Working Group 6 Microbiology of the food chain of the Technical Committee 275 Food analyses -Horizontal methods of CEN, the European Committee for Standardisation (CEN/TC 275/WG 6) is entrusted to this task [START_REF] Leclercq | Chapter 5 -CEN/ISO standards for both culture and molecular methods[END_REF]. These two structures develop, validate and standardize common reference methods under the CEN/ISO collaboration Vienna agreement. Until 1996, standardized reference methods were mainly based on recognized experts' opinion or "historically proven" methodologies without performance characteristics derived from proper interlaboratory studies. The current policy of ISO/TC 34/SC 9 and CEN/TC 275/WG 6 requires a full validation of reference methods based on collaborative studies for inclusion of resulting performance characteristics in CEN/ISO standards [START_REF] Anonymous | [END_REF].

In order to include performance characteristics in CEN/ISO standardized reference methods, a first European project, Project SMT4-CT96-2098 financed by European Commission under the fourth Framework Standards, Measurements and Testing Program (SMT4) was conducted between 1997 and 2000 to determine the performance characteristics (repeatability and reproducibility of quantitative methods or sensitivity, specificity, accordance and concordance of qualitative methods) of six CEN/ISO reference methods [START_REF] De Buyser | Validation of EN ISO standard methods 6888 Part 1 and Part 2: 1999--enumeration of coagulase-positive staphylococci in foods[END_REF][START_REF] Lahellec | Development of standard methods with special reference to Europe[END_REF][START_REF] Lombard | Experimental evaluation of different precision criteria applicable to microbiological counting methods[END_REF][START_REF] Schulten | Evaluation of ISO 7932 standard for the enumeration of Bacillus cereus in foods[END_REF]Scotter, Langton, Lombard, Lahellec, et al., 2001;Scotter, Langton, Lombard, Schulten, et al., 2001). These standardized methods concerned Bacillus cereus enumeration (EN ISO 7932:2004), Listeria monocytogenes detection and enumeration (EN ISO 11290-1:1996 andEN ISO 11290-2:1998), coagulase positive staphylococci enumeration (EN ISO 6888-1:1999 andEN ISO 6888-2:1999), Clostridium perfringens enumeration (EN ISO 7937:2004) and Salmonella detection (EN ISO 6579:2002) [START_REF] Anonymous | Microbiology of food and animal feeding stuffs --Horizontal method for the detection and enumeration of Listeria monocytogenes --Part 1: Detection method[END_REF][START_REF] Anonymous | Microbiology of food and animal feeding stuffs --Horizontal method for the detection and enumeration of Listeria monocytogenes --Part 2: Enumeration method[END_REF](Anonymous, , 1999a(Anonymous, , b, 2002(Anonymous, , 2004a, b), b). The determined performance characteristics have been included in the respective CEN/ISO Standards. They have allowed to (i) risk managers to know the performance of the methods that are used in official controls to verify conformity of foodstuffs to legal microbiological criteria, (ii) food operators to rely on methods used by food testing laboratories and fit for their purpose, (iii) food testing laboratories to verify their compliance to the method's performance characteristics in the frame of their EN ISO 17025 accreditation [START_REF] Anonymous | [END_REF], and (iv) validation/certification bodies to validate alternative proprietary methods against these reference methods, according to EN ISO 16140 and since 2016, EN ISO 16140-2 (Anonymous, 2003[START_REF] Anonymous | [END_REF].

Further to the validation of these first 6 standardized reference methods and for similar purposes, European Commission signed a specific grant agreement in December 2010 with CEN (known as CEN Mandate M/381) to finance the validation and standardization (or revision) of 15 methods for the main micro-organisms or their toxins in the field of food chain microbiology. These validations were in particular necessary to correctly set and implement microbiological criteria at European level, either already included in Commission Regulation (EC) No 2073/2005 (Anonymous, 2005a), or to be possibly included at a later stage in future amendments to this regulation (i.e. Norovirus, Hepatitis A, Bacillus cereus toxins). The 15 validation trials were carried out between 2012 and 2017 by a total of 150 laboratories in 35 countries with a budget of 3120000 euros, representing the largest worldwide validation effort of standardized methods in microbiology of the food chain.

In this special issue, the detailed validation studies of each reference method are described in separate articles, except for the reference method for detection of staphylococcal enterotoxins. An harmonized experimental design for the organization of collaborative studies has been followed [START_REF] Anonymous | Statistical methods for use in proficiency testing by interlaboratory comparison[END_REF][START_REF] Anonymous | [END_REF](Anonymous, , 2014)). The 15 reference methods which were validated are mostly microbiological methods, except two chemical methods, for quantification of histamine and Bacillus cereus toxins (cereulide) and one biochemical method, for detection of staphylococcal enterotoxins. Thus, two statistical approaches were used for the determination of performance characteristics: an approach designed for microbiological analysis and derived from EN ISO 16140-2 (Anonymous, 2016a, b) and an approach more suited for chemical methods, derived from ISO 5725-2 [START_REF] Anonymous | results --Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement method[END_REF].

  Alexandre Leclercq*, Convenor of CEN TC275/WG6, Institut Pasteur, Biology of Infection Unit, National Reference Centre and WHO Collaborating Centre for Listeria, Paris, France. Gwenola Hardouin, Secretary of CEN/TC 275/WG 6 & ISO/TC 34/SC 9, AFNOR, Saint-Denis, France. Bertrand Lombard, Chairman of ISO/TC 34/SC 9 (formerly), French Agency for Food, Environmental, Occupational Health & Safety (ANSES), Laboratory for Food Safety, Maisons-Alfort, France *Corresponding associate editor. Tel. +33(0)140613190. Fax: +33(0)140613567. Email: alexlec@pasteur.fr

Acknowledgments

The European and international validation of 15 main reference methods in the microbiology of the food chain was carried out under the framework of European Mandate No. M381 of DG SANTE and DG GROW (European Commission) and was allowed by the participation and coordination of 150 laboratories in the method characterisation and /or interlaboratory study.


L Pernod

B Lossouarn
email: boris.lossouarn@lecnam.net

J.-A Astolfi

J.-F Deü

PASSIVE VIBRATION DAMPING OF HYDROFOILS USING RESONANT PIEZOELECTRIC SHUNT

Keywords: Flow-induced vibrations, Resonant piezoelectric shunt, Passive vibration damping

Marine lifting surfaces undergo flow-induced vibrations leading to shorter life cycles due to structural fatigue and reduced acoustic performances. As such, accurate understanding of the fluid-structure response of marine structures, as well as vibrations control and damping, are critical to many maritime applications. In particular, this work investigates the potential of the electromechanical coupling inherent to piezoelectric materials for passive vibration damping of hydrofoils under hydrodynamic flows. An aluminium flat plate equipped with piezoelectric patches connected to a resonant shunt is considered. The structure is first tested under hydrodynamic flows for various Reynolds numbers to investigate its flow-induced vibrations. This allows to determine the natural frequency of interest to test the control solution. Second, an experimental modal analysis is carried out to determine the open and short circuit natural frequencies in order to compute the piezoelectric coupling factor. Indeed, the latter is related to the expected performance of the passive vibration damping strategy. Third, the values for the resistive and inductive components of the RL-shunt are inferred from the coupling factor and the natural frequencies. Last, the control solution is tested in still air and water in open and short circuits configurations. Comparisons of these two configurations are realised and the resonant shunt performance for vibration reduction of hydrofoils is estimated.

INTRODUCTION

Applications for high-speed ships fitted with hydrofoils, such as fast passenger-ferries and racing sailing boats, are progressively expanding and gaining economic importance. This leads to a renewed interest in the physical understanding of flow-induced vibrations of light flexible structures subjected to strong fluid-structure interactions. There is also a growing demand for vibration control and damping solutions. Indeed, flow-induced vibrations may trigger a sharp increase in the vibration amplitude when there is a coincidence between a natural frequency of the structure and a hydrodynamic excitation frequency. The consequences of such highamplitude vibrations are reduced acoustic performances (e.g. hydrofoil singing), and shorter life cycles [START_REF] Lelong | An Experimental Analysis of the Structural Response of Flexible Lightweight Hydrofoils in Cavitating Flow[END_REF][START_REF] Reese | Vibration and Damping of Hydrofoils in Uniform Flow[END_REF]. As such, it is of prime importance to develop methods for vibration damping.

Several researches have been completed on the high potential of piezoelectricity for vibration control and damping. In particular, Lossouarn et al. [START_REF] Lossouarn | Sensors and Actuators A: Physical Design of inductors with high inductance values for resonant piezoelectric damping Sensors Actuators A[END_REF] and Thomas et al. in 2012 [4] in 2017 successfully demonstrated the ability of passive piezoelectric shunts to reduce the amplitude of the vibrations of structures in air. Academic research is now turning to the application of this solution to the maritime domain [START_REF] Li | Passive Vibration Control of Flexible Hydrofoils Using Piezoelectric Material[END_REF]. The purpose of this work is to provide a first experimental prototype for the application of vibration damping using a resonant piezoelectric shunt in water. The methodology used to assess the performance of the solution and design the electrical components of the resonant shunt (inductance and resonance) from the natural frequencies in open circuit and closed circuit is first described. We then present a case of application on a flat aluminium plate.

VIBRATION DAMPING USING A RESONANT PIEZOELECTRIC SHUNT

Vibration damping using piezoelectricity consists in converting part of the vibratory mechanical energy into electrical energy, which is then dissipated through a resistor R. Moreover, it is possible to create an electric resonance with the addition of an inductance L to the electrical circuit. When this electric resonance matches one of the natural frequencies of the structure the energy transfer is increased. As a consequence, a passive resonant piezoelectric shunt behaves similarly to a tuned mass-damper [START_REF] Lossouarn | Sensors and Actuators A: Physical Design of inductors with high inductance values for resonant piezoelectric damping Sensors Actuators A[END_REF]. Figure 1 represents a model for a massspring system (mass m and stiffness K) coupled to a resonant piezoelectric shunt and subjected to a force F. U represents the displacement of the mass, q represents the electric charge displacement, C the capacitance of the piezoelectric patch, and e is the piezoelectric constant, R and L respectively being the resistance and the inductor of the shunt. Important electrical conditions and their corresponding natural frequencies are the "short circuit" and "open circuit" conditions. The former corresponds to zero voltage across the piezoelectric patch and a natural frequency ωsc, while the latter corresponds to zero electric charge displacement (q = 0) and to a natural frequency ω oc . The performances of the vibration damping solution are only dependant on the coupling factor kc defined by the short circuit and open circuit natural frequencies:

k c = √ ω oc 2 -ω sc 2 ω sc 2 (1)
Once the coupling factor is known, the electrical inductor and resistor of the resonant shunt have to be designed according to:

L= 1 Cω oc 2 (2) 𝑅𝑅= √ 3 2 1 𝐶𝐶𝜔𝜔 𝑜𝑜𝑜𝑜 2 (3)
These conditions correspond to the optimal values of L and R in order to set the electrical resonance on the mechanical resonance of interest [START_REF] Li | Passive Vibration Control of Flexible Hydrofoils Using Piezoelectric Material[END_REF]. Passive inductors are then produced according to these specifications using copper wire and appropriate magnetic circuits.

EXPERIMENTAL SETUP AND TECHNIQUES

Experimental Facility and Sensors

Experiments were carried out in the hydrodynamic tunnel at the Institut de Recherche de l'Ecole Navale (IRENav) in Brest, France, illustrated on Figure 2. The tunnel test section is 1 m long with a 192 mm square section. Available operational velocities in the test section range from 0 m/s to 12 m/s and available pressures range from 0.1 bar to 3 bar. Both are set up using a regulation system. Moreover, a honeycomb grid upstream of the test section allows for a 2% free stream turbulent intensity at the mid-section. Vibration measurements are realised using a scanning Doppler laser vibrometer located above the structure and a single point Doppler laser vibrometer located below the structure. For more information about the hydrodynamic tunnel and the experimental techniques, the interested reader may refer to [START_REF] Lelong | An Experimental Analysis of the Structural Response of Flexible Lightweight Hydrofoils in Cavitating Flow[END_REF][START_REF] Gaugain | Analyse expérimentale et simulations numériques de l'interaction fluidestructure d'un hydrofoil élastique en écoulements subcavitant et cavitant[END_REF].

Geometry and Materials

The structure considered in this work is a flat plate made of aluminium 5083 and equipped with four piezoelectric patches (two patches on each side of the plate). Base dimensions of the plate are 191 mm x 100 mm x 6 mm, with a 80 mm x 31 mm x 10 mm extension at its base, as illustrated on Figure 3. This extension is used to clamp the structure to the tunnel wall. Furthermore, the piezoelectric patches are embedded in the structure via two pockets machined on both sides of the plate (Figure 3 and Figure 4). Indeed, embedding the patches in the structure avoids creating local roughness, which may cause added turbulence or flow detachment if the roughness size is in the order of the boundary layer thickness. The piezoelectric patches are vacuum-glued in the pockets using epoxy, then a layer of paraffin is added to fill the residual depth of the pocket. Pathways are also machined in the plate and the extension to allow the passage of cables from the electrodes (two on each patch) to the components of the electrical control circuit.

Figure 5 presents the geometry and dimensions of the piezoelectric transducers, in which a PIC 255 piezoelectric ceramic (black rectangle) is encapsulated. The dimensions of the patch are 61 mm long, 35 mm wide and 0.8 mm thick, for a piezoelectric part 50 mm long, 30 mm wide and 0.5 mm thick.

RESULTS

Test Cases Under Hydrodynamic Flows

Experiments are first carried out with the plate placed in water at an incidence of 0º, and incident flow velocities ranging from 1 m/s to 10.5 m/s, i.e. moderate Reynolds numbers from 10 5 to 1.05x10 6 . The objective is to investigate the flow-induced vibrations of the structure in order to determine the natural frequency of interest to test the control solution. Indeed, as detailed in Section 2, one has to know the value of the frequency to be controlled to guide the design choices of the electrical circuit (inductor and resistor). Therefore, piezoelectric patches are inactive in this first series of tests. Figure 6 shows the vibration spectra of the plate for the range of incident velocities considered. The first three natural frequencies are visible for respective values of 26.4 Hz, 174.1 Hz and 265.6 Hz. The spectra also exhibit additional components with a strong dependence to the Reynolds number: the local maximum close to 50 Hz for a Reynolds number of 1.5x10 5 is progressively shifted towards the high frequencies when increasing the Reynolds number. This phenomenon is characteristic of a vortex shedding process associated to Von Kármán alleyways [START_REF] Lelong | An Experimental Analysis of the Structural Response of Flexible Lightweight Hydrofoils in Cavitating Flow[END_REF][START_REF] Reese | Vibration and Damping of Hydrofoils in Uniform Flow[END_REF][START_REF] Blake | Excitation of Plates and Hydrofoils by Trailing Edge Flows[END_REF][START_REF] Naudascher | Flow-Induced Vibrations: An Engineering Guide[END_REF]: The boundary layer is developing on both sides of the plate but meets with a singularity at the trailing edge. The boundary layer then starts to shed alternatively from both sides of the plate, leading to the formation of a vortex street in the turbulent wake. This alternate shedding causes a periodic fluctuation of the hydrodynamic loads applied on the plate and is thus a source of excitation associated to a shedding frequency fshed. In the present case, we can observe a strong fluid-structure interaction between the second mode of the structure (corresponding to the first torsion mode) and the shedding frequencies when their respective values coincide (around 174 Hz). This strong fluidstructure interaction is characterised by a sharp increase of the vibration amplitude. 7a presents the corresponding Reynolds-frequency diagram, which clearly highlights the linear evolution of the hydrodynamic excitation frequency with the Reynolds number, and therefore the velocity of the incident flow. This linear evolution is given by (4), with a constant Strouhal number close to 0.2 when vortex shedding occurs (d is the thickness of the plate and U0 the incident velocity) [START_REF] Blake | Excitation of Plates and Hydrofoils by Trailing Edge Flows[END_REF][START_REF] Naudascher | Flow-Induced Vibrations: An Engineering Guide[END_REF].

Sh= f shed d U 0 (4)
The linear evolution of the shedding frequencies with the Reynolds number according to Strouhal's law is also highlighted on Figure 7b, where the experimental data points correspond to the local maxima of the spectra presented on Figure 6. One can see that in the present case the Strouhal number is constant to 0.194, confirming the existence of vortex shedding. However, for some Reynolds numbers between 4x10 5 and 6x10 5 the shedding frequencies no longer follow Strouhal's law, but instead keep a constant value of 174 Hz, despite the increase in the number of Reynolds. This is the phenomenon of frequency lock-in around the first mode of torsion: Von Kármán's turbulent wake no longer evolves with its own dynamics (4) but is organized around the natural frequency of the structure [START_REF] Naudascher | Flow-Induced Vibrations: An Engineering Guide[END_REF][START_REF] Zobeiri | Effect of Hydrofoil Trailing Edge Geometry on the Wake Dynamics[END_REF]. As a consequence, the strong fluid-structure interaction previously mentioned corresponds to this lock-in phenomenon.

Test Cases in Still Air and Water

The first series of tests highlighted the existence of a strong fluid-structure interaction between the first torsion mode and the hydrodynamic excitation frequencies due to Von Kármán vortex shedding, resulting in a large increase in the vibration amplitude. As a consequence, this structure is an interesting test case for the application of the vibration damping solution (described in Section 2) to the first torsion mode. The objective is to reduce the amplitude of the vibrations thanks to a passive resonant piezoelectric shunt adjusted on the natural frequency of the structure where the strong fluid-structure interaction occurs. As a first attempt for vibration damping in water using a piezoelectric shunt, this solution is indeed easier to setup than a Reynolds-dependent shunt that would follow the frequency shift of the hydrodynamic excitation.

For this purpose, a second series of tests is carried out both in air and then in still water in order to determine the open and short circuit natural frequencies and evaluate the performances of the control solution in a simplified case. Since the flow no longer provides the excitation of the structure, an alternative source of excitation is used: an electrical voltage is imposed on one of the patches (using a function generator) which then generates a vibratory excitation on the structure through the inverse piezoelectric effect. The other three patches are for the moment kept inactive. The main advantage of this method lies in its ability to provide the frequency responses functions (FRF) of the structure as both the input (voltage imposed on the patch) and output (velocity measured by the vibrometer) are known. On the contrary, when the excitation is generated by the hydrodynamic flow, the input is unknown. Here, a sinus excitation swept across the entire frequency band between 1 Hz and 100 Hz is applied on one of the patches. The temporal signal of this excitation is shown on Figure 8a The frequency responses derived from the temporal signals are presented on Figure 9, where the first two modes of the structure are visible in air and the first three modes are visible in water. Corresponding values are reported in Table 1: the comparison of the natural frequencies in air and in water shows significant added mass effects, with a reduction of the in-air natural frequencies up to 58.8%. Last, a third series of tests was performed to evaluate the performances of the coupling solution with one of the patches providing the excitation, and the other patches activated for vibration damping. However, the coupling factors obtained (of the order of 3% for the first torsion mode) did not allow to observe a significant reduction of the amplitude of vibration. Such low coupling factors may be explained, on the one hand, by a problem of water sealing on the cables, and on the other hand by a poor placement of the patches to control the first torsion mode. To tackle this issue, two solutions may be considered. First, smaller patches could be used. This way, the location of the patches would be better suited for the control of the first torsion mode. Another solution is to control the first bending mode instead of the first torsion mode, as the coupling factors are more favourable for the bending. However, in order to get an electrical signal that is strong enough for clear measurements, it is preferable to increase the frequency of the first bending mode, for instance by decreasing the length of the plate from 191 mm to 100 mm. This second solution is currently under investigation.

CONCLUSIONS

This work presents the testing in water of an aluminium flat plate equipped with four piezoelectric patches. It constitutes a first experimental assessment of vibration damping in water using a resonant piezoelectric shunt. The dynamic response of the structure under hydrodynamic flow was first studied for various incident velocities, then the performances of the control solution were estimated. Prior characterization of the dynamic response of the structure is indeed needed to determine the short and open circuit natural frequencies in order to compute the coupling factors and the optimal values for the inductance and resistance components of the piezoelectric shunt.

Results showed a strong fluid-structure interaction phenomenon between the natural frequencies of the structure and the hydrodynamic excitation frequencies due to Von Kármán vortex shedding in the turbulent wake. In particular, the hydrodynamic excitation frequencies present a linear dependence to the Reynolds number, with a constant Strouhal of 0.194, with the exception of a frequency lock-in around the first torsion mode (around 174 Hz). An original method for obtaining the frequency responses of the structure (FRF) in still water has also been implemented using one of the patches to provide the excitation of the structure, which allow non-contact excitation and measurement. However, the coupling factors obtained during these first tests to control the first torsion mode were too low to observe a significant reduction of the vibration amplitude, in particular due to a non-optimal placement of the piezoelectric patches. New tests are currently underway with a modified structure to ensure a better placement of the piezoelectric patches. A numerical analysis is also being carried out in order to numerically estimate the coupling factors before testing in water.

Figure 1 :

 1 Figure 1: Purely mechanical model of a structure coupled to a resonant piezoelectric shunt

Figure 2 :

 2 Figure2: Hydrodynamic tunnel at IRENav,[START_REF] Gaugain | Analyse expérimentale et simulations numériques de l'interaction fluidestructure d'un hydrofoil élastique en écoulements subcavitant et cavitant[END_REF]

Figure 3 :

 3 Figure 3: Geometry and dimensions of the structure

Figure 4 :

 4 Figure 4: Flat plated equipped with the piezoelectric patches

Figure 5 :

 5 Figure 5: Geometry and dimensions of the piezoelectric patches

Figure 6 :

 6 Figure 6: Vibration spectra under hydrodynamic flow for different incident velocities

Figure

  Figure7apresents the corresponding Reynolds-frequency diagram, which clearly highlights the linear evolution of the hydrodynamic excitation frequency with the Reynolds number, and therefore the velocity of the incident flow. This linear evolution is given by (4), with a constant Strouhal number close to 0.2 when vortex shedding occurs (d is the thickness of the plate and

Figure 7 :

 7 Figure 7: (a) Reynolds-frequency diagram and (b) shedding frequency vs Reynolds number and lock-in

  and the corresponding temporal signal of the in-air and in-water responses are shown on Figure 8b.

Figure 8 :

 8 Figure 8: Temporal signals associated to (a) the excitation by sweep and (b) the in-air and in-water vibrations of the structure

Figure 9 :

 9 Figure 9: In-air and in-water frequency responses (without flow)

Table 1 :

 1 Comparison of the in-air and in-water natural frequencies and added mass effects

		f1	f2	f3
	fair [Hz]	63.3 295.9	-
	fwater [Hz] 26.4 174.1 265.6
	% fair	58.3 41.2	-

L. Pernod, B. Lossouarn, J.-A. Astolfi and J.-F. Deü