Slat aerodynamic noise reduction using dielectric barrier discharge plasma actuators

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Aerodynamic noise reduction from wing high-lift devices using dielectric barrier discharge plasma actuators – Part II (slat)

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Gabriel Pereira Gouveia da Silva Fernando Martini Catalano a

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, João Paulo Eguea , José Antônio Garcia Croce ,

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Article history: Received 11 August 2019 Received in revised form 12 November 2019 Accepted 10 December 2019 Available online xxxx

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Department of Aeronautical Engineering, University of São Paulo, Brazil b Federal Institute of Education, Science and Technology of São Paulo, Brazil

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Keywords: Electrohydrodynamics Airframe noise Active flow control EHD DBD

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The airplane, during the approach to landing, use high-lift devices to produce more lift at lower airspeeds, enabling the landing on shorter runways. In this flight condition, high-lift devices are amongst the main sources of aeroacoustic noise. The solutions found in the literature are mostly based on passive flow control and are reaching an asymptotic level of noise mitigation. Therefore, active flow control devices may have the potential for further improvements. Active flow control can be realized by dielectric barrier discharge plasma actuators. These devices generate a strong electric field that ionizes and accelerates the surrounding air, producing a wall jet. In this research, plasma actuators were installed at the cove and the cusp of a slat. Three geometries of actuators were tested. Acoustic array measurements in a wind tunnel have demonstrated the potential of plasma actuators for slat aerodynamic noise reduction, allowing a decrease of up to (3.3 ± 0.02) dB in the overall slat noise (reducing 12 dB of the dominant tone). However, many configurations were not able to reduce or even increased the overall noise, evidencing the necessity of properly optimizing the plasma actuator geometry, materials, and operational parameters to enhance the control authority of these devices, which remains an obstacle to real flight applications. © 2019 Elsevier Masson SAS. All rights reserved.

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1. Introduction Due to urban expansion (which increasingly brings urbanized regions closer to main airports), to growing air fleets and increasing aircraft operations, noise from airport activities has negatively affected a rising number of people, leading to a growing restriction on tolerable levels of noise by the authorities [1]. The last Committee on Aviation Environmental Protection (CAEP) report [2] sets a reduction target of 10 dB in the aeronautical noise level per operation by 2020 compared to the 2000 levels, based on the Advisory Council for Aviation Research and Innovation in Europe (ACARE) directives [3]. Until the 1970s aeronautical noise was predominantly produced by the reaction engines. However, this noise source has been continually reduced with the increase in the bypass ratio of these engines. Thus, at flight conditions where the engines are operating at low power (during the approach to landing, for example), airframe noise has become prevailing. Airframe noise is generated

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Corresponding author. E-mail addresses: [email protected] (G.P. Gouveia da Silva), [email protected] (J.P. Eguea), [email protected] (J.A.G. Croce), [email protected] (F. Martini Catalano). https://doi.org/10.1016/j.ast.2019.105642 1270-9638/© 2019 Elsevier Masson SAS. All rights reserved.

by the interaction between the external turbulent flow and the exposed geometrical discontinuities of the aircraft surfaces [4,5]. Among the sources of airframe noise, the high-lift devices and the landing gear are the most prominent [1,4]. Slat noise has frequencies up to 2500 Hz, with peaks in the rear arc, and flap noise has frequencies from about 4000 to 16000 Hz, with peaks in the forward arc at high frequencies and in the rear arc at low frequencies [6–8], being both mainly broadband sources. Whereas the flap side edge is considered a punctual sound source, the slat is a distributed source. Considering the acoustic power radiated per unit area, the flap side edge is a more intense source than the slat. However, when integrated along its length, the slat contribution to overall noise is more significant than that of more intense punctual sources. Thus, during the approach to landing, slat noise is dominant at low and medium frequencies when compared to flap side edge noise [9–11]. Due to the presence of independent noise generation mechanisms in overlapping frequency bands, slat noise is a complex aeroacoustic problem not yet clearly understood [10,12]. It can be classified into three overlapping components [9,13,14]: broadband, which is the main source of slat noise (dominant in real flight conditions), multiple tonal peaks of low frequency and multiple tonal

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peaks of high frequency (which does not occur in flight Reynolds, being a typical problem of scaled wind tunnel models). The flow structure within the slat cove is highly dependent on the geometry, deflection angle, gap, overlap, flow velocity and angle of attack of the multielement wing [15,16]. When extended, the slat has a cove that generates a region of separation, in which instabilities and vorticity are generated and can interact with neighboring surfaces, generating noise [15]. According to Choudhari and Khorrami [11] and Dobrzynski [14], a vortex develops in the slat cove, driven by the flow that passes through the slot. Between this vortex and the undisturbed flow, a free and unstable shear layer develops originating at the cusp and impacting the surface of the cove. These instabilities that lead to slat noise are probably generated in the shear layer of the slat, which is an amplifier of low and medium frequency disturbances [10,17,18]. This amplification occurs due to a feedback mechanism [11,19–21]: after passing through the cusp, the shear layer is wrapped in discrete spanwise vortices; these spanwise deformations are amplified with the downstream distance, resulting in predominantly three-dimensional vortical structures; the vortices approaching the shear layer reattachment region are strongly three-dimensional, and when interacting with the trailing edge, are distorted and broken in other vortices; a significant fraction of these vortices are strongly accelerated in the direction of flow, interacting with the trailing edge; the remaining vortices are trapped in the recirculation zone, being convected back to the cusp; this convection of three-dimensional vorticities trapped into the shear layer induces non-stationary eruptions of secondary vorticity along the boundary layer of the surface of the slat cove; the vortices that go to the slat cove end up reaching the cusp and further disturbing the shear layer, closing the feedback loop. This mechanism is similar to that of a cavity flow [22–24]. According to Jenkins et al. [15], Choudhari and Khorrami [20] and Lockard and Choudhari [25], the rapid distortion of vortices that occurs near the reattachment point and the ensuing propagation after passing through the trailing edge produce large fluctuations in pressure, thus being important sources of noise. Besides, low-frequency noise can also be produced, since the position of the vortex inside the cove is non-stationary [11,14]. It is believed that these mechanisms are responsible for the generation of the low-frequency broadband component, typical of slat noise. Beyond this mechanism, a vortex shedding at the slat trailing edge may occur, leading to high-frequency tonal components of the sound spectrum, which are typical of wind tunnel scaled models, being absent in real flight conditions [10,11,14,21,26,27]. Low-frequency tonal noise due to coherent laminar separation and vortex shedding at the slat cusp may also be produced in wind tunnel conditions [9,28]. The solutions currently employed to reduce high-lift noise are mostly based on passive flow control [5,6,8,11,14,16,17,29–31]. However, these solutions can be mechanically complex, heavy and difficult to manufacture and maintain, and can degrade aerodynamic performance [14]. Additionally, Yong et al. [32] remark that the developments in airframe noise reduction solutions adopted by the industry have reached an asymptotic level of improvement in recent years and that this trend indicates that the ACARE noise reduction goal will not be attained by 2020. Thus, Yong et al. [32] believe that alternative solutions should be explored, more specifically, solutions based on active flow control (plasma, air blowing, suction). The researches on active control applied to airframe noise reduction, however, are still in a primitive stage [32], requiring more researchers to look into this question to improve its technology readiness level (TRL). Active control is obtained when the manipulation of the airflow is made by adding energy to it. One way to accomplish this is by using dielectric barrier discharge plasma actuators (DBD-PAs). DBD

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Fig. 1. Schematic cross-section of a dielectric barrier discharge plasma actuator (out of scale).

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plasma actuators are completely electric devices, without pneumatic, hydraulic and moving parts, which directly convert electric energy into kinetic energy, inducing an airflow in the order of up to 10 m/s close to the wall, called ionic wind [33,34]. They are lightweight, very thin and fast-reacting, being ideal for aerodynamic control at a low power consumption cost [33,34]. In general, they are composed of two flat electrodes mounted on opposite sides of a dielectric (Fig. 1), which are subjected to a high AC voltage (usually from a few kV to 30 kV, with frequencies from 100 Hz to a few tens of kHz) [33,34]. When this AC voltage is high enough, the intense electric field produced ionizes the air above the covered electrode, forming an atmospheric cold plasma [35]. This ionized air, in the presence of the electric field (whose direction is defined by the geometry of the actuator), results in a body force acting to accelerate the air [35]. The asymmetric configuration of the DBD-PA creates a suction region on the exposed electrode causing air in the vicinity of the device to be drawn toward the wall, from where it is then ejected tangentially to the surface from the exposed electrode edge, over the region of the encapsulated electrode, thereby inducing a pseudo wall jet [36–38]. This DBD induced jet differs from the classic wall jet because, in the case of DBD, mass is not injected into the flow, only momentum [34,38]. The fact that the ionic wind produced is unidirectional and dependent on the orientation of the asymmetric electrodes implies that there is some asymmetry between the halves of the voltage cycle when operating the actuator [39]. Enloe et al. [39] demonstrated that about 97% of the momentum coupling occurs during the negative portion of the discharge cycle and related this behavior to the large differences in the spatial and temporal discharge structures between the positive and negative portions of the voltage wave applied to the electrodes. The negative half-cycles induce a higher horizontal velocity than the positive half-cycles causing the induced wind to be unidirectional despite the AC voltage applied [33–35,39–41]. The electrical and mechanical characteristics of the plasma and the induced jet depend strongly on several constructive and operational parameters. The main factors that affect the performance of DBD-PAs are the dimensions of the electrodes, the gap between the electrodes, the dielectric thickness, the dielectric material, the applied voltage, the AC frequency, the voltage waveform and if the operation is done in steady (duty cycle of 100%) or unsteady mode, where the modulation frequency and the duty cycle are also important. The influence of these operational parameters is highly non-linear and interdependent, which makes it very difficult to design, optimize or to model mathematically these devices. However, there are several works published regarding the isolated influence of each one of these parameters [28,33–37,42–52], which can guide the implementation of DBD-PAs for experimentation. In addition to the traditional configuration of DBD-PA, some geometric variations can be found in the literature [49,53–59]. The main differential of these configurations is the use of serrated electrodes to induce more intense jets (due to electric charge concentration) or to produce three-dimensional flows, which can be

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especially interesting for breaking coherence in two-dimensional noise sources. Most applications found in the literature refer to aerodynamic improvements (drag reduction, lift-to-drag ratio rise, stall delay) based on boundary layer separation and transition control through momentum addition or vortex generation [33,34,37,38,40,44,51, 60–72]. These features can also be used for aircraft attitude control through differential activation of the DBD-PAs installed at aerodynamic surfaces [47,73,74]. More specifically, a stall delay application of DBD-PA was explored by He et al. [75]. Since the plasma actuator installed on the leading edge of the wing keeps the flow attached at high angles of attack, it could replace the complex and heavy retraction and extension mechanisms of flaps and slats, which is especially advantageous in aircraft design, where light and simpler structures are preferable, along with the elimination of the high-lift noise problem [75]. Although the work of He et al. [75] is conceptually interesting, the idea would hardly be applicable in commercial aircraft, since it would be difficult for certification due to the risk of stalling at low altitudes, causing a catastrophic accident in case of DBD-PA failure during an approach to landing. Furthermore, in the current state of technology, plasma actuators have little control authority under actual flight conditions, since the energy they introduce is very small relative to the energy of the free flow to be manipulated. The traditional flap and slat systems are quite consolidated and reliable, being preferable in the design of a new aircraft. Thus, it is more interesting to find a solution that combines traditional high-lift devices with plasma actuators to attenuate their noise, so as not to compromise flight safety, being a commercially certifiable solution. Chen [12], Chappell et al. [76] and Chen et al. [77] used plasma actuators together with traditional slats to minimize the tonal peaks of the slat sound spectrum. The work developed by these researchers consisted of an experimental investigation on traditional DBD actuators installed along the slat cusp, operated by an LQG (Linear-Quadratic-Gaussian) controller. The researchers obtained 20 dB reduction in the dominant tonal peak. According to Chappell et al. [76], the work of Chen [12] is to date the only one in the literature that reports the reduction of slat noise through the use of dielectric barrier discharge, which was confirmed by searches carried out by the authors of the present research. However, the region of plasma actuator application in the work of Chen et al. [77] was only the slat cusp. This way, their results were mainly suppression of the tonal components of slat noise, with a slight effect on broadband noise. Chen [12] suggests in his doctoral thesis that future work be done with plasma actuators installed in more appropriate positions, such as on the suction surface of the slat or the cove surface near the trailing edge of the slat. These indications were considered by the authors during the experiments presented in this paper. Therefore, in summary, the objective of this research was to investigate if dielectric barrier discharge plasma actuators (with classical and non-usual geometric configurations) can be used to reduce slat aerodynamic noise. The differences between the work presented hereby and the similar works found in the literature lies in the fact that different geometries and installation positions were tested, aiming to alter the flow field and consequently the aeroacoustic noise produced differently.

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2. Experimental setup and procedures

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The experimental setups and procedures used to assess the ability of the DBD-PA as a solution to slat noise are presented in this section.

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Fig. 2. LAE-1 wind tunnel dimensions and acoustically treated regions (Adapted from Santana et al. [80]).

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2.1. Wind tunnel and microphone array

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The acoustic measurements were conducted at the LAE-1 wind tunnel facilities, at the University of São Paulo (Brazil). It is a closed-circuit low-speed wind tunnel, with a closed test section of 1.3 m height, 1.67 m width and 3.0 m length. The eight-blade fan driven by an 110 HP AC electric motor induces a maximum airspeed of 50 m/s, with low turbulence level (up to 0.21%) provided by a combination of two screen meshes, low-drag corner vanes, high-efficiency rotor blades and carefully designed low-angle diffusers [78–80]. The LAE-1 wind tunnel has melamine foam covers in the walls I and II (Fig. 2), a foam filling of the gap between the fan blades and the wall at the position III and an acoustic baffle at the position IV [80]. These treatments were able to reduce the background noise in 5 dB and practically remove the tonal peaks of the fan [80]. Since the LAE-1 is a hard-wall closed test section wind tunnel, the acoustic measurements were carried out using a phased array technique to improve the signal-to-noise ratio. The microphone array used consists of 61 G.R.A.S. 46 BD microphone sets (which combine a 1/4 pressure microphone 40 BD and a 1/4 preamplifier 26 CB), flush-mounted without grid at the test section side-wall, distributed in a modified spiral geometry (Fig. 3) optimized for achieving a high frequency range with minimum beamwidth and side-lobe contamination [81]. These microphones have a frequency range of 4 Hz to 70 kHz, a dynamic range of 166 dB, and a calibrated sensitivity of 1.5 to 2.6 mV/Pa (obtained with an NC-74 calibrator, which produces a 1.0 kHz, 1.0 Pa reference pressure fluctuation). This set of configurations and geometry warrants an array beamwidth (spatial resolution) smaller than 100 mm for frequencies above 400 Hz and a frequency resolution of 25 Hz [82]. The 61 microphone signals were synchronized and acquired with a National Instruments NI PXIe system composed of a PXIe 1082 chassis; a PXIe-8135 controller for synchronization, triggering and control of acquisition boards; four PXIe-4497 boards for IEPE voltage measurements up to 204.8 kHz and synchronized acquisition of 16 channels each; PXIe-8430 board for serial com-

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munication and PXIe-6341 board for multifunction acquisition of analog and digital I/O at 500 kHz. Data acquisition, processing, and storage are managed by a MATLAB set of codes created and described by Pagani Júnior [82]. The acquisition code receives the synchronized measurements from the NI PXIe system, with data from the microphones and from the DP-Calc 8705 Micromanometer, and manual inputs of temperature, atmospheric pressure, and relative humidity, read from analog instruments. All these data and information about the model are stored in a Hierarchical Data Format (HDF5) file. Then, these raw data files were processed with a frequency domain beamforming code also implemented by Pagani Júnior [82]. This code performs a Fourier transform of microphone time history, applying Welch’s methodology with 50% of block overlap and Hanning window, correcting the power by a factor of 8/3. This process is repeated for all the microphones in order to create a Cross Spectral Matrix (CSM), from which conventional beamforming calculations are made through a spatial filter within a selected potential source location, using steering vector normalization [82,83]. The dynamic pressure of the wind tunnel flow was measured by a Pitot tube and a static pressure tap both connected to a micromanometer DP-Calc 8705. It has a full-scale range of −1.245 kPa to 3.735 kPa, with an accuracy of 1% of the reading ±1 Pa. The micromanometer acquisition was made by the National Instruments PXIe system described previously.

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2.2. High voltage AC power supply and plasma actuators

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The high-frequency high-voltage AC power supply (HFHVPS) used was an in-house equipment composed by a high-frequency wave generator and a high-voltage transformer, both supplied by a commercial DC power supply. Details on the electric circuit of the HFHVPS are provided by Silva [84]. The HFHVPS output is practically linear with the DC voltage input for a given output frequency (with a coefficient of determination between 0.97 and 0.99) and can provide up to 23.0 kV AC at 6.0 kHz. The plasma actuators were made of hand-cut and hand-laid self-adhesive tapes of copper foil (0.045 mm thick) and polyimide film (0.054 mm thick per layer). All the DBD-PAs used had one layer of polyimide film as substrate and three layers of polyimide film as the dielectric between the electrodes. Three geometries were built: a traditional straight electrode configuration (Fig. 4), a square-serrated electrode configuration (Fig. 5) and a doubleserpentine serrated electrode configuration (Fig. 6). The serrated electrodes were made by etching the copper self-adhesive tape with a ferric chloride solution, the same process used for etching printed circuit boards. The spanwise extension of each plasma

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Fig. 6. Double-serpentine serrated electrode DBD-PA dimensions [mm].

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actuator varied according to the application tested: 140 mm for the smoke visualization experiment and 410 mm for the acoustic measurements. The two serrated geometries were chosen aiming to produce three-dimensional flow structures, to break spanwise coherence of the slat noise sources. Measurements in quiescent air showed that the straight electrode configuration was able to produce a 1.00 m/s induced jet, while the square-serrated electrode was able to produce a jet of 1.05 m/s at the peak and 1.43 m/s at the valley of the exposed electrode, and the double-serpentine serrated electrode was able to produce a jet of 1.28 m/s at the peak and 1.02 m/s at valley of the exposed electrode.

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2.3. Multielement wing and experimental procedures

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According to Huang et al. [21], the key strategies to reduce slat noise are: relief or impedance of the shear layer impact on the pressure side of the slat; thickening of the shear layer to disrupt instabilities; break up of spanwise coherence or facilitation of three-dimensionality; enhancement of the shear-layer receptivity to the acoustic disturbances; reduction of the streamwise flow speed by oblique shocks. Choudhari and Khorrami [20] also suggests that an optimal strategy to mitigate broadband slat noise should target the shear layer reattachment region, to reduce the amplitude of surface pressure fluctuations. Pereira et al. [18] determined the location of the shear layer reattachment point in the slat cove through the coherence between two consecutive hot-film sensors, finding that the reattachment position moves slowly toward the slat trailing edge with increasing angle of attack. The model in which Pereira et al. [18] performed their measurements is the same one that was used in the experiments of the research presented hereby. Thus, the results presented by Pereira et al. [18] were directly used for indicating the position in which the DBDPAs was to be installed, following the suggestion of Chen [12]. In order to have a proof of concept, a small scale smoke flow visualization experiment was executed before the implementation of the large scale acoustic tests. This visualization test was carried out in an open circuit vertical wind tunnel at the Aeronautical Engineering Department of the University of São Paulo. This wind tunnel is equipped with a Preston and Sweeting [85] smoke generator connected to a rake of tubes and operates at very low wind

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speed (in the order of 2.0 m/s). It has a test section of 700 mm length, 350 mm width and 150 mm depth, with fluorescent tubes in its lateral walls that create a light sheet in the plane of the smoke filaments. First, a scaled model of the complete multielement wing was made of ABS through FFF (Fig. 7). The model has a 150 mm span and a full-extended chord of 498 mm, and is attached to an aluminum shaft used for fixation within the test section. A straight electrode DBD-PA was handmade as described in section 2.2, with the dimensions presented in Fig. 4 and 140 mm length. The DBD-PA was installed at the slat cusp (Fig. 7). Due to the small scale of the multielement wing and to the impossibility in scaling the DBD-PA, the proof of concept was not performed for the case with DBD-PA installed at the shear layer reattachment point. The experiments consisted of a photographic register with the DBD-PA deactivated and activated for comparison of the streamlines (smoke filaments). The HFHVPS is the same presented in section 2.2. The operational parameters of the HFHVPS were set to V in = 19.7 kV, f = 6.0 kHz and D .C . = 100%. The wind speed was of about 2.0 m/s. The slat acoustic measurements were carried out in the wind tunnel facilities described in section 2.1. The DBD-PAs were installed individually at three positions (Fig. 9) along the slat of a two-dimensional multielement wing model (Fig. 8). This model is representative of a commercial aircraft wing. The wing model has 500 mm chord (stowed) and 1340 mm span. The main element chord is 350 mm, the flap chord is 150 mm and the slat chord is 85 mm. Single slotted flaps are attached to the main element by steel brackets with a deflection angle of 35◦ . The slotted leading edge slat is divided into three parts along its span. These parts are attached by steel brackets to the main element and the relative position between adjacent parts is maintained by positioning pins. The central part of the slat was made of ABS through FFF to have an insulating substrate for receiving the DBD-PA since the original model is made of conductive material (aluminum alloy). In order to evaluate the sound emissions from the central portion of the slat only, far from the model endings, the extreme portions of the slat were equipped with PLA slat cove fillers (Fig. 10), made through FFF, which have the shear layer contour for zero angle of attack (obtained through CFD) and zig-zag boundary layer trip strip at their suction surface. These devices were intended to reduce contamination in the spectrum from noise sources outside the region of interest (Fig. 10). Although typical angle of attack of takeoff and landing are much higher than zero, the tests were executed at zero angle of attack to evaluate the DBD-PA ability for noise reduction in the worst-case scenario, since according to the results obtained by Pereira [83], zero is the angle in which more intense noise is produced by the wing model used.

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Fig. 8. Multielement wing installed at the LAE-1 wind tunnel test section.

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Fig. 9. Positions in which the DBD-PA will be tested on the slat.

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First, the straight electrode DBD-PA, which is the simpler one, was tested at the three positions. Since positions 2 and 3 (Fig. 9) were not effective for noise reduction, the remaining geometries of DBD-PA were installed just at position 1. This was made to reduce the test matrix (Table 1), thus optimizing the use of wind tunnel time. The model was set to zero angle of attack and the wind tunnel to an average speed of 27.4 m/s (testing at higher speeds would be preferable, but structural and operational constraints of the experimental setup limited the test speed). For each DBD-PA configuration, the acquisition of the 61-microphone array data was made at a sample rate of 102.4 kHz during an acquisition time of 20.0 seconds (which ensures an error of ±0.02 dB in the overall sound pressure level, according to measurements made by Pereira [83]). All the combinations of electrode geometry, DBD-PA position, voltage input, frequency of AC voltage output, frequency of output modulation and duty cycle of output modulation presented in Table 1 were tested using this procedure. The modulation frequency of 306 Hz was chosen in order to match the unitary Strouhal, considering the slat chord, the wind speed and the modulation frequency (which was proved to be the more efficient for some applications [60,63,86]). The modulation frequency of 1835 Hz was

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Table 1 Slat test matrix.

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Straight electrode

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7.57 × 10 0.08 0◦ 1, 2 and 3 30 V (DC) 3.0, 4.0, 5.0 and 6.0 kHz 306 and 1835 Hz 50%, 75% and 100% Acoustic array

7.57 × 10 0.08 0◦ 1 30 V (DC) 3.0, 4.0, 5.0 and 6.0 kHz 306 and 1835 Hz 50%, 75% and 100% Acoustic array

7.57 × 10 0.08 0◦ 1 30 V (DC) 3.0, 4.0, 5.0 and 6.0 kHz 306 and 1835 Hz 50%, 75% and 100% Acoustic array

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Fig. 10. DBD-PA (a) and cove fillers (b) installed at the slat. Location of the region of interest for the beamforming calculations (c).

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Fig. 11. Shear layer reattachment displacement caused by the DBD-PA activation.

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chosen to match a sub-harmonic of the slat tonal noise of the slat measured by Pereira [83].

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The overall sound pressure level difference between the DBDPA activated and deactivated conditions are presented here for all the configurations tested. For the cases in which the DBD-PAs were able to reduce the overall sound pressure level, the integrated power spectral density (PSD) and the difference in PSD between the activated and deactivated conditions are presented for the best configurations in each position. For the cases in which the DBD-PAs only worsened the overall noise, the spectra of the worst configurations are presented. The slat flow visualization is also presented. 3.1. Flow visualization

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First, the proof of concept through smoke flow visualization is presented in Fig. 11. As can be seen in Fig. 11, the DBD-PA installed at the slat cusp was able to shift the reattachment position when activated. This shows the potential of the DBD-PA to inter-

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3.2. Acoustic array measurements

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fere directly with the slat noise generation mechanism. This ability was quantitatively assessed with the acoustic array measurements executed in a larger model.

3. Results and discussion

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Since the installation of the DBD-PA and the endings of its wiring at the slat were not flush-mounted (due to dimensional restrictions imposed by the FFF process in the scale used for the slat), one baseline condition for each location and DBD-PA geometry was established by taking acoustic measurements with the actuators installed but deactivated. Comparisons for evaluating plasma actuation effects were made in relation to the respective baseline condition, and not relative to the clean baseline configuration. These results are shown in Fig. 12. Fig. 12 shows the increase in noise caused by the installation of the DBD-PAs in relation to the clean baseline configuration. All the peaks are amplified by the presence of the DBD-PA and its wiring (even if the most portion of the wiring being passed inside the cove fillers). A possible explanation for it is that the installation of the DBD-PA thickened the cusp and the trailing edge and exposed part of the blunt wires to

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Fig. 12. Noise spectra of the baseline configurations, DBD-PA on each position of the slat, but deactivated. α = 0◦ , Re = 7.57 × 105 , M = 0.08. (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)

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the airflow at the bracket region, increasing noise created by vortex shedding from separated regions.

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After processing the acoustic beamforming from the raw data as described in section 2.1, the difference in the overall sound pressure level (OASPL) between each configuration and the baseline configuration (DBD-PAs installed but deactivated) was calculated. To avoid the contribution of the high frequency DBD-PA actuation noise in this analysis, the sound spectra were truncated at the plasma driven frequency and the OASPL calculated with this truncation is indicated by the symbol OASPL∗ . This procedure is valid since the slat noise frequency ranges under actual flight speed and dimensional scale conditions are much smaller than those produced by a wind tunnel scaled model (maintaining constant the Strouhal number), while the high frequencies of the DBDPA actuation noise are not altered by speed and dimension scales of a real flight condition. These OASPL∗ values are presented in Fig. 13. Then, sound spectra (integrated PSD) and PSD difference between activated and deactivated conditions are shown for the best and the worst cases, in order to provide an understanding of how the DBD actuation affected the slat noise generation mechanism. The straight electrode at position 1 produced the best results, slightly reducing broadband noise and considerably reducing tonal noise. The best result was produced by the DBD-PA operating at f = 3.0 kHz, modulated by a square wave with 1835 Hz and duty cycle of 50%. It was able to reduce 3.35 dB in the OASPL and 12 dB in the main peak (Fig. 14). The straight electrode at position 2 was not effective, having caused an increase in the OASPL

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Fig. 13. Overall sound pressure level difference between the conditions with DBD-PAs activated and deactivated.

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Fig. 14. Noise spectra: straight electrode DBD-PA on slat at position 1, V in = 30 V (V pp = 9.4 kV).

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Fig. 15. Noise spectra: straight electrode DBD-PA on slat at position 2, V in = 30 V (V pp = 11.1 kV for 4.0 kHz and V pp = 19.6 kV for 5.0 kHz). M = 0.08.

for almost all the operational configurations. The worst result was produced by the DBD-PA operating at f = 4.0 kHz, modulated by a square wave with 306 Hz and duty cycle of 75%. It has intensified both the broadband and the tonal noise, adding up to 6 dB in the PSD (Fig. 15). The straight electrode at the position 3 produced marginal benefits, slightly reducing the OASPL. The best result was produced by the DBD-PA operating at f = 4.0 kHz, modulated by a square wave with 1835 Hz and duty cycle of 50%, and at f = 5.0 kHz, modulated by a square wave with 306 Hz and duty cycle of 75%. It was able to reduce up to 4 dB in the PSD, but for most frequencies the difference was near zero (Fig. 16). The square serrated electrode at position 1 also produced marginal benefits, slightly reducing the OASPL. The best result was produced by the DBD-PA operating at f = 4.0 kHz in steady mode (D .C . = 100%). It was able to reduce up to 4 dB in the PSD, but for most frequencies, the difference was near zero (Fig. 17). The double-serpentine serrated electrode at position 1 was able to reduce up to 1.40 dB in the OASPL. The best result was produced by the DBD-PA operating at f = 3.0 kHz, modulated by a square wave with 1835 Hz and duty cycle of 50%. The OASPL reduction was mainly due to tonal peak attenuation, which was reduced in about 7.5 dB (Fig. 18). From these results, it can be seen that the best DBD-PA configurations to reduce slat noise were that installed at position 1 (slat

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cusp). Initially, it was expected that the serrated configurations would produce the higher slat noise attenuation. However, the experiments have shown that the straight electrode was the most effective. This expectation was due to the three-dimensional flow structures expected from the serrated configurations, that should be able to break spanwise coherence of pressure fluctuations. Although the straight electrode has been the most effective, it should be noted that this configuration also had the most intense baseline noise, especially at the most intense peak. If the DBD-PA was perfectly flush-mounted (which was impossible to manufacture in this research, due to dimensional restrictions imposed by the FFF process in the scale used for the slat), the noise reduction of this configuration could be even higher. The most effective result obtained for the straight electrode may be related to constructive issues since all the DBD-PAs tested were handmade. Straight electrodes are easier to cut and align, so the plasma discharge produced by them was more uniform and efficient. In the case of the serrated electrodes, the manual alignment of the electrodes was not perfect and may have severely impacted the DBD-PA performance. It has also to be highlighted that the most efficacious configuration was operating with a duty cycle of 50%, thus consuming half the power of steady mode operation and producing less actuation noise.

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Fig. 16. Noise spectra: straight electrode DBD-PA on slat at position 3, V in = 30 V (V pp = 11.1 kV).

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Fig. 17. Noise spectra: square-serrated electrode DBD-PA on slat at position 1, V in = 30 V (V pp = 11.1 kV).

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Fig. 18. Noise spectra: double-serpentine serrated electrode DBD-PA on slat at position 1, V in = 30 V (V pp = 9.4 kV).

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4. Conclusions and suggestions for future works

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The DBD-PAs tested along this research were able to reduce the aerodynamic noise from slats, thus showing its potential to affect the noise generation mechanisms associated with this source. When applied to the slat cusp, the straight electrode DBD-PA was able to reduce up to (3.3 ± 0.02) dB in the overall sound pressure level when operating at 3.0 kHz, modulated by a square wave with 1835 Hz and duty cycle of 50%. Applying the DBD-PA at the reattachment region showed no effectiveness in reducing slat noise. Other geometries of electrode applied to the slat cusp were somehow effective, but not as much as the straight electrode. This can be related to loss of efficiency in complex geometry DBD-PAs manually manufactured with copper foil and polyimide self-adhesive tapes. The control authority of the DBD-PAs tested is still poor for real flight conditions, but it may be improved by proper optimization of DBD-PA geometry, materials, operational parameters, and electric circuit. However, in the authors’ opinion, the technology of DBD-PAs is still far from being a feasible technical solution for aeronautical purposes. Besides its lack of control authority at real flight Reynolds, the materials used currently do not withstand plasma environments or very high voltages, and DBD-PA performance is too sensitive to manufacturing imperfections. The safety of DBD-PA operation is also a critical issue when considering it as an aeronautical embedded system. Production of very high voltages near fuel tanks can become a dangerous combination if unintended electric arcing occurs. Other obstacles to the DBD-PA application is the production of tropospheric ozone, which is a pollutant, and acoustic and electromagnetic actuation noise emissions. As suggestions for future works, the authors advise the interested researchers that additional flow measurements should be performed both to characterize the flow mechanisms involved in the acoustic phenomena reported hereby and to assess the impact on the aerodynamic performance, as well as ozone and electromagnetic emissions measurements. The authors also recommend that further research on material science and manufacturing methods should be carried out to produce more efficient plasma actuators. Declaration of competing interest The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

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Acknowledgements

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This work was supported by the National Council for Scientific and Technological Development (CNPq) [grant number 148947/2017-4]; the Foundation for the Enhancement of Research and Industrial Improvement (FIPAI) [grant number FB-016/17]; the Funding Authority for Studies and Projects (FINEP); and Embraer. The authors are grateful to the technicians José C. P. de Azevedo, Mário Sbampato and Osnan I. Faria, for their help in setting up and maintaining the experimental apparatus. The authors also acknowledge the engineers Daniel A. Giraldo, Laura B. Bolivar, and Lourenço T. L. Pereira, for their insights and their help in setting up and operating the experimental apparatus, and Dr. Carlos R. I. da Silva and Dr. Paulo C. Greco Júnior, for their contributions during the evaluation of the work presented hereby.

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References

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[1] M.J.T. Smith, Aircraft Noise, Cambridge University Press, 2009.

[2] International Civil Aviation Organization, On board a sustainable future: ICAO environmental report 2016, Technical Report, International Civial Aviation Organization, 2016. [3] P. Busquin, P. Argüelles, M. Bischoff, B.A.C. Droste, S.R.H. Evans, W. Kröll, J.-L. Lagardère, A. Lina, J. Lumsden, D. Ranque, S. Rasmussen, P. Reutlinger, S.R. Robins, H. Terho, A. Wittlöw, European Aeronautics: A Vision for 2020, Report, ACARE, 2001. [4] O. Zaporozhets, K. Attenborough, V. Tokarev, Aircraft Noise: Assessment, Prediction and Control, CRC Press, 2011. [5] T.L. Turner, R.T. Kidd, D.J. Hartl, W.D. Scholten, Development of a SMA-based, slat-cove filler for reduction of aeroacoustic noise associated with transportclass aircraft wings, in: Mechanics and Behavior of Active Materials; Structural Health Monitoring; Bioinspired Smart Materials and Systems, Energy Harvesting, vol. 2, ASME, 2013. [6] Y.P. Guo, M.C. Joshi, Noise characteristics of aircraft high lift systems, AIAA J. 41 (2003) 1247–1256. [7] M. Smith, S. Chow, Aerodynamic noise sources on aircraft high lift slats and flaps, in: 9th AIAA/CEAS Aeroacoustics Conference and Exhibit, American Institute of Aeronautics and Astronautics, 2003. [8] V.F. Kopiev, M.Y. Zaytsev, I.V. Belyaev, Investigation of airframe noise for a large-scale wing model with high-lift devices, Acoust. Phys. 62 (2016) 97–107. [9] W. Dobrzynski, K. Nagakura, B. Gehlhar, A. Buschbaum, Airframe noise studies on wings with deployed high-lift devices, in: 4th AIAA/CEAS Aeroacoustics Conference, American Institute of Aeronautics and Astronautics, 1998. [10] M.R. Khorrami, Understanding slat noise sources, in: Colloquium EUROMECH, vol. 449, Citeseer, 2003, pp. 9–12. [11] M. Choudhari, M. Khorrami, Slat cove unsteadiness: effect of 3d flow structures, in: 44th AIAA Aerospace Sciences Meeting and Exhibit, American Institute of Aeronautics and Astronautics, 2006. [12] P. Chen, Identification and Attenuation of Slat Noise, Doctoral thesis, University of Southampton, 2012. [13] T. Imamura, H. Ura, Y. Yokokawa, K. Yamamoto, A far-field noise and near-field unsteadiness of a simplified high-lift-configuration model (slat), in: 47th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, American Institute of Aeronautics and Astronautics, 2009. [14] W. Dobrzynski, Almost 40 years of airframe noise research: what did we achieve? J. Aircr. 47 (2010) 353–367. [15] L. Jenkins, M. Khorrami, M. Choudhari, Characterization of unsteady flow structures near leading-edge slat: Part I: PIV measurements, in: 10th AIAA/CEAS Aeroacoustics Conference, American Institute of Aeronautics and Astronautics, 2004. [16] Z. Ma, X. Zhang, Numerical investigation of broadband slat noise attenuation with acoustic liner treatment, AIAA J. 47 (2009) 2812–2820. [17] W. Dobrzynski, M. Pott-Pollenske, Slat noise source studies for farfield noise prediction, in: 7th AIAA/CEAS Aeroacoustics Conference and Exhibit, American Institute of Aeronautics and Astronautics, 2001. [18] L.T.L. Pereira, L.F. Rego, F.M. Catalano, D.C. Reis, E.L.C. Coelho, Experimental slat noise assessment through phased array and hot-film anemometry measurements, in: 2018 AIAA Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics, 2018. [19] M. Khorrami, D. Lockard, Effects of geometric details on slat noise generation and propagation, in: 12th AIAA/CEAS Aeroacoustics Conference (27th AIAA Aeroacoustics Conference), American Institute of Aeronautics and Astronautics, 2006. [20] M.M. Choudhari, M.R. Khorrami, Effect of three-dimensional shear-layer structures on slat cove unsteadiness, AIAA J. 45 (2007) 2174–2186. [21] H. Huang, W. Li peng, F. xin Wang, Slat noise suppression using upstream mass injection, J. Shanghai Jiaotong Univ. (Science) 18 (2013) 620–629. [22] M. Roger, S. Perennes, Low-frequency noise sources in two-dimensional highlift devices, in: 6th Aeroacoustics Conference and Exhibit, American Institute of Aeronautics and Astronautics, 2000. [23] K. Takeda, X. Zhang, P. Nelson, Unsteady aerodynamics and aeroacoustics of a high-lift device configuration, in: 40th AIAA Aerospace Sciences Meeting & Exhibit, American Institute of Aeronautics and Astronautics, 2002. [24] M. Terracol, E. Manoha, B. Lemoine, Investigation of the unsteady flow and noise generation in a slat cove, AIAA J. 54 (2016) 469–489. [25] D. Lockard, M. Choudhari, Noise radiation from a leading-edge slat, in: 15th AIAA/CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference), American Institute of Aeronautics and Astronautics, 2009. [26] K. Paschal, L. Jenkins, C. Yao, Unsteady slat wake characteristics of a 2-d highlift configuration, in: 38th Aerospace Sciences Meeting and Exhibit, American Institute of Aeronautics and Astronautics, 2000. [27] K. Takeda, G. Ashcroft, X. Zhang, P. Nelson, Unsteady aerodynamics of slat cove flow in a high-lift device configuration, in: 39th Aerospace Sciences Meeting and Exhibit, American Institute of Aeronautics and Astronautics, 2001. [28] X. Huang, S. Chan, X. Zhang, S. Gabriel, Variable structure model for flowinduced tonal noise control with plasma actuators, AIAA J. 46 (2008) 241–250. [29] R. Drobietz, I. Borchers, Generic wind tunnel study on side edge noise, in: 12th AIAA/CEAS Aeroacoustics Conference (27th AIAA Aeroacoustics Conference), American Institute of Aeronautics and Astronautics, 2006.

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[30] T. Imamura, H. Ura, Y. Yokokawa, S. Enomoto, K. Yamamoto, T. Hirai, Designing of slat cove filler as a noise reduction device for leading-edge slat, in: 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference), American Institute of Aeronautics and Astronautics, 2007. [31] M.R. Khorrami, E. Fares, B. Duda, A. Hazir, Computational evaluation of airframe noise reduction concepts at full scale, in: 22nd AIAA/CEAS Aeroacoustics Conference, American Institute of Aeronautics and Astronautics, 2016. [32] L. Yong, W. Xunnian, Z. Dejiu, Control strategies for aircraft airframe noise reduction, Chin. J. Aeronaut. 26 (2013) 249–260. [33] E. Moreau, Airflow control by non-thermal plasma actuators, J. Phys. D, Appl. Phys. 40 (2007) 605–636. [34] J.-J. Wang, K.-S. Choi, L.-H. Feng, T.N. Jukes, R.D. Whalley, Recent developments in DBD plasma flow control, Prog. Aerosp. Sci. 62 (2013) 52–78. [35] T.C. Corke, M.L. Post, D.M. Orlov, SDBD plasma enhanced aerodynamics: concepts, optimization and applications, Prog. Aerosp. Sci. 43 (2007) 193–217. [36] C.L. Enloe, T. McLaughlin, G.I. Font, J.W. Baughn, Parameterization of temporal structure in the single-dielectric-barrier aerodynamic plasma actuator, AIAA J. 44 (2006) 1127–1136. [37] J. Little, M. Nishihara, I. Adamovich, M. Samimy, High-lift airfoil trailing edge separation control using a single dielectric barrier discharge plasma actuator, Exp. Fluids 48 (2009) 521–537. [38] X. Zhang, H.-X. Li, Y. Huang, W.-B. Wang, Wing flow separation control using asymmetrical and symmetrical plasma actuator, J. Aircr. 54 (2017) 301–309. [39] C.L. Enloe, M.G. McHarg, T.E. McLaughlin, Time-correlated force production measurements of the dielectric barrier discharge plasma aerodynamic actuator, J. Appl. Phys. 103 (2008) 073302. [40] J. Roth, D. Sherman, S. Wilkinson, Boundary layer flow control with a one atmosphere uniform glow discharge surface plasma, in: 36th AIAA Aerospace Sciences Meeting and Exhibit, American Institute of Aeronautics and Astronautics, 1998. [41] J.R. Roth, D.M. Sherman, S.P. Wilkinson, Electrohydrodynamic flow control with a glow-discharge surface plasma, AIAA J. 38 (2000) 1166–1172. [42] C.L. Enloe, T.E. McLaughlin, R.D.V. Dyken, K.D. Kachner, E.J. Jumper, T.C. Corke, M. Post, O. Haddad, Mechanisms and responses of a dielectric barrier plasma actuator: geometric effects, AIAA J. 42 (2004) 595–604. [43] J. Pons, E. Moreau, G. Touchard, Asymmetric surface dielectric barrier discharge in air at atmospheric pressure: electrical properties and induced airflow characteristics, J. Phys. D, Appl. Phys. 38 (2005) 3635–3642. [44] J.R. Roth, X. Dai, Optimization of the aerodynamic plasma actuator as an electrohydrodynamic (EHD) electrical device, in: 44th AIAA Aerospace Sciences Meeting and Exhibit, American Institute of Aeronautics and Astronautics, 2006. [45] T.C. Corke, M.L. Post, D.M. Orlov, Single-dielectric barrier discharge plasma enhanced aerodynamics: concepts, optimization, and applications, J. Propuls. Power 24 (2008) 935–945. [46] M. Forte, J. Jolibois, J. Pons, E. Moreau, G. Touchard, M. Cazalens, Optimization of a dielectric barrier discharge actuator by stationary and non-stationary measurements of the induced flow velocity: application to airflow control, Exp. Fluids 43 (2007) 917–928. [47] M.P. Patel, T.T. Ng, S. Vasudevan, T.C. Corke, C. He, Plasma actuators for hingeless aerodynamic control of an unmanned air vehicle, J. Aircr. 44 (2007) 1264–1274. [48] T. Abe, Y. Takizawa, S. Sato, N. Kimura, Experimental study for momentum transfer in a dielectric barrier discharge plasma actuator, AIAA J. 46 (2008) 2248–2256. [49] F.O. Thomas, T.C. Corke, M. Iqbal, A. Kozlov, D. Schatzman, Optimization of dielectric barrier discharge plasma actuators for active aerodynamic flow control, AIAA J. 47 (2009) 2169–2178. [50] R.H.M. Giepman, M. Kotsonis, On the mechanical efficiency of dielectric barrier discharge plasma actuators, Appl. Phys. Lett. 98 (2011) 221504. [51] C.A. Borghi, A. Cristofolini, G. Neretti, P. Seri, A. Rossetti, A. Talamelli, Duty cycle and directional jet effects of a plasma actuator on the flow control around a NACA0015 airfoil, Meccanica 52 (2017) 3661–3674. [52] M. Abdollahzadeh, J. Pascoa, P. Oliveira, Comparison of DBD plasma actuators flow control authority in different modes of actuation, Aerosp. Sci. Technol. 78 (2018) 183–196. [53] H. Aono, S. Yamakawa, K. Iwamura, S. Honami, H. Ishikawa, Straight and curved type micro dielectric barrier discharge plasma actuators for active flow control, Exp. Therm. Fluid Sci. 88 (2017) 16–23. [54] N. Asaumi, S. Matsuno, T. Matsuno, M. Sugahara, H. Kawazoe, Multi-electrode plasma actuator to improve performance of flow separation control, Int. J. Gas Turbine Propuls. Power Syst. 9 (2017). [55] H. Nishida, K. Nakai, T. Matsuno, Physical mechanism of tri-electrode plasma actuator with direct-current high voltage, AIAA J. 55 (2017) 1852–1861. [56] C.-C. Wang, S. Roy, Geometry effects of dielectric barrier discharge on a flat surface, in: 49th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, American Institute of Aeronautics and Astronautics, 2011. [57] J.A.G. Croce, Aumento da eficiência de um dispositivo eletro-hidrodinâmico através da alteração das características geométricas do eletrodo ativo, Ph.D. thesis, Escola de Engenharia de São Carlos, 2014.

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[58] M. Belan, F. Messanelli, Compared ionic wind measurements on multi-tip corona and DBD plasma actuators, J. Electrost. 76 (2015) 278–287. [59] G. Gao, K. Peng, L. Dong, W. Wei, G. Wu, Parametric study on the characteristics of a SDBD actuator with a serrated electrode, Plasma Sci. Technol. 19 (2017) 064010. [60] T. Corke, B. Mertz, M. Patel, Plasma flow control optimized airfoil, in: 44th AIAA Aerospace Sciences Meeting and Exhibit, American Institute of Aeronautics and Astronautics, 2006. [61] M.P. Patel, Z.H. Sowle, T.C. Corke, C. He, Autonomous sensing and control of wing stall using a smart plasma slat, J. Aircr. 44 (2007) 516–527. [62] C. Porter, T. McLaughlin, C. Enloe, G. Font, J. Roney, J. Baughn, Boundary layer control using a DBD plasma actuator, in: 45th AIAA Aerospace Sciences Meeting and Exhibit, American Institute of Aeronautics and Astronautics, 2007. [63] N. Benard, J. Jolibois, E. Moreau, Lift and drag performances of an axisymmetric airfoil controlled by plasma actuator, J. Electrost. 67 (2009) 133–139. [64] S. Grundmann, C. Tropea, Experimental damping of boundary-layer oscillations using DBD plasma actuators, Int. J. Heat Fluid Flow 30 (2009) 394–402. [65] D. Caruana, Plasmas for aerodynamic control, Plasma Phys. Control. Fusion 52 (2010) 124045. [66] A.J. Lombardi, P.O. Bowles, T.C. Corke, Closed-loop dynamic stall control using a plasma actuator, AIAA J. 51 (2013) 1130–1141. [67] L.-H. Feng, T.-Y. Shi, Y.-G. Liu, Lift enhancement of an airfoil and an unmanned aerial vehicle by plasma Gurney flaps, AIAA J. 55 (2017) 1622–1632. [68] P. Joseph, a. Leroy, S. Baleriola, D. Nelson-Gruel, Y. Haidous, S. Loyer, P. Devinant, S. Aubrun, Dynamics of circulation control using surface plasma actuators for aerodynamic load alleviation on wind turbine airfoils, S19-Mécanique pour l’énergie éolienne, 2017. [69] G.P.G. Silva, J.P. Eguea, F.M. Catalano, Aerodynamic improvement of a swept wing using a dielectric barrier discharge plasma actuator at the wing tip, in: 3◦ Simpósio do Programa de Pós-Graduação em Engenharia Mecânica, Universidade de São Paulo, 2018. [70] A. Ebrahimi, M. Hajipour, Flow separation control over an airfoil using dual excitation of DBD plasma actuators, Aerosp. Sci. Technol. 79 (2018) 658–668. [71] X. Meng, H. Hu, X. Yan, F. Liu, S. Luo, Lift improvements using duty-cycled plasma actuation at low Reynolds numbers, Aerosp. Sci. Technol. 72 (2018) 123–133. [72] A. Ebrahimi, M. Hajipour, K. Ghamkhar, Experimental study of stall control over an airfoil with dual excitation of separated shear layers, Aerosp. Sci. Technol. 82–83 (2018) 402–411. [73] K. Ramakumar, Flow control and lift enhancement using plasma actuators, in: 35th AIAA Fluid Dynamics Conference and Exhibit, American Institute of Aeronautics and Astronautics, 2005. [74] J. Laten, Dielectric barrier discharge plasma actuators for aircraft maneuvering control, in: 17th AIAA Aviation Technology, Integration, and Operations Conference, American Institute of Aeronautics and Astronautics, 2017. [75] C. He, T.C. Corke, M.P. Patel, Plasma flaps and slats: an application of weakly ionized plasma actuators, J. Aircr. 46 (2009) 864–873. [76] S. Chappell, Z. Cai, X. Zhang, D. Angland, Slat noise feedback control with a dielectric barrier discharge plasma actuator, in: 6th AIAA Flow Control Conference, American Institute of Aeronautics and Astronautics, 2012. [77] P. Chen, X. Zhang, S. Chappell, Z. Cai, D. Angland, Attenuation of noise from an airfoil equipped with a high-lift device using plasma actuators, in: ERCOFTAC Bulletin, 2013. [78] F.M. Catalano, The new closed circuit wind tunnel of the aircraft laboratory of University of São Paulo, in: 16th Brazilian Congress of Mechanical Engineering, vol. 6, 2001, pp. 306–312. [79] F.M. Catalano, The new closed circuit wind tunnel of the Aircraft Laboratory of University of Sao Paulo, Brazil, in: 24th International Congress of the Aeronautical Sciences ICAS, 2004. [80] L.D. Santana, M. Carmo, F.M. Catalano, M.A.F. Medeiros, The update of an aerodynamic wind-tunnel for aeroacoustics testing, J. Aerosp. Technol. Manag. 6 (2014) 111–118. [81] W.D. Fonseca, J.P. Ristow, D.G. Sanches, S.N.Y. Gerges, A different approach to Archimedean spiral equation in the development of a high frequency array, in: SAE Technical Paper Series, SAE International, 2010. [82] C. do Carmo PAGANI JÚNIOR, Aeroacoustic source mapping of a slat in a closedsection wind tunnel using beamforming with DAMAS deconvolution, Ph.D. thesis, São Carlos School of Engineering, University of São Paulo, São Carlos, 2014. [83] L.T. Pereira, Experimental slat noise assessment by means of phased array and hot-film anemometry measurements, Master’s thesis, São Carlos School of Engineering, University of São Paulo, São Carlos, 2018. [84] G.P.G. Silva, Dielectric barrier discharge plasma actuators applied to high-lift devices for aerodynamic noise reduction, Master’s thesis, Escola de Engenharia de São Carlos, 2019. [85] J.H. Preston, N.E. Sweeting, An improved smoke generator for use in the visualisation of airflow, particulary boundary layer flow at high Reynolds numbers, ARC/R&M-2023, 1943. [86] F.O. Thomas, A. Kozlov, T.C. Corke, Plasma actuators for cylinder flow control and noise reduction, AIAA J. 46 (2008) 1921–1931.

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