N. Benard

Optical visualization and electrical characterization of fast-rising pulsed dielectric barrier discharge for airflow control applications

Flow control consists of manipulating flows in an effective and robust manner to improve the global performances of transport systems or industrial processes. Plasma technologies, and particularly surface dielectric barrier discharge (DBD), can be a good candidate for such purpose. The present experimental study focuses on optical and electrical characterization of plasma sheet formed by applying a pulse of voltage with rising and falling periods of 50 ns for a typical surface DBD geometry. Positive and negative polarities are compared in terms of current behavior, deposited energy, fast-imaging of the plasma propagation, and resulting modifications of the surrounding medium by using shadowgraphy acquisitions. Positive and negative pulses of voltage produce streamers and corona type plasma, respectively. Both of them result in the production of a localized pressure wave propagating in the air with a speed maintained at 343 m/s (measurements at room temperature of 20 °C). This suggests that the produced pr...

Time-dependent volume force produced by a non-thermal plasma actuator from experimental velocity field

The electrohydrodynamic volume force produced by the Coulomb force acting on charged species in weakly ionized gas can improve the aerodynamic performances of academic and industrial turbulent flows. In this paper, a single dielectric barrier discharge is investigated with a focus on the experimental characterization of the time-resolved topology of the produced electrohydrodynamic volume force. The distribution of force over the volume of gas is calculated from velocity measurements by resolving simplified Navier–Stokes equations. Comparisons between the present calculated body force and data from the open literature confirm the accuracy of the method used. This study reveals that the unsteady force shows large fluctuations with an alternation of positive and negative longitudinal forces. The glow and streamer discharge regimes contribute differently to the electrohydrodynamic volume force. Both regimes promote a positive volume force longitudinal to the flow and a negative volume force in the transverse direction. However, the momentum transfer is significantly larger during the glow regime. A negative volume force (70% of the positive force amplitude) is observed following the glow phase, when there is no discharge. This negative volume force results from the local flow deceleration due to viscous influence at the wall and turbulent diffusion in the flow.

Capabilities of the dielectric barrier discharge plasma actuator for multi-frequency excitations

Natural instability mechanisms are inherent in most laminar and turbulent flow configurations. Usually, these instabilities result in the formation of flow structures occurring at diverse spatial and time scales. An effective control requires an actuator able to bring momentum transfer over a wide range of frequencies to act on these instabilities. Promising results are expected for such control strategy because, according to stability theory, a small amplitude perturbation can be large enough to produce significant effects even at high Reynolds number. Moreover, simultaneous production of small perturbations at several frequencies can enhance or cancel non-linear interactions; this opens alternative methods for flow control. The focus of this study is to demonstrate the ability of plasma actuators to introduce flow perturbations at single and dual frequencies by simply adjusting the waveform of the voltage applied to the plasma actuator. The flows produced by a dielectric barrier discharge supplied by burst, superposition and ring modulations are described in temporal and frequential domains. The results confirm the potential of non-thermal plasma actuators to produce highly unsteady flows at single, double or multiple frequencies.

Electric wind produced by a surface dielectric barrier discharge operating in air at different pressures: aeronautical control insights

The effects of the ambient air pressure level on the electric wind produced by a single dielectric barrier discharge (DBD) have been investigated by Pitot velocity measurements. Pressures from 1 down to 0.2 atm were tested with a 32 kVp–p 1 kHz excitation. This preliminary study confirms the effectiveness of surface DBD at low pressure. Indeed, the induced velocity is strongly dependent on the ambient air pressure level. Quite surprisingly the produced airflow presents a local maximum at 0.6 atm. The measured velocities at 1 atm and 0.2 atm are 2.5 m s −1 and 3 m s −1 , respectively while 3.5 m s −1 is reached at 0.6 atm. The position of the maximal velocity always coincides with the plasma extension. Mass flow rate calculations indicate that the DBD is effective in real flight pressure conditions. (Some figures in this article are in colour only in the electronic version)

A large-scale multiple dielectric barrier discharge actuator based on an innovative three-electrode design

For about 10 years, surface dielectric barrier discharges (DBDs) have been widely used as plasma actuators in subsonic airflow control applications. However, the extension length of a single surface DBD is limited to about 2 cm, which could restrict its use to small-scale applications. One way to extend the plasma actuation surface consists of using several single surface DBDs in series, energized by zero phase delayed or phase shifted high voltages. However, the mutual interaction between successive discharges affects the benefits of such standard multi-DBD actuators. This paper deals with a new design electrode for large-scale flow control applications. It consists of replacing each single two-electrode DBD by a three-electrode DBD where the third electrode acts as a shield between two successive DBDs. Experimental measurements by laser doppler velocimetry, pressure probe and time-resolved particle image velocimetry show that the mutual interactions can be strongly reduced, resulting in a constant electric wind velocity above the multi-DBD actuator.

Flow Control by Dielectric Barrier Discharge Actuators: Jet Mixing Enhancement

This study is focused on a new approach to control the mixing at the exhaust of a jet nozzle. Two dielectric barrier discharges are used to enhance the turbulent quantities in the jet wake. An axisymmetric air jet is equipped with a 22-degree-angle diffuser that houses two dielectric barrier discharge actuators, symmetrically placed along the lips of the diffuser. Streamwise, cross-stream planes, and turbulent spectra for free air jet velocities from 10 to 40 m/s are investigated by stereoscopic particle image velocimetry and laser Doppler velocimetry up to 8 jet diameters. The airflow is separated naturally along the diffuser wall and the results demonstrate that a full (at 10 m/s) or partial (at 20 m/s) flow reattachment can be performed, this process being associated with the shedding of coherent structures. The turbulent spectra suggest that the frequency of the vortex shedding is dictated by the electrical frequency applied to the actuators and that the flow structures can be manipulated up to 40 m/s. The time-averaged data confirm that a significant enhancement of the jet spreading, a jet core length reduction, and a turbulent kinetic energy increase are performed up to 30 m/s for a quasi-steady actuation. Unsteady actuations are also investigated and the results highlight that a Strouhal number from 0.25 to 0.32 is particularly effective to enhance the turbulent kinetic energy in the overall flowfield.

Unsteady aspect of the electrohydrodynamic force produced by surface dielectric barrier discharge actuators

The time-resolved electrohydrodynamic force produced by single dielectric barrier discharge (DBD) actuators used for airflow control is computed from electric wind velocity measurements. Two actuator designs are investigated: a plate-to-plate and a wire-to-plate surface DBD because each of them produces a different discharge current. Results show that: (1) the high voltage active electrode shape plays a key role in the plasma physics, (2) the body force is highly unsteady with fluctuations up to about ten times its time-averaged value, and (3) the typical plate-to-plate DBD produces a positive force during the positive half-cycle and a negative force during the negative half-cycle when both cycles result in a positive force with the wire-to-plate DBD.

Electrohydrodynamic force produced by a wire-to-cylinder dc corona discharge in air at atmospheric pressure

Wire-to-cylinder corona discharges are studied to better understand the electrohydrodynamic (EHD) phenomena that govern the performances of electric propulsion systems. First, theory associated with EHD thrusters is presented in order to be compared with experimental results. Secondly, direct thrust measurements are carried out to optimize the electrical and geometrical parameters of such devices. The main results are as follows: (1) the discharge current I is proportional to the square root of the grounded electrode diameter and to 1/d2 where d is the electrode gap; (2) for d???20?mm, the mobility of negative ions is higher than that of positive ions while the mobility of both ions is equal for higher gaps; (3) therefore, for gap ?30?mm, positive and negative coronas results in the same current-to-thrust conversion; (4) the current-to-thrust conversion is equal to 33?N?A?1 per centimetre of gap, and it is proportional to the gap; (5) the thruster effectiveness ? increases with , decreases with the square root of thrust and reaches about 15?N?kW?1 for d?=?40?mm; (6) the force computed from experimental velocity profiles is overestimated compared with the values measured with a balance, showing that this method cannot be used for thrust determination.

Computational Approach to Estimating the Effects of Blood Properties on Changes in Intra-stent Flow

In this study various blood rheological assumptions are numerically investigated for the hemodynamic properties of intra-stent flow. Non-newtonian blood properties have never been implemented in blood coronary stented flow investigation, although its effects appear essential for a correct estimation and distribution of wall shear stress (WSS) exerted by the fluid on the internal vessel surface. Our numerical model is based on a full 3D stent mesh. Rigid wall and stationary inflow conditions are applied. Newtonian behavior, non-newtonian model based on Carreau-Yasuda relation and a characteristic newtonian value defined with flow representative parameters are introduced in this research. Non-newtonian flow generates an alteration of near wall viscosity norms compared to newtonian. Maximal WSS values are located in the center part of stent pattern structure and minimal values are focused on the proximal stent wire surface. A flow rate increase emphasizes fluid perturbations, and generates a WSS rise except for interstrut area. Nevertheless, a local quantitative analysis discloses an underestimation of WSS for modelisation using a newtonian blood flow, with clinical consequence of overestimate restenosis risk area. Characteristic viscosity introduction appears to present a useful option compared to rheological modelisation based on experimental data, with computer time gain and relevant results for quantitative and qualitative WSS determination.

Nanosecond pulsed sliding dielectric barrier discharge plasma actuator for airflow control: Electrical, optical, and mechanical characteristics

Plasma actuators used for active flow control are widely studied because they could replace mechanical actuators. Industrial applications of these plasma actuators sometimes require a large surface plasma sheet in view of increasing the interaction region between the discharge and the incoming flow. Instead of using a typical two-electrode nanosecond pulsed dielectric barrier discharge for which the interaction region is limited to about 20 mm, this study proposes to characterize a nanosecond sliding discharge based on a three-electrode geometry in order to increase the extension length up to the electrode gap. This sliding discharge is compared to the typical nanosecond dielectric barrier discharge by means of electrical, optical, and mechanical diagnostics. Electrical characterization reveals that the deposited energy can be widely increased. Time-resolved Intensified Charge Coupled Device (iCCD) images of the discharge development over the dielectric surface highlight that the intensity and the propaga...