Régine Weber

Plasma morphology and induced airflow characterization of a DBD actuator with serrated electrode

Plasma morphology and airflow induced by a dielectric barrier discharge (DBD) actuator, whose exposed electrode geometry is designed with a serrated configuration, are investigated in quiescent air and compared with a DBD actuator consisting of electrodes designed with a standard linear strip configuration. ICCD imaging, electrical measurements and three-component laser Doppler velocimetry were carried out to compare various features of these two actuators. With the serrated configuration, ICCD images of the discharge show that streamers are bent, whereas with the linear configuration they are straight. These curved streamers induce a three-dimensional flow topology, which is confirmed by friction line visualization and velocity measurements. Whereas a two-dimensional wall-jet is induced with the linear configuration, a transverse velocity component is measured with the serrated configuration, implying the creation of spanwise-periodic vorticity. Phase-averaged velocity measurements allow the temporal variation of this transverse velocity to be highlighted. On both sides of a tooth, it has qualitatively the same variation as the longitudinal velocity with respect to the negative or positive half-cycles of the high voltage signal. Moreover, with the same electrical operating parameters, the measured longitudinal velocity was higher, particularly at the tips.

Negative Spark Leaders on a Surface DBD Plasma Actuator

Plasma actuators are currently under investigations in order to improve the aerodynamic performances of vehicles. Such devices induce airflow of few kilometers per hour called ionic wind. A previous study showed that ionic wind velocity is limited by a particular regime of the discharge where bright and long sparks appear. In this paper, we present some typical images of these spark leaders.

Experimental Study of the Flow Induced by a Sinusoidal Dielectric Barrier Discharge Actuator and Its Effects on a Flat Plate Natural Boundary Layer

Since the mid-1990s, electrohydrodynamic actuators have been developed for modifying on subsonic airflows. The principle of plasma action is the use of the direct conversion of electrical energy into kinetic energy in order to act on the flow boundary layer. This paper presents our contribution to such an investigation concerning an electrohydrodynamic actuator consisting of several sinusoidal dielectric barrier discharges. First, the ionic wind induced by this actuator was measured with a pressure sensing probe. The induced flow velocity increased with the applied voltage and frequency. The particle image velocimetry system without external airflow showed the presence of induced swirls, generated by the ion movement in plasma. Then the action of this actuator on a flat plate boundary layer in parallel flow at zero incidence was studied in a subsonic wind tunnel. Experiments were performed for 15 m/s and 22 m/s. They showed that electric discharges (±8 kV, 1 kHz) acting on a laminar flow tripped the laminar-to-turbulent transition. Moreover, higher applied voltages (up to ±12 kV, 1 kHz) were necessary for modifying turbulent boundary layers.

Drag reduction of a 3D bluff body using plasma actuators

The aim of this study is to reduce the drag of a simplified car geometry using surface dielectric barrier discharge actuators. Experiments were conducted in a wind tunnel for a low Reynolds number (6.7.105) with the Ahmed body reference (rear slant angle of 25°, zero yaw angle). The effect of steady and unsteady actuation on the flow topology was investigated carrying out 2C-PIV and 1D hot wire measurements. The efficiency of the actuators was characterised by stationary balance measurements. Drag reductions up to 8% were obtained by suppressing the separation bubble above the rear window. The results suggest that plasma actuators are simple to implement on a model and can provide useful information for automotive aerodynamics through parametric studies with parameters relevant for flow control (position, surface, frequency and duty cycle of the pulsed actuation).

Transition control using a single plasma actuator

The aim of this study is to modify, by using surface plasma actuators, the laminar-to-turbulent transition location of a Blasius boundary layer developing on a flat plate mounted in an opened wind tunnel. Measurements of flow velocities were performed by hot wire anemometry. Results show that an actuator placed upstream the natural transition zone enables the promotion or the delay of the transition onset, depending on the location, the voltage amplitude, and the frequency of the high voltage electrical parameters.

Le problème de Helmoltz pour des obstacles peu réguliers

The classical Helmholtz model for wakes behind an obstacle in perfect fluid mechanics boils down in complex analysis into a Riemann problem with free boundary, the free boundary being the wake and being such that the Riemann map parametrizes it as arclength parametrization. We generalize a method due to Coifman and Meyer [CM] to solve this problem for a class of non-smooth obstacles.

Study of a DBD plasma actuator dedicated to airflow separation control

Since about ten years, dielectric barrier discharge (DBD) was studied as electro-hydrodynamic (EHD) actuator for airflow control. A DBD surface discharge generates nonthermal plasma allowing to modify the boundary layer of airflow. The active control enables fast action on airflow. A thin flexible asymmetric DBD actuator was used in our study, each elementary DBD was made with two copper electrodes of 35 mum in thickness and 6 mm in width. Dielectric was a multilayer configuration using Kaptonreg and Mylarreg. Two lengths of electrode were used in applications mentioned below: 150 mm and 900 mm. The DBD actuator was characterized by means of electric and optical measurements: discharge currents, voltages and dissipated power of DBD actuator; spectroscopic measurements were also performed. All these measurements were done for several frequencies of power supply. For flow separation controls experiments, firstly, we performed tests on a 1 m length flat plate with an elliptic leading edge placed in an open wind tunnel. This wind tunnel has a test section of a 2 m times 0.5 m times 0.5 m (LtimesHtimesW). Several DBD actuators with 150 mm length electrodes were placed on the upper surface of the flat plate. The action of DBD actuator enables to obtain a more stable laminar boundary layer and to delay the laminar-turbulent transition. Secondly, a 1 m chord and 1.10 m span wing-like airfoil (BMVR130) was used to perform measurements. This airfoil was placed in a wind tunnel whose test section has dimensions of 5 m times 2 m times 2 m. DBD actuators with electrode length of 900 mm were installed on the extrados of profile every 30 mm from x/c = 0.02 to x/c = 0.8. However, only a few elementary DBDs (up to 4) operated simultaneously. The experiments were carried out for velocities up to 15 m/s (Re = 106) and for angles of attack ranging from 8deg to 16deg. Flow visualizations were performed with a PIV system, the drag and lift coefficients were deduced by aerodynamic balance measurements. At 10 m/s (Re = 670,000), the flow was fully reattached for the angles of attack from 8deg to 12deg. A lift increase of about 5% could be observed.