Pierre Audier

Plasma propagation of a 13.56?MHz asymmetric surface barrier discharge in atmospheric pressure air

The propagation of an rf asymmetric surface barrier discharge in atmospheric pressure air has been investigated. Measurements of the pulse-modulated 13.56 MHz voltage and current together with ICCD images of the plasma were recorded to study the visible plasma structure with respect to the rf pulses, time within the pulses and the rf waveforms. When exposing images over full rf pulses, which comprise over 150 oscillations of the applied voltage, clearly defined filamentary structures are observed indicating a strong memory effect. The discharge intensity decreases exponentially with distance from the electrode edge, and the average propagation length increases linearly with the applied voltage. Similar to some lower frequency asymmetric surface dielectric barrier discharges, two distinct breakdown events occur during one period of the voltage waveform. The number of filaments is found to be the same for both breakdown events, and collective effects are observed in both discharges.

Experimental investigation of a surface DBD plasma actuator at atmospheric pressure in different N2/O2 gas mixtures

This paper presents an investigation of the influence of nitrogen and oxygen on the behavior of a surface dielectric barrier discharge (SDBD) used for active flow control. The SDBD operated in a controlled atmosphere under several N2/O2 gas mixture ratios. For each gas mixture, the consumed power was measured as a function of voltage amplitude. Then, for a given applied high voltage, the plasma morphology was recorded and commented and lastly, ionic wind velocity measurements were performed. Results show that the induced ionic wind velocity is mainly due to oxygen negative ions during the negative half-cycle. Nevertheless, the contribution of nitrogen to velocity is not negligible during the positive half-cycle. Moreover, the propagation of negative spark filaments during the negative half-cycle is linked to the proportion of O2 in the gas mixture. Increasing this proportion beyond 20% leads to a shift in the saturation effect to lower voltages and to a decrease in the maximum ionic wind velocity value.

Influence of the Slant Angle of 3D Bluff Bodies on Longitudinal Vortex Formation

This paper presents the experimental results and analytical arguments concerning simplified geometries of the rear window and windscreen of automotive vehicles in order to contribute to a better understanding of the swirling structure formation and vortex bursting processes. Static pressure distributions and skin friction line visualizations on both sides of the edge where the swirling structure is generated on the rear window of an Ahmed body are presented for different slant angles. Results show the influence of the slant angle on the swirling structure formation and further show that the vortex bursting process can be promoted by small rear window angles. These results are then extrapolated with the help of analytical demonstrations to the windscreen configuration to demonstrate that large windscreen slopes would have the same disintegration effect.

Unsteady Forcing of a Post-stall Flow Over a NACA0012 Airfoil by a Surface DBD Actuator

of the shear layer rollup and wake vortex shedding can usually be observed. This study highlights the actuator eects by unsteady actuation with forcing frequencies selected according to the natural frequency of the wake vortex shedding. Mean aerodynamic force measurements, PIV and hot-wire anemometry in the vicinity of the shear layer and in the far wake were performed to characterize the benets of the control. For a deep post-stall airow, results show that forcing frequencies closer to the wake vortex shedding frequency lead to achieve a high increase in lift.

Influence of the N2/O2 volumetric ratio on the behavior of a SDBD plasma actuator

This paper is focused on studying the in uence of nitrogen and oxygen on the behavior of a surface dielectric barrier discharge (SDBD) actuator. Experimental characterizations have been conducted under several ratio of N2/O2 gas mixtures and in ambient air. Ionic wind velocity pro les have been measured for di erent positions above the dielectric surface, electrical power measurements and ICCD imaging of plasma laments in oxygen, nitrogen and air are presented. Experimental results show that ionic wind velocity is higher in pure oxygen than in pure nitrogen and thus con rm the main role of oxygen ions. They also show that the presence of water molecules in ambient air induces a stronger ionic wind velocity than in dry air. ICCD imaging illustrates di erent shapes of plasma depending on the gas composition.

ICCD Imaging of a Plasma Filament in a Circular Surface-Dielectric-Barrier-Discharge Arrangement

The development of plasma filaments in a single-side surface dielectric-barrier discharge is investigated. Morphological characteristics are presented for anodic and cathodic regimes. Intensified-charge-coupled-device pictures have been acquired at different specified times of the discharge. They show the time evolution of filaments, which appear about 100 μs before the beginning of each half period. Moreover, the correlation with current peaks underlines that filaments are not ignited simultaneously in our specific conditions.

Filaments in a Surface Dielectric Barrier Discharge Operating in Altitude Conditions

Positive and negative discharges occurring in a surface dielectric barrier discharge are investigated for different altitude conditions. Fast imaging reveals the expansion of the glow zone and streamer when the altitude increases from ground level up to 9000 m which corresponds to cruising altitudes of commercial flights.

Propagation Structure of a 13.56-MHz Asymmetric Surface Barrier Discharge Plasma in Atmospheric-Pressure Air

The global structure of a pulse-modulated 13.56-MHz asymmetric surface barrier discharge plasma has been investigated in atmospheric-pressure air. Intensified-CCD images were exposed over 20- μs radio-frequency pulses for three input power cases. The discharges do not appear to move significantly during the pulses, even though they are integrated over more than 200 oscillations of the applied voltage, and this allows for the observation of organized filamentary structures. The quantity and propagation length of the discharges are observed to increase with increasing input power, and the onset of branching occurs at a uniform propagation length. Subsequent to branching, a distinct decrease in luminosity is observed as the branches realign to propagate parallel to the original discharge. The spacing between the tips of the filaments is very similar to that of small indentations present on the surface of the dielectric barrier.