Search

An experimental study aimed at the characterization of energy deposition of nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuators was carried out. Special attention was given on the effect of the thickness and material used for dielectric barrier. The selected materials for this study were polyimide film (Kapton), polyamide based nylon (PA2200), and silicone rubber. Schlieren measurements were carried out in quiescent air conditions in order to observe density gradients induced by energy deposited. Size of heated area was used to qualify the energy deposition coupled with electrical power measurements performed using the back-current shunt technique. Additionally, light intensity measurements showed a different nature of discharge based upon the material used for barrier, for a fixed thickness and frequency of discharge. Finally, a characterisation study was performed for the three tested materials. Dielectric constant, volume resistivity, and thermal conductivity were measured. Strong trends between the control parameters and the energy deposited into the fluid during the discharge were observed. Results indicate that efficiency of energy deposition mechanism relative to the thickness of the barrier strongly depends upon the material used for the dielectric barrier itself. In general, a high dielectric strength and a low volumetric resistivity are preferred for a barrier, together with a high heat capacitance and a low thermal conductivity coefficient in order to maximize the efficiency of the thermal energy deposition induced by an ns-DBD plasma actuator. [ABSTRACT FROM AUTHOR]

An experimental investigation was carried out on the effect of unsteady periodic control on a separated laminar shear layer. Time resolved Particle Image Velocimetry (tr-PIV) was used to characterize a backward-facing step (BFS) flow ( Re h = 3600), periodically perturbed by a nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuator. Ensemble averaged vector fields indicate a decrease of reattachment length with increasing actuation non-dimensional frequency, reaching a minimum at a St h = 0.32. Further increase of forcing non-dimensional frequency, up to 0.4, resulted into an increase of the reattachment length, which nevertheless remained shorter than the non-actuated case. Spectral analysis of the fluctuating fields revealed a change of the amplified frequency range for the actuated cases with respect to the base flow. Proper Orthogonal Decomposition (POD) analysis showed that actuation leads to a redistribution of energy among coherent spatial modes. Stability diagrams were calculated from mean velocity field data for each case via Linear Stability Theory (LST). Results indicate that stability of each actuated case changes with respect to the non-actuated case. Moreover, looking into the calculated growth rate for all the cases a more stable flow regime is observed for the cases of most successful reduction in terms of reattachment length. The effect of a pulsed periodic perturbation on the control of a laminar shear layer promotes the development of large structures, i.e. K-H vortices, due to inviscid-viscous interaction. These convect downstream resulting in a mean flow deformation (MFD) which causes a change of stability. New unstable frequencies are excited and promote the redistribution of energy among modes. This ultimately affects the efficiency of actuation in promoting transition from laminar to turbulent flow. [ABSTRACT FROM AUTHOR]

An experimental investigation of the working principles of a nanosecond-pulsed dielectric barrier discharge (ns-DBD) plasma actuator has been conducted. Special emphasis is given on the thermal effects accompanying the rapid deposition of energy associated with this kind of actuation. A ns-DBD plasma actuator has been operated in quiescent air conditions as well as in a flat plate laminar boundary layer, with external flow velocity of 5 and 10 m s−1. Schlieren imaging and particle image velocimetry have been used to characterize the actuation. Additionally, the back-current shunt technique has been used for current measurements, from which energy input (per pulse) is calculated. Cases of 10-, 20- and 50-pulse bursts are tested. Schlieren imaging in still air conditions shows the formation of a high-temperature region in the vicinity of the discharge volume. The spatial extent of the visible ‘hot spot’ depends upon the number of pulses within the burst, following a power law. Schlieren imaging of the span-wise effect of the plasma actuator reveals weak compression waves originating from the loci of discharge filaments. The thermal ‘hot spots’ exhibit significant three-dimensionality. Particle image velocimetry is used to measure the velocity field resulting from the ns-DBDs acting on a laminar boundary layer. The disturbance leads to formation of a Tollmien–Schlichting wave train, with spectral content in good agreement with linear stability theory. It is observed that the group length of the wave train is proportional to the number of pulses within the burst. [ABSTRACT FROM AUTHOR]