Effects of Ns-DBD plasma actuators on turbulent shear layers

The effects of nanosecond pulse driven dielectric barrier discharge (ns-DBD) plasma actuators on turbulent shear layers is examined experimentally on a mixing layer and backward facing step (BFS). The ns-DBD control mechanism is believed to be primarily thermal in contrast to most flow control actuators, including ac-DBDs, which rely on periodic or pulsed momentum to excite shear layer instabilities. Control authority using thermal perturbations has been demonstrated in various high speed shear flows yet many questions on fundamental physics and scaling remain unanswered. This work aims to provide insight into both ns-DBDs and thermal mechanisms in general for high amplitude aerodynamic flow control. Previous studies suggest the efficacy of ns-DBDs (and likely thermal perturbations in general) is strongly dependent on initial shear layer conditions, namely some measure of the initial thickness. In an effort to support this hypothesis, boundary layer suction is applied to a splitter plate upstream of a turbulent mixing layer origin. This successfully reduces the initial mixing layer momentum thickness, but does not result in substantial ns-DBD control authority. These results and the experimental conditions are documented in detail. Application of ns-DBD forcing to the turbulent shear layer downstream of a BFS having even smaller initial thickness produces the expected control authority at significantly lower pulse amplitude than employed in the mixing layer case. This supports the importance of initial shear layer thickness (rather than state) for estimating potential control authority by ns-DBD and thermal perturbations in general. In the context of past studies, this also suggests that the efficacy of thermal perturbations is likely dependent on the scale of energy deposition relative to the initial shear layer conditions. The dependence (or lack thereof) of this undefined amplitude parameter on Reynolds number based on some measure of initial shear layer thickness is currently unclear.