Separation control over small backward-facing steps using nanosecond plasma actuation in transonic flows

To assess the impact of small steps that may exist in the engine nacelle's intake duct on its aerodynamic characteristics, this study investigates the flow over a backward-facing step with a height of the order of 10−3 m on a flat plate. The research aims to examine the effects of small-height steps on the flow field of the flat plate and the corresponding total pressure loss under transonic inflow conditions. Additionally, the study explores the control characteristics of the separated flow downstream of the step and the changes in total pressure resulting from nanosecond pulse surface dielectric barrier discharge (NS-SDBD) plasma actuation, discussing the flow control mechanisms involved. The flow field simulation is performed using the two-dimensional unsteady Reynolds-averaged Navier–Stokes method, with the NS-SDBD plasma actuation described by phenomenological mathematical equations introduced into the energy equation. The validity of the model is verified by comparing it with experimental results. The research findings show that for the same step height, the flow field at Ma=1.2 produces a recirculation zone with a smaller area and additional wave structures (expansion waves and reattachment shock waves) compared to Ma=0.8. At the same Mach number, the length of the recirculation zone and the distance from the reattached shock foot (position) to the step are linearly related to the step height. The separation and reattachment shock waves downstream of the step cause total pressure losses of approximately 1.0–3.0%. Under single high-pressure pulse actuation, the NS-SDBD plasma actuator generates a pressure wave system consisting of quasi-planar waves and semicircular waves. This pressure wave system propagates supersonically within the first few micrometers after the actuator is turned on and then gradually decays to the local speed of sound. The pressure wave system induces an instantaneous velocity of approximately 35–40 m/s, mainly distributed at the peaks of the semi-circular and quasi-planar wave regions. Significant starting vorticity is observed at the exposed/covered electrode seams. The pressure wave, the instantaneous induced velocity, and the starting vorticity generated by the NS-SDBD plasma actuation effectively promote momentum mixing between the separated region downstream of the step and the mainstream, thereby reducing the area of the flow separation zone. At Ma=0.8, the size of the separation bubble is reduced by 38.6%. At Ma=1.2, the interaction between the pressure wave and the reattachment shock wave causes it to exhibit unsteady characteristics, changing its time-averaged strength and position, and reducing the total pressure loss by 26.7%.