A comparative study of alternating current and nanosecond plasma actuators in flow separation control

A combined numerical and experimental study is performed to elucidate the difference in flow separation control mechanisms between alternating current (ac) and nanosecond (ns) plasma actuators. The focus is on the visualization of detailed flow actuation process and establishment of relationship between flow control authority and characteristics of different actuators. Time-resolved particle image velocimetry (PIV) system is used to capture the transient separated flow over a stalled NACA 0015 airfoil at low Reynolds number (Re) of 6.3 x 10(4) and response of the flow to plasma actuation. Reynolds-averaged two-dimensional computation is performed for both low and high Reynolds number cases at Re = (0.063, 0.75) x 10(6) to complement experiment. Utilizing the elaborate Reynolds stress turbulence model in conjunction with an empirical body force model for surface ac plasma discharge and a sophisticated self-similar formulation for ns pulsed discharge, we have been able to numerically reproduce the experimentally observed intricate mechanisms, e.g. discretization of unstable shear layer into individual vortices, formation of large-scale spanwise vortex, strong entrainment effect of vortices and flow regularization procedure. Moreover, the information from simulation allows us to make a number of important observations which have no experimental counterparts. The detailed simulation shows that the convection of residual heat from ns discharge with cross flow makes ns plasma actuator more flexible in perturbing far downstream fluid in comparison with other actuators, and makes it superior to other devices in some high Re number flows. Also observed is the occurrence of strong flow resonance and frequency lock-in at a proper range of forcing frequency. In addition, the continuous ac plasma actuator appears to have complex control mechanisms, depending on flow conditions. (C) 2019 Elsevier Ltd. All rights reserved.