On the reattachment mechanism of nanosecond pulsed plasma actuation for laminar separation

The local and global flow responses to thermal perturbations induced by a nanosecond dielectric barrier discharge (NS-DBD) actuator for flow separation mitigation were investigated using three-dimensional scale-adaptive simulations (SAS). A NACA 0015 airfoil was analyzed at a chord-based Reynolds number of 2.5×105 (20 m/s) and an angle of attack of 14°, for a low-energy actuation case with H+0 = 0.45 and an actuation frequency of F+ = 2.54. The study provides detailed temporal and spatial analyses of the shear layer's local response to the initial pulse and the global flow response to the first 40 pulses, focusing on the mechanisms of vortex formation and flow reattachment, respectively. Regarding the vortex formation mechanism, evaluating the terms in the vorticity transport equation reveals that the underlying process closely resembles that observed in the two-dimensional case. It is primarily driven by vorticity generation through vorticity dilatation and baroclinic torque rather than by the excitation of local flow instabilities. This process generates spanwise vortices, which travel downstream along the airfoil surface. A crucial aspect of the flow reattachment mechanism involves small-scale perturbations that distort the spanwise vortices, stimulating the transition from laminar to turbulent flow and ultimately preventing flow separation. This mechanism cannot be captured correctly by two-dimensional simulations.