Three-dimensional plasma actuation for faster transition to turbulence

We demonstrate that a 3D non-linear plasma actuation method creates secondary instabilities by forming lambda vortices for a spatially developing turbulent boundary layer flow over a flat plate. Both bypass transition and controlled transition processes are numerically investigated using wall resolved modal discontinuous Galerkin based implicit large eddy simulation. The largest momentum thickness based Reynolds numbers $\left(R{e}_{\theta }\right)$ tested are 1250 and 1100 for the bypass transition and the controlled transition, respectively. The 3D actuation method is based on a square serpentine plasma actuator (Durscher and Roy 2012 J. Phys. D: Appl. Phys. 45 035202). The transition is achieved via oblique wave transition by perturbing the flow at a frequency of 1 kHz with amplitude of 10% of the freestream velocity. Although the flow is perturbed at a single frequency, the instabilities arising due to the nonlinear interaction between the consecutive lambda vortices, creates subharmonic lambda vortices (half of the fundamental frequency), which finally break down into fully turbulent flow. These interactions have been thoroughly studied and discussed. Since the actuation creates oblique wave transition it will allow faster transition compared to the standard secondary instability mechanism with similar disturbance amplitude reducing the amount of energy input required for flow control. (paper)