Control of Travelling Crossflow Vortices Using Volume Forcing

Abstract

In the present DNS work we focus on the most unstable travelling crossflow vortex (CFV) mode in a 3-d boundary layer, and examine the applicability of unsteady volume-force actuation to directly tackle the invoked nonlinear state. The actuators with forcing nearly in or against the crossflow direction are arranged in a spanwise row and operated with the same frequency as that of the oncoming mode. The unsteady volume forcing is designed as a model for actuation by plasma actuators. It is shown that unsteady forcing with a suitable phase can efficiently counteract the vortical motion of the nonlinear travelling CFVs, and hence delay the onset of secondary instabilities. However, a detrimental steady mode is also excited. The superposition of the steady and travelling modes renders some parts of the mean CFVs more unstable. By employing two additional actuator rows downstream with combined steady and unsteady forcing, the growing steady and travelling modes can be further attenuated, respectively, yielding enhanced delay of the complex transition with travelling primary 3-d structures.