Linear and Non-Linear Dynamics of a Micro-ramp Wake

Micro-ramps are deployed to prevent boundary layer separation by creating a momentum excess close to the wall. Through Direct Numerical Simulations (DNS) of the base, instantaneous and mean flow, we identify that the perturbation dynamics in the wake of the microramp play an essential role in creating the near-wall momentum excess. To identify the origin of the perturbations, we deploy BiGlobal stability analysis on the laminar base flow. We demonstrate that the amplification of the most unstable linear mode is closely related to the time-averaged amplitude of the unsteady perturbations. The flow structure corresponding to this mode has a varicose symmetry with respect to the symmetry plane and matches with the early development of the hairpin vortices in the instantaneous flow field. It is concluded that the varicose instability supported by the laminar base flow represents the mechanism that generates the hairpins.