Investigation of Laminar Separation Bubbles Using Experiments, Theory and DNS

A combined approach based on high quality wind-tunnel experiments and high-fidelity direct numerical simulations investigating a laminar separation bubble on a flat plate is presented. In the experiments, the favorable to adverse pressure gradient under an inverted modified NACA 643-618 airfoil generates a separation bubble on the flat plate. Flow control suppresses separation on the airfoil and the time-averaged flow field is matched to the boundary conditions of the simulations. The DNS exhibits a mean separated region larger than the experiments. Discrepancies between the simulation and the experiments are due to an earlier onset of transition, attributed to non-zero free-stream turbulence in the experiments. Introduction of very low-level of isotropic, vortical FST in the DNS accelerates transition, and decreases the mean separated region, matching remarkably well with the experiments. Detailed analysis of the experimental measurements and the simulation data based on Fourier analysis, proper orthogonal decomposition and instantaneous flow visualizations is presented to provide further insight into the characteristic features of the unsteady flow.