On the Physics and Control of Laminar Separation Bubbles Using Experiments, Theory and DNS*

A laminar separation bubble on a flat plate is investigated using a combined approach of wind tunnel experiments and high-fidelity direct numerical simulations. The favorable to adverse pressure gradient under a displacement body, an inverted modified NACA 64(3)-618 airfoil at a chord Reynolds number of Re=90k, generates a separation bubble on the plate. In the experiment, flow control on the displacement body ensures laminar flow and the time-averaged flow field is matched with the boundary conditions in the simulations. Without free-stream turbulence in the DNS, the mean separated region on the flat plate exceeds the experiment, where disturbances in the free-stream facilitate an earlier onset of transition. Introduction of low-levels of isotropic, vortical FST in the DNS accelerates transition, and decreases the mean separated region, matching remarkably well with the experiments. Dominant coherent structures in the separated shear layer are identified using proper orthogonal decomposition, Fourier analysis and instantaneous flow visualization from DNS. Linear stability theory based on the time averaged flow field identifies the most dominant structures to be the inviscid shear-layer instability.