Interaction of Crossflow with a Laminar Separation Bubble

Crossflow development and its impact on a Laminar Separation Bubble (LSB) was investigated experimentally on a NACA 64₃ − 618 airfoil at a sweep angle of Γ = 45° and a chordwise Reynolds number of Re_C = 600 000. Discrete Roughness Elements (DREs) imposed a dominant spanwise wavelength and successfully amplified the stationary crossflow instability along the suction side of the airfoil. A Type III secondary instability was identified in strong stationary crossflow at AoA = −8°. The boundary layer is fully attached at these conditions and there is no LSB. For weak crossflow at AoA = 0°, three-dimensional structures were observed in the LSB forming in the adverse pressure gradient. Laminar-to-turbulent transition in the LSB remains dominated by the 2D Kelvin-Helmholtz instability in the separated shear layer. However, the spanwise coherent vortical structures, typical for unswept LSBs, are significantly distorted due to the impact of the crossflow and associated instabilities.