Coherent Flow Structures Downstream of a Laminar Separation Bubble - a Numerical Investigation

Vortices arising from the Kelvin-Helmholtz instability are formed within the shear layer above a laminar separation bubble. The present work shows an analysis of their evolution based on data from large eddy simulation of a planar NACA0018 wing at Re = 80000 and alpha = 4 degrees. The spectra of fluctuations around the wing show that vortex shedding on the upper and on the lower side interacts but occurs at different frequencies which are both detectable around the entire wing. The wake flow is dominated by flow the lower side since the separation occurs further downstream in the featured conditions. By phase averaging of the flow field it is demonstrated that on the upper side traces of the vortices in terms of turbulence intensity and pressure drop exist up to the trailing edge. On the other hand, the origin of the vortices is shown to be well upstream of the separation zone.