Numerical study of the instability mechanism in transitional separating–reattaching flow

Laminar separated flows are known to become unstable at relatively low Reynolds numbers. As a result, both the mean and instantaneous flow patterns are highly influenced by instabilities leading to transition to turbulence. Large-Eddy Simulation (LES) is employed to investigate the primary and secondary instabilities of a separated boundary layer transition on a flat plate with a blunt leading edge. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed to compute the subgrid-scale stresses more accurately in the transitional flow case. Statistics of the LES are found to be in acceptable agreement with the available experimental data. Based on the characteristic frequency from the velocity and pressure spectra, the LES results confirm that transition starts with the primary 2D instability originating from the free shear in the bubble as the free shear layer is inviscidly unstable via the KelvinHelmholtz mechanism. The flow visualisation together with the spectral analysis for the velocity components and pressure give strong indication of the dominance of the helical-pairing instability which could be mainly responsible for the breakdown to turbulence.