Frequency dependence of unsteady separation in a turbulent boundary layer

Abstract

Large-Eddy Simulations were carried out to characterize the unsteady separation induced by a dynamic pressure gradient on a turbulent boundary layer. Mean and phase-averaged quantities were described in a previous meeting. Here we report about the behaviour of higher-order statistics. The turbulent kinetic energy (TKE) reveals peculiar differences between the medium (k=1) and low (k=0.2) frequency cases. For the lowest k, TKE matches well the steady case before the separation bubble at the most adverse pressure gradient phase, and its evolution in time is in-phase with the free-stream forcing. As the frequency increases, the lag generated by hysteresis effect causes the maximum of TKE to be shifted in phase, and the region of high TKE before the separation bubble is significantly smaller. Both at high and low frequency, the region of high TKE after the bubble is completely absent, due to the advection of the slow-moving fluid region as shown by the phase evolution of TKE. An advection-like phenomenon plays a significant role even at the lowest k. It was not captured by the phase-averaged flow analysis, and seems to be characterized by different dynamics compared to the medium frequency case.