Turbulence characteristics of separated flows

Flow separation often referred to as a mean flow phenomenon, characterized by flow reversal, and the underlying characteristics of the turbulence in separated flows has not been fully addressed in previous studies. To fill this gap, a systematic study has been conducted to investigate flow separation from the perspective of turbulence statistics, particularly the energy spectrum. A series of direct numerical simulations (DNS) have been performed for both attached Couette-Poiseuille (CP) flows and separated flows over backward-facing steps (BFS) and periodic hills (PH). The CP flows are designed to have mean velocity profiles similar to those of separated flows exhibiting flow reversal. It is shown that the energy spectrum of the attached CP flows shares similar features with canonical channel flow, where energetic motions scale with their distance from the wall. In contrast, the flow within the separation bubbles in the BFS and PH cases is energized by the overlying shear layer, and the bounding wall no longer plays a significant role in determining its energetics. The eddy viscosity field is analyzed using the DNS strain-rate tensors and Reynolds stress tensors. It is demonstrated that the eddy viscosity increases within the separation bubble, but the response lags behind the separation. Finally, integral analysis of the skin friction shows that the turbulent production term is significantly affected by flow separation.