Influence of the transition of a laminar separation bubble on the downstream evolution of strong adverse pressure gradient turbulent boundary layers

The evolution of boundary layers subject to adverse pressure gradient (APG) is strongly influenced by the upstream history. Hence, the detailed understanding of APG flows in general or downstream of reattachment of a separation bubble requires a clear acquaintance of the relation between the upstream flow structures and the downstream ones. In this work the results for three different direct numerical simulations (DNS) of APG flows are analyzed to scrutinize the relaxation of these flows having distinct development histories. The three cases have the same overall characteristics – laminar separation, transition, and turbulent reattachment – with respect to the imposed APG and the Reynolds number Re_δ99⁰ at the inlet. However, in the first case no additional perturbations are imposed to trigger transition, in the second case the flow is tripped by a trip wire and in the third case a periodic wake is superimposed on the flow. The detailed information provided by DNS is critical to evaluate the current turbulence models and develop new ones. Hence, main points that are discussed are the Reynolds stress budgets, two-point spatial correlations, and spanwise spectra. Attention is in particular directed to a comparison between the three different cases. The most important conclusion is that the flow downstream of the reattachment decorrelates faster with the flow at the transition position if transition to turbulence completes within the separation bubble than in the case the transition completes after reattachment.