DNS and LES of the Transition Process in a Laminar Separation Bubble

Abstract

A region of strong local adverse pressure gradient acting on a laminar flat-plate boundary layer can produce a closed fully laminar separation bubble for sufficiently small pressure rise and Reynolds number. However, such a flow field is hydrodynamically highly unstable and transition will occur in the region of adverse pressure gradient. Due to an interaction with the potential flow, the transition process may even suppress laminar separation completely.

Direct numerical simulation (DNS) of oblique transition in a steady laminar separation bubble is carried out. The mean flow deformation is found to play an important role even upstream of the transition location. However, with proper treatment of the upper boundary it is possible to take the upstream influence into account and to obtain results that are height independent in accordance with the physical model of an unbounded domain. Hence, DNS results can serve as a reference for an analysis of the large-eddy simulation (LES) technique. Such an analysis of LES is carried out based on a (scale) separation step associated with an explicit filter and a (subfilter-scale) modeling step to obtain closure. It is shown that filtering of the Navier-Stokes equations is not a formality and that the subgrid-scale model has to be adopted to the filter accordingly. Numerical results for a discrete filter together with a relaxation-term model lead to guidelines for the choice of the explicit filter and the desired action of the turbulence model.