Analysing Near-Wall Behaviour in a Separating Turbulent Boundary Layer by DNS

Abstract

The near-wall behaviour of a separating and reattaching turbulent boundary layer has been analysed using data from a direct numerical simulation (DNS). The focus of the evaluation is on the improvement of boundary conditions for large-eddy simulations (LES) in separating and reattaching turbulent flows. The DNS has been designed according to an experiment of Kalter and Fernholz, 1994. The comparison of the results between the DNS and the experiment is fully satisfying (Man-hart and Friedrich, 2001). Instantaneous velocity profiles are analysed in terms of instantaneous inner coordinates showing a remarkable deviation from conventional wall scaling. The model of Werner and Wengle, 1989 is shown to produce satisfying prediction of the wall shear stress for forward flow events and a systematic underprediction of the wall shear stress for backward flow events. This happens because linear wall scaling is no more valid in situations with strong pressure gradients. A new formulation including the instantaneous pressure gradient gives a considerable improvement of the prediction of the wall shear stress in backward flow events before separation, in the separation zone and after reattachment.