DIRECT NUMERICAL-SIMULATION OF A SEPARATED TURBULENT BOUNDARY-LAYER

A separated turbulent boundary layer over a flat plate was investigate d by direct numerical simulation of the incompressible Navier-Stokes e quations. A suction-blowing velocity distribution was prescribed along the upper boundary of the computational domain to create an adverse-t o-favourable pressure gradient that produces a closed separation bubbl e. The Reynolds number based on inlet free-stream velocity and momentu m thickness is 300. Neither instantaneous detachment nor reattachment points are fixed in space but fluctuate significantly. The mean detach ment and reattachment locations determined by three different definiti ons, i.e. (i) location of 50% forward flow fraction, (ii) mean dividin g streamline (phi = 0), (iii) location of zero wall-shear stress (<(ta u)over bar>(w) = 0), are in good agreement. Instantaneous vorticity co ntours show that the turbulent structures emanating upstream of separa tion move upwards into the shear layer in the detachment region and th en turn around the bubble. The locations of the maximum turbulence int ensities as well as Reynolds shear stress occur in the middle of the s hear layer. In the detached flow region, Reynolds shear stresses and t heir gradients are large away from the wall and thus the largest press ure fluctuations are in the middle of the shear layer. Iso-surfaces of negative pressure fluctuations which correspond to the core region of the vortices show that large-scale structures grow in the shear layer and agglomerate. They then impinge on the wall and subsequently conve ct downstream. The characteristic Strouhal number St = f(in)(delta)/U -0 associated with this motion ranges from 0.0025 to 0.01. The kinetic energy budget in the detachment region is very similar to that of a p lane mixing layer.