PREDICTION OF TURBULENT WALL SHEAR FLOWS DIRECTLY FROM WALL

The fully elliptic Reynolds-averaged Navier-Stokes equations have been used together with Lam and Bremhorst's low-Reynolds-number model, Che n and Patel's two-layer model and a two-point wall function method inc orporated into the standard k-epsilon model to predict channel flows a nd a backward-facing step flow. These flows enable the evaluation of t he performance of different near-wall treatments in flows involving st reamwise and normal pressure gradients, flows with separation and flow s with non-equilibrium turbulence characteristics. Direct numerical si mulation (DNS) of a channel flow with Re = 3200 further provides the d etailed budgets of each modelling term of the k and epsilon-transport equations. Comparison of model results with DNS data to evaluate the p erformance of each modelling term is also made in the present study. I t is concluded that the low-Reynolds-number model has wider applicabil ity and performs better than the two-layer model and wall function app roaches. Comparison with DNS data further shows that large discrepanci es exist between the DNS budgets and the modelled production and destr uction terms of the epsilon equation. However, for simple channel flow the discrepancies are similar in magnitude but opposite in sign, so t hey are cancelled by each other. This may explain why, even when emplo ying such an inaccurately modelled epsilon-equation, one can still pre dict satisfactorily some simple turbulent flows.