Contribution towards the second-moment closure modelling of separating turbulent flows

Six cases of turbulent flows over backward-facing steps and in sudden plane and axisymmetric expansions at a range of Reynolds numbers were analyzed computationally, using two variants of high-Re number second-moment closures and a new model which accounts separately for low-Re number, wall blockage and pressure reflection effects, thus allowing integration up to the wall. Attention was focused on a back-step flow at a step-height Reynolds number of 5000, for which detailed data were recently supplied both by experiments (Jovic, S. and Driver, D. M., Backward-facing step measurements at low Reynolds number, Re-H=5000, TM 108807, NASA, 1994, 1993) and by direct numerical simulation (Le, H., Moin, P. and Kim, J., Direct numerical simulation of turbulent dow over a backward-facing step. J. Fluid Mech. 1997, 330, 349. The new model (shown earlier to reproduce well several classes of equilibrium and nonequilibrium wall-parallel flows) improved the predictions of most mean and turbulent dow properties, and particularly the turbulent stress budget in all three characteristic dow subregions: separating, reattachment and recovery zones; Anomalous performances of some popular Reynolds-stress models are also analyzed and possible causes of the models' deficiencies and their elimination are discussed. It is demonstrated that the introduction of a new term in the invariant form into the E-equation to suppress the excessive growth of the turbulence scale obviates the anomaly and better reproduces the streamline pattern. (C) 1998 Elsevier Science Ltd. All rights reserved.