A Procedure for Using DNS Databases

A second moment closure (SMC) computation is compared in detail with the direct numerical simulation (DNS) data of Le et al. (1997) for the backstep flow at Re = 5100 in an attempt to understand why the intensity of the backflow and, consequently, the friction coefficient in the recirculation bubble are under-estimated. The data show that this recirculation bubble is far from being laminar except in the very near wall layer. A novel “differential a priori” procedure was used, in which the full transport equation for one isolated component of the Reynolds stress tensor was solved using DNS data as input. Conclusions are then different from what would have been deduced by comparing a full simulation to a DNS. In particular, the e-equation, usually blamed for faults in model predictions, has been found to give excellent results in this case. In fact, the main problem comes from the uv -equation which predicts a too high turbulent force. A modification, by including the gradients of mean flow in the transport model, has then been attempted and has cured 50 percent of the backflow discrepancy.