Comparison of RANS and IDDES solutions for turbulent flow and heat transfer past a backward-facing step

Abstract

Numerical simulations were carried out under conditions of the benchmark-quality experiments of Vogel&Eaton (1985), where nominally 2D fluid dynamics and heat transfer past a backward-facing step in a channel with expansion ratio of 1.25 was investigated at the Reynolds number of 28,000 (based on the step height and the upstream centreline velocity). Comparative computations were performed using an in-house finite-volume code SINF/Flag-S and the ANSYS Fluent, running the codes with same grids. Two approaches were used for turbulence modelling. First, the Menter SST turbulence model was used to perform refined 2D and 3D RANS steady-state computations. The 3D analysis was undertaken to evaluate influence of boundary layers developing on the sidewalls of the experimental channel. The data obtained has resulted in the conclusion that the side wall effects disturbing spanwise uniformity of wall friction and heated wall temperature in the test configuration were of the same order or less than the skin friction coefficient and the Stanton number measurement errors. Then, 3D time-dependent computations were carried out using the vortex-resolving IDDES method being a hybridization of RANS and LES. The IDDES results obtained with the two codes are in a satisfactory agreement, especially for the finer grid of 17.3 million cells. Comparing with the experimental data, the IDDES approach produces the best agreement for the wall friction, whereas the RANS solutions show superiority in predictions of the local Stanton number distribution.