Near-Wall RANS Modelling in LES of Heat Transfer in Backward-Facing Step Flows Under Conditions of Constant and Variable Fluid Properties

Abstract

Two backward-facing step (BFS) flow configurations associated with the heat transfer under the conditions of constant and variable fluid properties were investigated computationally by means of LES and a zonal Hybrid LES/RANS (HLR) method. The latter scheme couples a RANS (Reynolds-Averaged Navier-Stokes) model with large-eddy simulation (LES) within a two-layer framework. A differential near-wall eddy-viscosity model resolves the wall layer and the LES model the remainder of the flow domain. As an introductory heat-transfer case a fully-developed channel flow at Re number Rem = 24000 (DNS: Abe et al., 2004) was computed. In both presently investigated BFS cases the flow is subjected to increasingly enhanced wall heating. Whereas the first considered case (ReH = 28000, ER = 1.25), treated experimentally by Vogel and Eaton (1985) - reference LES is due to Keating et al., 2004, deals with a passive scalar transport, the high-intensity heat flux introduced into the flow domain through the step wall in the second investigated configuration (ReH = 5540, ER = 1.5; reference LES by Avancha and Pletcher, 2002; corresponding isothermal experiment by Kasagi and Matsunaga, 1995) leads to large temperature gradients causing a strong variation of the flow properties. An important feature of the latter flow is a substantial increase of the friction coefficient magnitude with the wall heating intensification in both the flow reversal and recovery region, associated with the local flow acceleration in the immediate wall vicinity. The results obtained by the present simulations with respect to the mean velocity and temperature fields, friction factor and Stanton number variations follow closely the reference experimental and LES databases.