LARGE-EDDY SIMULATION OF HEAT TRANSFER OVER A BACKWARD-FACING STEP

A large-eddy simulation (LES) of the flow over a backward-facing step was conducted to investigate the heat transfer phenomena in the reattachment zone. The Navier-Stokes equations for an incompressible fluid with the temperature field considered as a passive scalar are solved using a second-order accurate scheme in space and time. An original coupling is used with a previous simulation to impose a fully turbulent flow at the entrance of the domain. The mixed scale subgrid model is used for the momentum equations while a scalar subgrid diffusivity model is employed for the temperature equation. The Reynolds number based on the velocity at the entrance of the domain and the step height is 7,432. The mean velocity field shows clearly that the shear layer issued from the step impacts the wall defining a recirculation zone, in which the reversed flow spreads into the original shear layer. A second recirculation region is found behind the step and acted like an obstacle for the first reversal flow. The mean reattachment length is well correlated to the maximum Nusselt number. The examination of the mean temperature field proves that the mixing behind the step is weak. The temperature fluctuations are important in the shear layer and in a zone issued from the reattachment point and stretched toward the region of reversed flow. Two main frequencies are identified: a high one due to the vortex shedding of the shear layer and a low one corresponding to the drift of the reattachment point.