Analysis Of Temperature Fluctuations In ATurbulent Flow Over A Backward-facing Step

Abstract

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. In order to have a fully turbulent flow in the domain, an auxiliary simulation is carried out to provide unsteady velocity and temperature fields at the entrance to the backward-facing step. 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 bulk velocity (Ub) at the entrance and the step height (h) is 66 100. The mean reattachment point was predicted at x=5.4h which is close to the value calculated by Moss et al. [5] for the same expansion ratio and for a turbulent flow. 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. Just behind the step, a second recirculation region acts like an obstacle for the first reversal flow. The examination of the mean temperature field proves that the mixing behind the step is weak. From the shear layer to the reattachment zone, a large region exists containing warm fluid, but the fluid just behind the step remains cold. Apart from the fact that the temperature fluctuations are important in the shear layer, another region of high fluctuations was found to issue from the reattachment point and stretch toward the region of reversed flow.