Numerical investigations of forced laminar and turbulent wall jets over a heated surface

The effect of high amplitude forcing on laminar and turbulent wall jets over a heated flat plate is analyzed. Highly accurate Direct Numerical Simulations (DNS) are used in the laminar case to investigate the dominant transport mechanisms. When forcing is applied, the skin friction is reduced markedly and the wall heat transfer is increased, in contrast to the prediction of the Reynolds analogy, which states proportionality between both quantities. Detailed examination of the unsteady flow field showed that the concepts of eddy viscosity and eddy thermal diffusivity can be applied to analyze unsteady laminar flows and to explain the effect of highly unsteady phenomena. For the investigation of the turbulent wall jet, a new Flow Simulation Methodology (FSM) is employed in the limit of unsteady BANS (Reynolds averaged Navier-Stokes) simulations. With this novel approach, the simulation of large, coherent structures in the turbulent flow field very closely parallels the laminar simulations. Following the idea of Large Eddy Simulation (LES), the large coherent motion is computed directly, while the effect of the small scale, random motion is modelled. In FSM, a state-of-the-art two-equation turbulence model is used. Forcing the turbulent wall jet results in a reduction of the skin friction and an increase in wall heat transfer. The mechanisms responsible for these mean flow changes show a remarkable similarity to the mechanisms found in the laminar case. This is confirmed by close examination of the large coherent motion and its effect on the turbulent mean flow. Using this approach, several questions regarding the character of the turbulent wall jet could be answered