Large-eddy simulation of the interaction of wall jets with external stream

Large eddy simulations are performed for a wall jet with an external stream. The external stream is in the form of a heated boundary layer. This is separated from a cold wall jet by a thin plate. The Reynolds number based on the displacement thickness, for the incoming boundary layer is 2776. A series of jet velocity ratios in the range M=Uj/U∞=0.30-2.30, is considered. The wall jet and outer stream velocities are Uj and U∞, respectively. The jets with M⩽1.0 develop von-Karman type shed vortices in the wake region. The higher velocity ratio jets with M>1.0 undergo Kelvin-Helmholtz instability and develop closely spaced counter-clockwise rolling structures. These structures determine the mean flow field behaviour and near wall heat transfer. At any given streamwise location adiabatic film-cooling effectiveness for M<1.0 increases rapidly with increasing M. For M>1.0 it decays slowly with further increase in M. For M<1.0 heat transfer from the hot outer stream to the wall depends on two factors; mean wall normal velocity and wall normal turbulent heat flux. For M>1.0 only a wall normal turbulent heat flux is responsible for heat transfer to the wall. The scaling behaviour shows that the near wall flow scales with wall parameters for all values of M. However, scaling in the outer region is highly dependent on M. The flow develops towards a boundary layer in the farfield for M<1.0 and towards a wall jet for the highest velocity ratio M=2.30.