Direct Numerical Simulation of Unstably Stratified Turbulent Channel Flow

Direct numerical simulations of the fully developed horizontal channel flow under unstable density stratification were carried out to investigate interactive shear and buoyancy effects on the turbulent momentum and heat transport. As the Grashof number is increased, buoyant thermal plumes are generated. The large-scale thermal convection involving the thermal plumes diminishes the quasi-coherent streamwise vortices, which are known to play a major role in the transport mechanism of near-wall turbulence. The destruction of the streamwise vortices result in the increased bulk mean velocity and the decreased turbulent friction coefficient. The vertical fluid motion of thermal plumes drastically changes the transport mechanism of the Reynolds shear stress. The thermal plumes are spatially aligned in the streamwise direction, and the low-speed streaks and vortical structures are concentrated in the region where the thermal plume starts to rise. The Prandtl number effects on the turbulent kinetic energy are also studied when the thermal plumes emerge.