Buoyancy effects in an unstably stratified turbulent boundary layer flow

Direct numerical simulation has been performed to investigate the effect of buoyancy on an unstably stratified turbulent boundary layer with the Oberbeck-Boussinesq approximation. The simulation results show that the mean values of the streamwise velocity and scalar fields are increased in the near-wall region but decreased in the outer layer under the effect of buoyancy, which leads to significant increases in the skin-friction drag and heat transfer. In addition, it is found that the unstable thermal stratification results in large increases in the intensities of the near-wall streamwise vortices and high- and low-speed streaks, and a reduction in the mean diameter of the vortical structures. Moreover, the turbulent coherent structures become less organized due to the stratification effect. With respect to the neutral boundary layer flow, the outer vortical structures tend to bias the direction of the principal extensional strain towards the vertical plane in the unstably stratified flow. Published by AIP Publishing.