Buoyancy effects on low-Reynolds-number turbulent flow in a horizontal square duct

Direct numerical simulations of fully developed low-Reynolds-number turbulent flow in a horizontal square duct heated from below are performed at Richardson numbers 0 Ri 1.03 to investigate the buoyancy effects on coherent structures near the walls, i.e. streamwise vortices and associated streaks, and on turbulence-driven secondary flow of Prandtl's second kind. It is found that cross-streamwise thermal convection which is represented by single large-scale circulation appears to affect coherent structures and the mean secondary flow for Ri greater than or similar to 0.02. As Ri is increased near-wall coherent structures are observed to appear more frequently in the region near one of the two corners on the wall, since they are swept along the wall towards the corner by the large-scale convection. The localization of the near-wall structures affects the profile of skin friction and heat transfer rate on the wall.