We studied the flow organization and heat transfer properties in two-dimensional and three-dimensional Rayleigh–Bénard cells that are imposed with different types of wall shear. The external wall shear is added with the motivation of manipulating flow mode to control heat transfer efficiency. We imposed three types of wall shear that may facilitate the single-roll, the horizontally stacked double-roll, and the vertically stacked double-roll flow modes, respectively. Direct numerical simulations are performed for fixed Rayleigh number $Ra = 10^{8}$ and fixed Prandtl number $Pr = 5.3$ , while the wall-shear Reynolds number ( $Re_{w}$ ) is in the range $60 \leqslant Re_{w} \leqslant 6000$ . Generally, we found enhanced heat transfer efficiency and global flow strength with the increase of $Re_{w}$ . However, even with the same magnitude of global flow strength, the heat transfer efficiency varies significantly when the cells are under different types of wall shear. An interesting finding is that by increasing the wall-shear strength, the thermal turbulence is relaminarized, and more surprisingly, the heat transfer efficiency in the laminar state is higher than that in the turbulent state. We found that the enhanced heat transfer efficiency at the laminar regime is due to the formation of more stable and stronger convection channels. We propose that the origin of thermal turbulence laminarization is the reduced amount of thermal plumes. Because plumes are mainly responsible for turbulent kinetic energy production, when the detached plumes are swept away by the wall shear, the reduced number of plumes leads to weaker turbulent kinetic energy production. We also quantify the efficiency of facilitating heat transport via external shearing, and find that for larger $Re_{w}$ , the enhanced heat transfer efficiency comes at a price of a larger expenditure of mechanical energy.

We present a numerical study of vorticity production and transport in the two-dimensional Rayleigh–Bénard (RB) convection. Direct numerical simulations are carried out in the Rayleigh number (Ra) range 105≤Ra≤106, the Prandtl number (Pr) of 0.71, and the aspect ratio (Γ) of the convection cell range 0.75≤Γ≤6. We found that the flow structure and temperature distribution vary with Γ greatly due to multiple vortices interaction. Further investigation on the vorticity production and transport reveals that, in the RB convection, in addition to the vorticity production due to wall shear stress, buoyancy produces significant vorticity in the bulk region. The produced vorticity is transported via advection and diffusion. An interesting finding is that the main vortices and the corner vortices can be visualized via the contour of buoyancy-produced vorticity. Although a vigorous definition of the vortex is still lacking in the community, our efficient vortex visualization approach in the RB convection may shed light on further research toward vortex identification. We also found that the spatial distribution of vorticity flux along the wall is positively correlated with that of the Nusselt number (Nu), suggesting the amount of vorticity that enters the flow is directly related to the amount of thermal energy that enters the flow.