  We studied the flow organization and heat transfer properties in
two-dimensional and three-dimensional Rayleigh-B\'enard 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 $60 \le Re_{w} \le 6000$.
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.
