  In this combined experimental and numerical study on thermally driven
turbulence in a rectangular cell, the global heat transport and the coherent
flow structures are controlled with an asymmetric ratchet-like roughness on the
top and bottom plates. We show that, by means of symmetry breaking due to the
presence of the ratchet structures on the conducting plates, the orientation of
the Large Scale Circulation Roll (LSCR) can be locked to a preferred direction
even when the cell is perfectly leveled out. By introducing a small tilt to the
system, we show that the LSCR orientation can be tuned and controlled. The two
different orientations of LSCR give two quite different heat transport
efficiencies, indicating that heat transport is sensitive to the LSCR direction
over the asymmetric roughness structure. Through a quantitative analysis of the
dynamics of thermal plume emissions and the orientation of the LSCR over the
asymmetric structure, we provide a physical explanation for these findings. The
current work has important implications for passive and active flow control in
engineering, bio-fluid dynamics, and geophysical flows.
