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

Controlling heat transport and flow structures in thermal turbulence using ratchet surfaces