Controlling Heat Transport and Flow Structures in Thermal Turbulence Using Ratchet Surfaces

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 ratchetlike 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, biofluid dynamics, and geophysical flows.

Figure 1

(a) A sketch of the convection cell with the asymmetric ratchets on the top and bottom plates. The cell can be tilted to the left or the right side. (b) Tilting angle $\beta =-3.2°$ resulting in a flow in clockwise direction. (c) Tilting angle $\beta =+3.2°$ resulting in a flow in the counterclockwise direction. (d) Nusselt number Nu as a function of Ra in the smooth and rough cells ($\mathrm{Pr}=4.3$). (e) Nu enhancement as a function of Ra for the two cases. The open symbols correspond to experimental data, the closed symbols correspond to numerical data, and the triangles represent the data for the smooth wall case.

Figure 2

Volume rendering of the temperature field, showing plume dynamics at $\mathrm{Ra}=5.7\times {10}^9$ and $\mathrm{Pr}=4.3$ from the DNS for (a) the smooth wall situation; (b) case $A$; (c) case $B$. The corresponding movies are available in the Supplemental Material [34].

Figure 3

Cumulative histogram of the normalized plume area $A_p/A$ for case $A$, case $B$, and smooth case. Inset shows the histograms of the same. Case $B$ shows the highest number of plume emissions.

Figure 4

Shadowgraph visualization of the spatial distribution of thermal plumes at $\mathrm{Ra}=5.7\times {10}^9$ and $\mathrm{Pr}=5.7$ for (a) case $A$, and (b) case $B$. The large scale circulation roll direction is clockwise in (a), and counterclockwise in (b). The corresponding movies are available in the Supplemental Material [34].

Figure 5

The measured probability for the LSCR orientation to remain in case $A$ as a function of tilting angle at $\mathrm{Ra}=5.7\times {10}^9$. A value of 1 means that the LSCR orientation is always in case $A$, whereas a value 0 means that it is in case $B$. In the transition regime, each measurement is repeated 10 times.