Effects of boundary asymmetry in turbulent Rayleigh-Benard convection*

We present study of the effects of boundary asymmetry in turbulent Rayleigh-Benard convection. This asymmetry was introduced by using roughness elements with different roughness aspect ratios λ on the top and the bottom plates, i.e., λ_t=1.0 (top plate) and λ_b=4.0 (bottom plate). The experiment was carried out in a cylindrical cell with an aspect ratio 1 and a diameter of 19.2 cm. The height of the roughness elements was 8 mm. The Rayleigh number Ra was in the range of 3.5×10⁸≤Ra≤ 8.2×10⁹ and the Prandtl number was Pr=4.34. It is found that the scaling exponents of the Nusselt number and the Reynolds number with Ra in the asymmetric cell is larger than those obtained in a symmetric cell with λ_t=λ_b=1.0 and smaller than those obtained in a symmetric cell with λ_t=λ_b=4.0. Measurement of the temperature profile along the cell's centerline suggests the boundary asymmetry not only changes the transport properties, but also leads to deviation of the mean temperature T_bulk from the averaged temperature of the top and bottom plate T_m. In addition, this deviation is Ra-dependent. The change of T_bulk might be due to the different plume dynamics at the asymmetric boundaries with increasing Ra.