Effects of rough surfaces on annular centrifugal Rayleigh–Bénard convection

Two-dimensional direct numerical simulations are performed to explore the effects of rough surfaces on the temperature fields, heat transport, and zonal flow in the annular centrifugal Rayleigh–Bénard convection (ACRBC) with cold inner and hot outer cylinders co-rotating axially, up to a Rayleigh number of Ra = 4.7 × 10⁹. Three combinations of roughness are considered, that is, two rough surfaces (Case A), rough-inner/smooth-outer (Case B) and smooth-inner/rough-outer (Case C). It is shown that, the rough walls cause a bias in the temperature fields towards the values set on the rough walls at high Ra, while have a minimal impact at low Ra. This effect can be attributed to the rough walls promoting the generation and detachment of plumes, which act as the primary heat carriers in turbulent convective heat transfer. Furthermore, the presence of rough surfaces leads to enhanced heat transport at high Rayleigh numbers. Notably, two universal regimes are distinguished by the critical Rayleigh number Ra_c≈10⁹: in regime I, where roughness elements protrude through the thermal boundary layer (BL), heat transport is significantly enhanced, resulting in a considerable increase in scaling exponent; in regime II, the scaling exponent reaches a saturation point, returning to a value similar to the smooth case. Similar findings are also observed in the classical Rayleigh–Bénard convection (Zhu et al., 2017). The decrease in scaling exponent can be attributed to the intense mixing caused by secondary vortices within the roughness elements, leading to a thin and uniformly distributed thermal BL along the rough surfaces, resembling the behavior observed in the smooth case. It is also found that roughness on the outer wall promotes the emergence of zonal flow, while roughness on the inner wall weakens it. These findings provide valuable insights into the effect of wall roughness on temperature fields, heat transport, and zonal flow in ACRBC.