Turbulent convection at rough surfaces covers a large variety of heat transfer processes in nature and engineering. However, in particular the transport of heat near the rough surface is still not well understood. We present measurements of the near-wall temperature field in turbulent Rayleigh-Bénard convection with rough walls, and for reference also with smooth walls. The measurements have been undertaken in a large-scale convection experiment, the “Barrel of Ilmenau.” Our experiments covered Rayleigh numbers in a domain $5.4\times {10}^9<\text{Ra}<9.6\times {10}^{11}$ and two different aspect ratios $\mathrm{\Gamma }=1.1$ and 2.9. The working medium was air with a Prandtl number $\text{Pr}=0.7$. Using very tiny micro-thermistors of $150\mu \mathrm{m}$ diameter and $350\mu \mathrm{m}$ length, we conducted highly resolved measurements of the temperature field near the heated bottom plate. Our measurements show that, as was already observed in high Prandtl number fluids, the ratio between the thickness of the thermal boundary layer ${\delta }_{\text{th}}=H/\left(2\text{Nu}\right)$ and the roughness height $h$ plays a crucial role for the near-wall temperature field, and thus for the convective heat transport. If ${\delta }_{\text{th}}/h>1$, the temperature field at the rough surface does not differ from that at the smooth one. If this ratio falls below ${\delta }_{\text{th}}/h=1$, both the mean temperature field and the temperature fluctuations start to change and to differ from that at the smooth surface. These variations are virtually independent of the Rayleigh number in the parameter domain we investigated.

• Received 20 November 2023
• Accepted 24 January 2024

DOI:https://doi.org/10.1103/PhysRevFluids.9.023501