Thermal convection on rough surfaces

• Applicant: Professor Dr.-Ing. Claus Wagner
• Subject Area: Fluid Mechanics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 495678007

Thermal convection on a rough surface is a general model that is better suited to describe the physical processes in e. g. devices for heat transfer, at the cooling of electronic circuits or studying the heat exchange between an urban area and the atmosphere than other models based on smooth surfaces. Even though, a multitude of publications exists on this field, in particular, the local transport in the close vicinity of a heated or cooled wall with roughness is not well understood. This is due to the fact that only very few convection experiments exist, which permit local measurements of the transport quantities with a sufficiently high spatial and temporal resolution near the wall and that direct numerical simulations modelling the high Rayleigh numbers of the proposed experiments actually require an excessive effort. In the present research project, the local velocity and temperature fields near a heated/cooled surface with roughness will be measured in the large-scale convection experiment "Barrel of Ilmenau". Due to the large dimensions of the test section, the diameter amounts to 7.1 m, the height amounts to 8.0 m, such measurements can be done up to a Rayleigh number as high as Ra = 10^{12}. On the other hand, the spatial resolution of the measurements is still sufficiently high, to be capable to validate the wanted functional relations of the transport quantities in the near-wall fluid layer with a good certainty. In the course of the project, various surface structures including uniform elements and such ones staggered in height will be investigated. In the result of their investigations, the scientists will evaluate the relevance of various flow phenomena, like the increase of the rate of thermal plumes, the “thinning” of the thermal boundary layer on top of the roughness elements, or the transition of the thermal boundary layer towards a turbulent state, on the variation of the convective heat transport already observed in the past. Moreover, measurement data at the various surface structures will be compared and common features with respect to their effect on the local flow field and the heat transport will be found. In the ideal case, this search leads to a physical model that might be applicable similarly universally, like the model of hydraulic roughness at pipe or shear flows.

• DFG Programme: Research Grants