Turbulent secondary motions over spanwise heterogeneous surfaces

• Applicant: Professorin Dr.-Ing. Bettina Frohnapfel
• Subject Area: Fluid Mechanics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 423710075

Walls exposed to turbulent flow can modify the flow resistance and heat transfer between solid and fluid through their surface properties. In the case of surfaces that only exhibit heterogeneity in the spanwise direction, i.e. transverse to the main flow direction, turbulent secondary flows of Prandtl’s second kind occur, which can strongly modify the flow and temperature field. These secondary flows are visible as vortices in the mean flow field, which are oriented in the direction of flow. In addition, Kelvin-Helmholtz instabilities can occur over spanwise heterogeneous walls, leading to spanwise oriented vortex structures. The aim of the present proposal is to quantify the importance of these flow phenomena for the combined momentum and heat transfer. The focus is on the question of how it is possible to realize a stronger increase in heat transfer than in flow resistance with heterogeneous surfaces. This ratio is expressed by the Reynolds analogy factor. While rough walls generally increase the flow resistance more than the heat transfer, spanwise heterogeneous structures have the advantage of not generating any additional pressure resistance at individual roughness elements. This makes them ideal candidates for increasing the Reynolds analogy factor. There are indications in the specific literature that this is possible in principle, which is also supported by initial results from the first funding phase. The proposed numerical research project aims to investigate this topic in detail on the basis of direct numerical simulations.

• DFG Programme: Research Grants
• Co-Investigators: Dr.-Ing. Davide Gatti; Dr.-Ing. Kay Schäfer