GALCIT Colloquium

Riblets are a surface texture that reduce skin-friction drag in turbulent flow, and can now be found on in-service aircraft. Riblet features are smaller than the smallest vortices of turbulence. On the fuselage of a passenger aircraft, riblet spacing is about 100 microns. Riblet performance is notoriously sensitive to the fine details of their micro-structure, with optimal performance thought to require sharp tips, which are impossible to manufacture and maintain in practice. Thus, their successful application requires careful lifetime management of performance benefits, balanced against manufacturing, installation and maintenance costs. Key to this balancing act is our ability to accurately predict riblet performance given the inevitable micro-structure imperfections. To this end, I will discuss our group's flow-physical modeling of the interaction between detailed riblet shapes and the near-wall vortices of turbulence; the outcome is a consistent improvement in accuracy of performance predictions across diverse riblet shapes.

Predicting rough-wall heat transfer has been a longstanding challenge, especially when new surface topographies are encountered. The heat-transfer coefficient of accreted ice on aircraft is different from that of engineered heat-exchanger surface textures. The best we can do are empirical correlations, which are not reliable. It is widely known that rough-wall heat transfer is not analogous to skin friction, i.e. not Reynolds analogy, but, then, what is it? With access now to the detailed temperature and flow fields near roughness features, I will show that heat transfer peaks at regions of the surface that are exposed to the oncoming flow, and, at these regions, a local version of Reynolds analogy survives. These insights allow us to develop a simple physics-based model of heat transfer that accounts for topography and working-fluid variations.