Vortex-speed selection within the transitional region of rotating boundary-layer flows

We discuss the existing theoretical prediction that convective disturbances traveling between 70% and 80% of the local surface speed are the most amplified within the boundarylayer flows over rotating cones and spheres. Although most experimental observations of such flows report stationary vortices excited by surface roughnesses, we note that Kobayashi & Arai [Laminar Turbulent Transition, IUTAM Symposium 1990] report the appearance of slow vortices with a relative speed of 76% over rotating spheres under particular conditions. We associate this observation with the theoretical prediction for maximum amplification rate and suggest that, in situations where perfectly smooth, clean surfaces are used, such slowly rotating vortices may be observed over disks, cones and spheres. The analysis is then extended to the general class of BEK flows where such slow vortices are again predicted.