Stability of Concave Boundary Layers: Overview of Stability Mechanism and Recent Findings

A series of experimental measurements of flow and heat transfer under streamwise Go¨rtler vortices shows conclusively that the local surface heat transfer rates can exceed that of the turbulent boundary layer even in the absence of turbulence. We have observed unexpected behavior of heat transfer in a laminar boundary layer on a concave wall at low nominal velocity, a configuration ignored in the literature. In this situation, precise measurements of the wall heat flux show that the heat transfer enhancement is extremely elevated, above that corresponding to the case of a turbulent boundary layer on a flat plate. The nonlinearly developing steady streamwise vortex (primary instability) heat transfer can already bridge the local laminar to turbulent heat transfer values in the absence of turbulence. The analysis shows that for a range of velocities less than a certain critical velocity, the transitional boundary layer is dominated by centrifugal instability. However, the steady streamwise vortices, like steady Taylor vortices between coaxial rotating cylinders, are susceptible to secondary instabilities of the varicose and sinuous modes. In experiments both modes appear to coexist and cause waviness of the primary streamwise vortices. Other results confirm this discussion based on analysis of the influence of a forcing upstream disturbance.