Boundary layer transition over a concave surface caused by centrifugal instabilities

The laminar-to-turbulent transition process over a concave surface is studied using direct numerical simulations (DNS). It is found that the flow passing over such surface is highly susceptible to develop centrifugal instabilities in the form of Görtler vortices. Transition is triggered by means of wall-roughness elements which also serve to preset the wavelength of the Görtler vortices that remains constant during their streamwise development. This allows to obtain a clear spanwise characterization of the Görtler boundary layer, and its breakdown into turbulence. The different regions encountered in the transition process, i.e., linear, nonlinear, transition and fully turbulent, are identified and characterized. Parametric studies are presented showing that the transition starting point is delayed when the radius of curvature is increased, however, it occurs at the same critical Görtler number. Additionally, an increase of the height of the wall-roughness elements advances the transition onset upstream. Moreover, a linear relation between the critical Görtler number and the Reynolds number based on the wall-roughness streamwise location has been found. Furthermore, it is observed that, compared to a wall-roughness bump element, a bump-dimple geometry is more efficient for exciting the transition process.