On excitation of Görtler vortices due to scattering of free-stream vortices on surface non-uniformities

The study is devoted to experimental investigation of processes of excitation of non-stationary Gortler vortices in a boundary layer on a concave wall by vortices convected in free stream. In the present set of experiments, performed at controlled disturbance conditions, several cases of possible receptivity mechanisms have been considered: (i) scattering of 2D freestream vortices on localized 3D surface non-uniformities, (ii) scattering of 3D freestream vortices on localized 2D surface non-uniformities, (iii) scattering of 2D freestream vortices on uncontrolled 3D base-flow non-uniformities and (iv) scattering of 3D freestream vortices on natural 2D base-flow non-uniformities. Cases (i) and (ii) represent localized types of the vortex-receptivity mechanisms, while cases (iii) and (iv) correspond to distributed type of vortex-receptivity mechanisms. Accurate measurements are carried out in a broad range of experimental parameters. It is found that in cases (i), (ii), and (iii) no generation of measurable Gortler vortices is observed. However, the interaction of 3D convected vorticity with the growing boundary layer (case (iv)) does lead to excitation of rather intensive Gortler vortices. In this case the distributed receptivity mechanism is able to change significantly the growth rates of the exciting unsteady Gortler vortices. The corresponding receptivity coefficients are evaluated.