Influence of forced perturbations in the stagnation region on Görtler instability

The growth of forced perturbations generated by a wire grid in a concave boundary layer at supercritical Gortler numbers is experimentally investigated. Under certain conditions (sufficiently high wire Reynolds numbers), two pairs of Gortler vortices, instead of the single pair seen in previous experiments, have been observed in the concave boundary layer behind each triggering wire. Two generation mechanisms are examined for these two pairs of vortices associated with Benard-Karman streets and leading-edge vortices. It is shown that whereas the Benard-Karman streets are generated behind the wires, they do not penetrate the boundary layer and, thus, cannot trigger two pairs of Gortler vortices in the unstable boundary layer. The leading-edge vortices, on the contrary, influence the generation of Gortler vortices and control their wavelengths. However, there is a limit to this mechanism: At very high values of wire Reynolds number Re d , the flow behind the wires becomes turbulent, and Gortler vortices cannot be generated. It is also found that the wavelength of Gortler vortices is constant and independent of the diameter of the wire that triggers them