Influence of forced perturbations in the stagnation region on Gortler instability

The growth of forced perturbations gene rated by a wire grid in a concave b oundary layer at supercritical Gortler numbers is experimentally investigat ed. Under certain conditions (sufficiently high wire Reynolds numbers), two pairs of Gortler vortices, instead of the single pair seen in previous exp eriments (Ajakh, A., Kestoras, M, D,, Peerhossaini, H., and Bahri, F., "An Experimental Study of Gortler Vortices," Proceedings of the annual Conferen ce of the French Society of Heat Transfer, Elsevier, Paris, 1995, pp, 272-2 77), have been observed in the concave boundary layer behind each triggerin g wire. Two generation mechanisms are examined for these two pairs of vorti ces 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 pa irs of Gortler vortices in the unstable boundary layer. The leading-edge vo rtices, on the contrary, influence the generation of Gortler vortices and c ontrol their wavelengths. However, there is a limit to this mechanism: tit very high values of wire Reynolds number Red, the flow behind the wires bec omes 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.