ON THE SPECTRAL DISTRIBUTION OF THE MODES IN NONLINEAR GORTLER INSTABILITY

Longitudinal vortices generated by Gortler instability in a concave wa ll boundary layer are studied experimentally. The vortices were trigge red by a wire grid upstream of the test-section leading edge. The vort ices are visualized and their velocity field is measured in four axial positions. The measurements show weakly nonlinear growth and also non linear saturation of perturbations. Wall-normal and spanwise gradients of the velocity field calculated from the velocity measurements show that the spanwise gradients grow faster than the wall-normal gradients . It is thus inferred that the sinuous mode associated with the spanwi se gradient will be the dominant instability mode. Spectral analysis o f the spanwise profiles of the streamwise velocity show that seven mod es suffice to represent the velocity field in the nonlinear regime. Ho wever, the amplitudes of the modes present in the velocity profiles va ry with distance from the wall as with distance from the leading edge. (C) 1998 Elsevier Science Inc. All rights reserved.