Bump-Induced Transition in Compressible High Reynolds Number Flow: Experimental Results and Correlation with Linear Stability Analysis

The effect of surface bumps on boundary-layer transition was systematically studied in this joint experimental and numerical work in combination with the influence of variations in streamwise (global) pressure gradient, freestream Mach number (up to 0.77) and chord Reynolds number (up to 10 · 106). The experiments were conducted in a (quasi-) two-dimensional flow in the Cryogenic Ludwieg-Tube Göttingen. Quasi-two-dimensional bumps, with a sinusoidal shape in the streamwise direction, fixed length and three different heights, were installed on a two-dimensional flat-plate model. The model was equipped with temperature-sensitive paint and pressure taps for the measurement of surface temperature and pressure distributions, respectively, which also served as inputs for laminar boundary-layer computations. The linear instability characteristics of the boundary layer were analyzed by both compressible local stability theory and parabolized stability equations. For the case of the small bump, over which the boundary layer did not undergo separation, the N-factors of Tollmien-Schlichting waves from local stability theory were also correlated with the experimental transition locations. This enabled to study the dependency of the transition N-factor on Mach number, Reynolds number and global pressure gradient in the presence of the bump