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

In the presence of surface imperfections, the extent of the laminar flow regions on natural laminar flow (NLF) aircraft components can be significantly reduced, as compared to design. The present work focuses on surface bumps, which can occur, e.g., at or between structural joints, ribs and stringers. Bumps can induce a marked amplification of boundary-layer instabilities and thus lead to premature transition to turbulence. The influence of bumps on boundary-layer transition was systematically studied in this work in combination with the effect of variations in the following parameters: streamwise pressure gradient, freestream Mach number and chord Reynolds number. The experiments were conducted in a quasi-two-dimensional flow in the Cryogenic Ludwieg-Tube Göttingen, where chord Reynolds numbers up to 10 million, freestream Mach numbers up to 0.77 and various streamwise pressure gradients were examined. Essentially two-dimensional bumps, with a sinusoidal shape in the streamwise direction (positive half of a sine), fixed length and three different heights, were mounted on a two-dimensional flat-plate configuration. The wind-tunnel model was instrumented with a temperature-sensitive paint for non-intrusive, global transition detection, and also with pressure taps for the measurement of the surface pressure distributions. Boundary-layer transition was found to occur at a more upstream location with increasing bump height-to-length ratio. Transition induced by the small bump was shown to be dependent on streamwise pressure gradient, freestream Mach number and chord Reynolds number. In the case of bumps with larger height-to-length ratios, however, the influence of the aforementioned parameters on bump-induced transition was significantly reduced. The measured surface pressure distributions served also as an input for boundary-layer computations, and the calculated boundary-layer velocity profiles were analyzed according to linear, local stability theory. The results of the stability analysis were then correlated with the transition locations measured in the experiments. Detailed results will be presented in the final version of this contribution, where the importance of considering the effect on transition of pressure gradient, Mach number and Reynolds number in criteria for allowable size of bumps on NLF surfaces will be also discussed.