Experimental study of bump effects on boundary-layer transition in compressible high Reynolds number flow

Abstract

The influence of surface bumps on boundary-layer transition was experimentally investigated in the present work. The experiments were conducted in a (quasi-) two-dimensional flow at low to high subsonic Mach numbers and chord Reynolds numbers up to 10 million in the low-turbulence Cryogenic Ludwieg-Tube Göttingen. Various streamwise pressure gradients relevant for natural laminar flow surfaces were implemented. Quasi-two-dimensional bumps, with a sinusoidal shape in the streamwise direction (positive half of a sine), fixed length and three different heights, were installed on a two-dimensional flat-plate model. The model was equipped with temperature-sensitive paint for non-intrusive transition detection and with pressure taps for the measurement of the surface pressure distributions. Boundary-layer transition was shown to occur at a more upstream location with increasing bump height-to-length ratio. This was mainly due to the local adverse pressure gradient on the downstream side of the bump, which was particularly pronounced in the case of the bump with the largest height-to-length ratio, thereby inducing boundary-layer separation (as verified via oil-film visualizations). In the case of the bump with the smallest height-to-length ratio, bump-induced transition was found to be dependent on global pressure gradient, Mach number and Reynolds number; however, the influence of these parameters on transition induced by bumps with larger height-to-length ratios was significantly reduced. The sensitivity of boundary-layer transition to the effect of the bumps was shown to be more pronounced with stronger global flow acceleration and at smaller Mach numbers.