Surface Temperature Effects on Boundary-Layer Transition at Various Subsonic Mach Numbers and Streamwise Pressure Gradients

The effect on transition of a non-adiabatic surface was systematically studied in the present experimental work in combination with the influence of variations in Mach number and pressure gradient. The investigations were carried out in a (quasi-) two-dimensional flow at four different subsonic Mach numbers and chord Reynolds numbers up to 13 million. Various streamwise pressure gradients and wall temperature ratios were examined. The experiments were conducted in the low-turbulence Cryogenic Ludwieg-Tube Gottingen on a two-dimensional flat-plate configuration designed for an essentially uniform pressure gradient on the model upper surface. The model was instrumented with a temperature-sensitive paint to measure globally and non-intrusively the surface temperature and thus the boundary-layer transition. A marked influence of a variation in the wall temperature ratio on transition was observed for all considered Mach numbers, being this effect more pronounced at lower Mach numbers. The measured transition locations were also correlated with the results of linear local stability analysis. Smaller disturbance amplification factors were found at transition for larger Mach numbers and, in most of the examined cases, for smaller wall temperature ratios and stronger flow acceleration.