Experimental investigation of the effect on boundary-layer transition of forward-facing steps, pressure gradient, and a non-adiabatic surface

Abstract

Experiments were conducted in the Cryogenic Ludwieg-Tube Göttingen to study the influence of surface imperfections, streamwise pressure gradient, Reynolds number, Mach number, and a non-adiabatic surface on boundary-layer transition. The temperature-sensitive paint technique has been used to detect transition. The influence on boundary-layer transition of surface thermal gradients was examined using a two-dimensional model with a natural laminar flow (NLF) airfoil as cross-section. Various combinations of flow conditions and angles of attack were employed to investigate different stability situations at chord Reynolds numbers up to 13 Mio. The unfavorable impact of surface temperatures larger than the adiabatic-wall temperature was identified to be dependent on the considered boundary-layer-stability situation. For certain stability situations, it was shown that a non-adiabatic model surface can strongly influence the boundary-layer transition location. When transition is induced by a positive (adverse) pressure gradient, however, the sensitivity to the wall-temperature ratio is reduced. Forward-facing steps of different height were installed on two spanwise-invariant wind tunnel models and their effect on transition was examined at high chord Reynolds numbers, various streamwise pressure gradients, and Mach numbers from 0.35 to 0.77. Transition was found to gradually move towards the step location with increasing step Reynolds numbers and relative step height. Larger flow acceleration had a favorable influence on the transition Reynolds number even in the presence of forward-facing steps, but its effect was less pronounced as step Reynolds number and relative step height were increased. The plots of the relative change in transition location as a function of step Reynolds number and relative step height gave good correlation of the results obtained with both wind tunnel models.