Steffen Risius

Effect of Forward-Facing Steps on Boundary Layer Transition at a Subsonic Mach Number

Experiments were conducted at a subsonic Mach number in the Cryogenic Ludwieg-Tube Gottingen to study the influence of surface imperfections on boundary layer transition. 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 and various streamwise pressure gradients. Transition, detected non-intrusively by means of the Temperature-Sensitive Paint technique, was found to gradually move upstream 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 also increased the sensitivity of boundary layer transition to the surface excrescences. The plots of the relative change in transition location as a function of step Reynolds number and relative step height gave good correlations of the results obtained with both wind tunnel models. The correlations were practically independent of the streamwise pressure gradient. Criteria for allowable tolerances on low-sweep Natural Laminar Flow surfaces can now be established from the functional relations determined in this work.

Experimental investigation of Mach number and pressure gradient effects on boundary layer transition in two-dimensional flow

The influence of Mach number (\(M = 0.35\)–0.65), chord Reynolds number (\(Re_c=6\times 10^6\) to \(10\times 10^6\)) and pressure gradient (\(dc_p/dx = -0.6\)–0.07 m\(^{-1}\)) on laminar-turbulent boundary layer transition was experimentally investigated in the Cryogenic Ludwieg-Tube Gottingen (DNW-KRG). For this investigation the existing two-dimensional wind tunnel model, PaLASTra, which offers a quasi-uniform streamwise pressure gradient, was modified in order to reduce the size of the flow separation at its trailing edge. The streamwise temperature distribution and the location of laminar-turbulent transition were measured by means of temperature-sensitive paint (TSP). It was found that the transition Reynolds number exhibits a linear dependence on the pressure gradient, characterized by the Hartree parameter, and that an increasing Mach number leads to a linear decrease of the transition Reynolds number. The latter effect is likely due to an increase of the total pressure turbulence level with Mach number in DNW-KRG. The measured pressure and temperature distributions served as input for boundary layer calculations and linear-stability analysis. N-factors were calculated according to compressible and incompressible stability theory. At zero pressure gradient a critical N-factor of approximately 9.5 and 9.0 was found for incompressible and compressible calculations, respectively.