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 Göttingen (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.