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

Abstract

Drag reduction by means of Natural Laminar Flow technology is a promising way to reduce wall shear stress, which can significantly reduce fuel consumption of commercial aircraft. Tollmien-Schlichting (T-S) waves are the dominating mechanism to inducing 'natural' transition in low free-stream disturbance environments for boundary layers developing on 'smooth' two-dimensional surfaces. At these conditions the linear stability theory, in particular the e^N-method, predicts a stabilizing effect of compressibility with an increase in Mach number. However, there is no experimental verification of this effect for Mach numbers up to transonic conditions. In this work we show experimental evidence that may contradict a stabilizing effect of compressibility. A systematic investigation of pressure gradients (dcp/dx = -1.2 – 0.25 1/m), Mach numbers (Ma = 0.35 – 0.77) and chord Reynolds numbers (Rec = 3.5 – 16.0 million, with c = 0.2 m) will be presented.