Experimental investigation of unit Reynolds number and pressure gradient effects on two-dimensional boundary layer transition

Abstract

Natural laminar flow technology is a promising way to reduce wall shear stress and fuel consumption of commercial aircraft. In low free-stream disturbance environments, as encountered in flight, Tollmien-Schlichting (T-S) waves are the dominating mechanism to trigger 'natural' laminar-turbulent transition for boundary layers developing on smooth two-dimensional surfaces. However, most conventional wind tunnels have disturbance levels much higher than in flight which can cause the mechanism of 'natural' transition to be changed or bypassed. Stability theory has been used to demonstrate that on a flat plate in subsonic flow an increasing unit Reynolds number leads to an increase in transition Reynolds number, which is also referred to as 'unit Reynolds number effect'. However, there is only limited experimental verification of this effect for highly subsonic conditions in low turbulence environments. In this work we show a systematic experimental investigation of pressure gradients (dcp/dx = -1.2 to 0.25 1/m), Mach numbers (M = 0.35 to 0.65) and unit Reynolds numbers (Re=17.5E6 to 80.0E6 1/m).