Combination of a time- and space-resolved measurement technique for laminar-turbulent transition detection

Abstract

Laminar flow technology is a promising way to significantly reduce aerodynamic drag. The underlying mechanisms that lead to laminar-turbulent transition are important knowledge required for accurate design development of low-drag aircraft. Still, detailed information on both boundary-layer stability and transition, is challenging to acquire simultaneously in experiments at flight-relevant flow conditions. This research combines two measurement techniques to develop a method for simultaneous measurement of boundary-layer stability and transition at flight-relevant Mach and Reynolds numbers. Measurements were conducted in the Cryogenic Ludwieg-Tube Göttingen (DNW-KRG) for chord Reynolds numbers ranging from 3.5 to 8 million, Mach numbers from 0.3 to 0.75 and various stream-wise pressure gradients. A 2D wind-tunnel model (chord length 0.2 m), designed to have a large area of uniform pressure gradient on the examined surface, is equipped with both temperature-sensitive paint (TSP) for non-intrusive transition detection and surface hot-films for highly time-resolved measurement of boundary-layer disturbances. For a large part of the tested conditions, both measurement techniques show a noteable overlap in the transition regions. For the first time Tollmien-Schlichting waves could be identified with frequencies above 30 kHz for flow conditions relevant for transport aircraft at high Mach numbers, agreeing with linear stability theory. The described measurement technique enables an experimental proof to predicted mechanisms leading to transition for flight relevant flow conditions.