Tollmien-Schlichting and Crossflow Instability Mode Selection on Swept Wings

Abstract

Transport aircraft consume significant fuel and produce commensurate pollution. In-creasing the extent of laminar flow over wings is one strategy to reduce skin friction drag and thus decrease fuel consumption. The boundary layer on swept wings can transition to turbulence due to Tollmein-Schlichting (TS) waves and stationary crossflow vortices. The growth of each instability is well-modeled by stability theory codes, particularly those driven by the parabolized stability equations. However, the realization of a particular mode or a mix of both is highly dependent on mode selection: the combined effects of the receptivity process and initial growth rates. This phenomenon has not been rigorously explored. What is needed is an experiment that demonstrates the possible modes in a range of sweep, pressure distribution, and disturbance conditions to guide simulations regarding what instabilities may exist in what conditions. To achieve this, a slotted, natural-laminar flow airfoil model and a mount that provides variable sweep and pitch were designed and installed in the Klebanoff-Saric Wind Tunnel. Modes were forced according to their receptivity properties. When the TS mode was amplified, 2-D roughness strips and sound pressure were applied; crossflow was forced with DRE arrays. Under single-amplified conditions, TS and crossflow modes were individually selected in the boundary layer. When both modes were amplified, TS and crossflow were selected in-dividually and simultaneously; modes coexisted in the boundary layer. Mixed-mode forcing produced weak stationary crossflow vortices and strong TS waves that nonlinearly inter-acted and led to an earlier breakdown compared to single-mode forcing. Altogether, these results mapped out the possible modes within the range of parameters, building confidence in stability calculations that can be extended to design-operating conditions.