Effect of isolated micron-sized roughness on transition in swept-wing flows

Boundary-layer transition-to-turbulence studies are conducted in the Arizon a State University Unsteady Wind Tunnel on a 45-deg swept airfoil. The pres sure gradient is designed so that the initial stability characteristics are purely crossflow dominated. Flow-visualization and hot-wire measurements s how that the development of the crossflow vortices is influenced by roughne ss near the attachment line. Comparisons of transition location are made be tween a painted surface (distributed 9-mu m peaks and valleys on the surfac e), a machine polished sur face (0.5-mu m rms finish), and a hand-polished surface (0.25-mu m rms finish). Then isolated 6-mu m roughness elements are placed near the attachment line on the airfoil surface under conditions of the final polish (0.25-mu m rms). These elements create an enhanced packet of stationary crossflow waves, which results in localized early transition . The diameter, height, and location of these roughness elements are varied in a systematic manner. Spanwise hot-wire measurements are taken behind th e roughness element to document the enhanced vortices. These scans are made at several different chord locations to examine vortex growth.