Effect of Isolated Micron-Sized Roughness on Transition in Swept-Wing Flows

Abstract

Boundary-layer transition-to-turbulence studies are conducted in the Arizona State University Unsteady Wind Tunnel on a 45-deg swept airfoil. The pressure gradient is designed so that the initial stability characteristics are purely crossflow dominated. Flow-visualization and hot-wire measurements show that the development of the crossflow vortices is influenced by roughness near the attachment line. Comparisons of transition location are made between a painted surface (distributed 9-μm peaks and valleys on the surface), a machine-polished surface (0.5-μm rms finish), and a hand-polished surface (0.25-μm rms finish). Then isolated 6-μm roughness elements are placed near the attachment line on the airfoil surface under conditions of the final polish (0.25-μm 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 the roughness element to document the enhanced vortices. These scans are made at several different chord locations to examine vortex growth