Role of Micron - Sized Roughness in Swept - Wing Transition

Stability and transition experiments are conducted in the Arizona State University Unsteady Wind Tunnel on a 45° swept airfoil. The pressure gradient is designed so that transition and stability are purely crossflow-dominated. Flow visualization and hot-wire measurements show that the development of the crossflow vortices is influenced by roughness near (not at) 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 - 9 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 amplify a centered stationary crossflow vortex and its neighbors, resulting in localized early transition. The diameter and height 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.