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

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-/xm peaks and valleys on the surface), a machine-polished sur-face (0.5-/xm rms finish), and a hand-polished surface (0.25-/im rms finish). Then isolated 6-/im roughness elements are placed near the attachment line on the airfoil surface under conditions of the final polish (0.25-/zm 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. Nomenclature c = chord D = diameter of roughness k = height of roughness N = linear stability amplification factor, L(uf/ufQ) Rec = chord Reynolds number Rek = roughness Reynolds number x = distance along chord, m jctr/c = transition location y = wall-normal coordinate, m z = distance along span, m a = angle of attack A = stationary crossflow vortex wavelength along span Act = stationary crossflow vortex wavelength CD = traveling-wave frequency I.