Riblet effects on Gortler vortex development over a concave surface.

摘要

Riblet effects on Gortler vortex development in laminar and transitional flows were measured using three-dimensional laser-Doppler anemometry. Three freestream velocities were tested such that riblet spacing corresponded to Gortler vortex (GV) wavelengths which presumably would be either strongly amplified, weakly amplified, or attenuated by centrifugal forces on a curved surface. Experiments revealed the development of paired, counterrotating vortices within the riblet valleys over both flat and concave surfaces. For riblet spacings equivalent to weakly or non-amplified GV wavelengths, GV development on a curved surface was above the riblets and was delayed by the addition of riblets. The GV wavelength was larger than the riblet spacing, but was approximately the same as for the smooth surface. For the strongly amplified case, streamwise vorticity developed earlier than for the smooth plate and was confined to paired, counterrotating, riblet-valley vortices with wavelength equal to the riblet spacing. Boundary layer transition was accelerated by the addition of riblets to the curved surface for the strongly amplified case, but was delayed for the other two cases. Thus, riblets with spacing equal to strongly amplified GV wavelengths accelerate boundary layer transition; whereas, riblet spacings equal to smaller, less strongly amplified or attenuated GV wavelengths delay transition. Surface skin friction was increased in all test cases by the addition of riblets.