Bypass Transition in Boundary Layers Including Curvature and Favorable Pressure Gradient Effects

Abstract

Recent experimental studies of two-dimensional boundary layers undergoing bypass transition have been reviewed to attempt to characterize the effects of free-stream turbulence level, acceleration, and wall curvature on bypass transition. Results from several studies were cast in terms of “local” boundary layer coordinates (momentum and enthalpy thickness Reynolds numbers) and compared. In unaccelerated flow on flat walls, skin friction coefficients were shown to match those from a laminar integral solution before transition and quickly adjusted to match those from a fully turbulent correlation after transition. Stanton number data also matched a correlation in the laminar region, but do not match correlation values so well in the turbulent region. The data showed that the relationship between skin friction coefficient and momentum thickness Reynolds number is unaffected by streamwise acceleration. Stanton numbers were strongly affected by acceleration, however, indicating a breakdown in Reynolds analogy. Concave curvature caused the formation of Görtler vortices, which strongly influenced the skin friction. Convex curvature had an opposite, and lesser effect. The location and length of the transition region generally followed the expected trends as free-stream turbulence level, curvature, and acceleration were varied; the onset location and the transition length were extended by acceleration and convex curvature and reduced by concave curvature and enhanced turbulence. When individual cases were compared, some inconsistencies were observed. These inconsistencies indicate a need to characterize the flows to be compared more completely. Better spectral and length scale measurements of the free-stream disturbance would help in this regard. Within the transition region, the intermittency data from all the cases on flat walls (no curvature) were consistent with an intermittency distribution from the literature. Turbulent spot production rates were shown to be mostly dependent on free-stream turbulence, with a noted increase in spot production rate due to concave curvature and little effect of convex curvature. The acceleration effect on spot production rate was small for the cases studied.