Boundary layer transition with and without lateral divergence

Abstract

Information on the transition processes in turbomachine boundary layers is important in computing schemes for predicting blockage, flow losses, separation, etc. Actual transition on turbomachine blades does not conform to the usual correlations based on two?dimensional flows. Lateral flow divergence is known to affect boundary?layer growth and presumably could influence the transition process as well. This thesis describes an experimental investigation of transition in the presence of lateral flow divergence and convergence. Extensive measurements of transition were first made in two?dimensional flows to re?examine current concepts regarding intermittency, etc. The results suggest that a modified continuous breakdown hypothesis for spot formation fits the experimental results better than the hypothesis of concentrated breakdown. Correlations, which appear to be definitive, have been obtained for the start of transition in terms of pressure gradient and turbulence level. The end of transition appears to be independent of pressure gradient and depends only on the turbulence level. The introduction of lateral divergence and convergence has a much greater influence on the start of transition than pressure gradient alone, divergence tending to hasten the start of transition and convergence to delay it. The end of transition, which was independent of pressure gradient in two?dimensional flows, also appears to be uninfluenced by lateral divergence and to be uniquely a function of turbulence level. The similarity law for variation of intermittency through transition appears to be valid for flows with and without pressure gradients as well as for flows with and without divergence or convergence. The variation of skin friction through the transitional zone also appears to follow the same similarity law. The properties of spot formation appear to be more in accord with a modified continuous breakdown hypothesis than with that of concentrated breakdown.