Free-stream turbulence and concave curvature effects on heated, transitional boundary layers

An experimental investigation of transition in concave-curved boundary layers at two free-stream turbulence levels (0.6 and 8.6 percent) was performed. For the lower free-stream turbulence intensity case, Gortler vortices were observed in both laminar and turbulent flows using liquid crystal visualization and spanwise velocity and temperature traverses. Transition is thought to occur via a vortex breakdown mode. The vortex locations were invariant with time but were nonuniform across the span in both the laminar and turbulent flows. The upwash regions between two vortices were more unstable than were the downwash regions, containing higher levels of u' and u'v’, and lower skin friction coefficients and shape factors. Turbulent Prandtl numbers, measured using a triple-wire probe, were near unity for all post-transitional profiles, indicating no gross violation of Reynolds analogy. No streamwise vortices were observed in the higher turbulence intensity case. This may be due to the high eddy viscosity, which reduces the turbulent Gortler number to subcritical values, thus eliminating the vortices, or due to an unsteadiness of the vortex structure that could not be observed by the techniques used. Based upon these results, predictions that assume two-dimensional modeling of the flow over a concave wall with high free-stream turbulence levels, as on the pressure surface of a turbine blade, seem to be adequate-there is no time-average, three-dimensional structure to be resolved. High levels of free-stream turbulence superimposed on a free-stream velocity gradient (which occurs within curved channels) cause a cross-stream transport of momentum within the flow outside the boundary layer. The total pressure within this region can rise above the value measured at the inlet to the test section.