Conditionally sampled measurements in a heated turbulent boundary layer: curvature and free-stream turbulence effects

The effects of the introduction and removal of concave curvature on the wake region of a uniform-heat-flux turbulent boundary layer are studied. Experiments are conducted with negligible streamwise pressure-gradient and with two levels of free-stream turbulence intensity (FSTI), 0.6% and 8%. The boundary layer first naturally grows over a concave wall then proceeds onto a flat, recovery wall. Measurements are separated, based on temperature fluctuation level, into regions of laminar (external flow which is being entrained) and turbulent (flow with wall-generated vorticity) flows. Results in the low-FSTI case show that the intermittent region where this separation can be done is as thick as the 99.5% momentum boundary layer and is centered at the edge of the boundary layer. The effect of sustained concave curvature on this region appears to be minimal. Removal of the concave curvature, is experienced by the boundary layer as it passes from the curved wall to the recovery profiles constructed as ensemble averages of the laminar flow retain their potential-flow velocity distribution. This flow is more unsteady than is the laminar flow external to this region, however. With increased levels of FSTI (8%), removal of curvature effects a rise in intermittency, contrary to observations in the low-FSTI case. The differences is in the nature of the external flow entrained into the boundary layer. As a consequence, elevated shear stresses are found within the entrained free-stream zones of the high-FSTI case.