Heat Transfer in Separated Flows at High Levels of Free-Stream Turbulence

In the present paper, a comparative analysis of the influence of free-stream turbulence on the separated flows past obstacles is given. As the obstacles, a downward-facing step, a flat rib installed at different orientations to the free-stream direction, a system of several ribs, and a cross-flow trench with vertical or inclined walls are considered. The experimental results obtained in the present study are compared to data previously reported by other workers. The structure of the separated flow at an enhanced level of free-stream turbulence is compared to the flow under low-turbulence conditions in terms of the characteristic length of the separation zone, mixing-layer parameters, and pressure distributions. The emphasis is on the thermal characteristics, including the profiles of temperature across the shear layer, the distributions of temperature over the streamlined surface, and the local and mean heat-transfer coefficients. It is shown that the effect of enhanced free-stream turbulence on the separated flow is much more pronounced than that on the boundary-layer flow over a flat surface. For separated flow, this effect is manifested more clearly behind rib than behind step. The largest heat-transfer intensification ratios due to external turbulence were found in the cross-flow trench and in the system of ribs.