Unsteady Stagnation-Point heat transfer due to the motion of freestream vortices

The effects of freestream vortices on the flow and heat transfer in a stagnation-point boundary layer are considered for the case of a circular cylinder exposed to an approaching pair of counter-rotating line vortices imbedded in an otherwise uniform crossflow. Series expansions are used to formulate a description of the unsteady hydrodynamic and thermal boundary layers in the neighborhood of the stagnation point, and the resulting governing equations are solved numerically for several vortex flow situations. Perturbations in stagnation-point heat transfer of ±40% are found to be induced by vortex pairs that have strength and separation distance suggested by recent measurements and experiments. In addition, reversed flow in the boundary layer at the stagnation point is found to be produced in cases of practical interest. The results indicate that the effects of freestream vortices may be significant in situations of current engineering interest, such as the interaction of unsteady wakes of turbomachinery blades with downstream surfaces.