Vortical Airfoil Wake Structure in Non-Uniform Approach Flow

Phase-resolved spatial map of the streamwise velocity from LDV measurements in the wake of an airfoil pitching at high reduced frequency placed inside a two-stream shear layer appears to show the signature of only a single sign vortex per oscillation cycle. This is curious as it is counter to the alternating-sign vortex pair per oscillation cycle that is characteristic of the wake structure in uniform flow. To elucidate the flow physics present in the experiment, computations are carried out in an attempt to mimic the conditions of the experiment. The computed wake structure provides a clue to the actual vortical field and how this particular wake vortex arrangement can lead to incorrect inference based on the incomplete information from phase-resolved map of streamwise velocity.