Dynamics of co-rotating vortex pairs in the wakes of flapped airfoils

Omer Savas, University of California, Berkeley

The behavior of a pair of co-rotating vortices in the wake of a flapped airfoil is experimentally studied in a water towing tank. Three rectangular airfoils with an aspect ratio of 6 and flap spans of 0%, 30%, and 67% are used in the experiments. Reynolds numbers based on total circulation of the vortices range from 18,000 to 65,000. Velocity field and its gradient are obtained from PIV images using an adaptive Lagrangian parcel tracking algorithm. Several observations are noted in the vortex wake. All vortices show a Lamb-Oseen circulation distribution when they are clearly identifiable. Further, vortices from the flapless wing exhibit Lamb-Oseen velocity and vorticity distributions which grow slowly. Merger of the co-rotating vortex pair is observed at all circulation Reynolds numbers. A key observation is that the merger occurs at around 0.8 orbit period of a co-rotating vortex pair. The merger process appears to be inviscid and three-dimensional. First order statistics of the flow field remain invariant during the merger. The higher order moments of the vorticity distribution show strong time dependence which implies three dimensionality of the flow resulting from vortex stretching. The strengths of the individual vortices before merger are constant, and the total circulation after merger remains constant within the range of observations. The trajectory of the center of vorticity remains unaffected by the merger process. Kinetic energy and angular momentum of the flow are conserved during the merger. The merger is preceded by a splitting of the weaker vortex into filaments. Depending on the relative strengths of the vortices, the splitting can be in the radial direction, the axial direction, or somewhere in between. Mechanisms contributing to the merger dynamics are discussed.

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Last Modified: October 14, 1997