Session Ci - Vortex Dynamics.
ORAL session, Sunday, November 23
309, Moscone Center
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 1.8\cdot 10^4 to 6.5\cdot 10^4. Velocity vector fields and their gradients are derived from PIV images using an adaptive Lagrangian parcel tracking algorithm. The wakes behave inviscidly at these Reynolds numbers. Merger of the co-rotating vortex pair is observed at all Re_\Gamma. A key observation is that the merger occurs within the time required for one vortex to rotate around the other. The merger appears 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.