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Two vertical, rotating flaps are used to generate a vortex pair beneath a free surface. Vortex pair formation, propagation, and interaction with a free surface are described. Numerical simulations for inviscid flow about a constant upwash sheet of vorticity beneath a free surface agree with experiment up to the time that turbulent mixing occurs during interaction with the free surface. The spacing between the vortex pairs then becomes larger than the calculated spacing. The experiments and lines of marked particles included in the simulations show fluid ingestion and transport toward the free surface.
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Willmarth et al. (1989) studied this question.
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