THE INTERACTION OF SPATIALLY MODULATED VORTEX PAIRS WITH FREE SURFACES

Spatially modulated vortex pairs were generated below a free surface b y two counter-rotating flaps whose edges approximate a sinusoid. The s urface interactions of the vertically approaching vortex pairs were vi sualized by the shadowgraph technique. Two limiting cases were investi gated in detail: the interaction with a surfactant-rich (contaminated) surface and with a surfactant-poor ('clean') surface. In the latter c ase shadowgraph images showed that the underlying vortex core formed a line of circular surface depressions. Subsequent measurements of the temporally evolving velocity fields using digital particle image veloc imetry (DPIV) of the vortex pair cross-sections and the subsurface pla ne confirmed the connection process of the main vortex core with the s urface. As a result of the connection the initially modulated vortex t ube was broken into a line of U-vortices. In the presence of surfactan ts this connection could not be observed; rather a Reynolds ridge (or stagnation line) was formed and a very weak connection of the secondar y separation vortex could be seen in the shadowgraphs as well as measu red with the time-resolved DPIV technique. A prerequisite for connecti on of the vortex with the surface is that the flow's kinematics force the vortex core, that is, regions of concentrated vorticity, toward th e surface. The ensuing locally concentrated viscous flux of surface-pa rallel vorticity through the surface is balanced by a local surface de celeration. Surface-normal vorticity appears on each side of the decel erated region whose gradually increasing circulation is directly balan ced by the loss of circulation of the surface-parallel vortex. However , the shear forces caused by small amounts of surface contamination an d its associated subsurface boundary layer inhibit the connection proc ess by preventing the essential viscous flux of parallel vorticity thr ough the surface. Instead, the subsurface boundary layer is associated with a flux of parallel vorticity into the surface which then concent rates into the observable secondary separation vortex.