TURBULENT VORTEX RING FREE-SURFACE INTERACTION

The interaction of turbulent vortex rings that approach a clean water surface under various angles is experimentally investigated. The tempo ral evolution of the vortex rings with an initial Reynolds number of R e-0 = 7500 is characterized by the laminar/turbulent transition and as ymptotic relaminarization of the flow. Using the shadowgraph technique , two major flow cases were identified as a result of the vortex-ring/ free-surface interaction: a trifurcation case that results from the in teraction during the transition stage, and a bifurcation case that evo lves during the filly-developed turbulent stage. In contrast to the la minar interaction, the turbulent bifurcation pattern is characterized by the reconnection and mutual interaction of many small-scale structu res. Simultaneous digital particle image velocimetry (DPIV) and shadow graph measurements reveal that the evolution of the small-scale struct ures at the free surface is strongly dominated by the bifurcation patt ern, which in turn is a consequence of the persisting laminar sublayer in the core regions of the reconnected turbulent vortex loops.