Turbulent Vortex Ring I Free Surface Interaction

Graduate Aeronautical Laboratories, Californ ia Institute ol Technology, Pasadena. CA 91125 The interaction of turbulent vortex rings that approach a clean water surface under various angles is experimentally investigated. The temporal evolution of the vortex rings with an initial Reynolds number of Re0 = 7500 is characterized by the Laminar/ turbulent transition and asymptotic relaminarization of the flow. Using the shadowgraph technique, two major flow cases were identified as a result ~f the v~rtex-ri~gl free-surface interaction: a trifurcation case that results from the tnteractwn durmg the transition stage, and a bifurcation case that evolves during the fully-developed turbulent stage. In contrast to the Laminar interaction, the turbulent bifurcation pattern is characterized by the reconnection and mutual interaction of many small-scale structures. Simultaneous digital particle image velocimetry ( DPN) and shadowgraph meastlrements reveal that the evolution of the small-scale structures at the free surface is strongly dominated by the bifurcation pattern, which in tum is a consequence of the persisting laminar sublayer in the core regions of the reconnected turbulent vortex loops.