Eighteenth Symposium on Naval Hydrodynamics (1991) / Chapter Skim
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Vortex Ring Interaction with a Free Surface
Vortex Ring Interaction with a Free Surface
Pages 479-490
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From page 479... ...
Results are also presented on the interaction of vortex rings with clean and contaminated free surfaces at inclined incidence. The phenomenon of vortex lines breaking and attachment to the free surface is documented.
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From page 480... ...
Near the jet exit, vortex rings are generated that produce some surface deformations and waves, but the most dramatic signature is produced when these vortex rings open and reconnect with the free surface. To explore this mechanism in more detail, Bernal and Kwon1O and Kwonll experimented with a single ring moving parallel and at inclined incidence relative to the surface, respectively.
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From page 481... ...
Comparison with experiments was made by adjusting the vortex ring circulation and core parameter to obtain the same circulation and initial propagation speed as in the tests. For additional details on the experimental setup and numerical simulations the reader is referred to Song et al.20 Flow visualization Typical flow visualization results of the interaction at normal incidence are shown in Figure 2.
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From page 482... ...
Flow visualization of the interaction of a vortex ring with a free surface at normal Incidence. Top is a shadowgraph image of the surface, Bottom, underwater flow visualized by hydrogen bubbles.
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From page 483... ...
The solid lines are the calculated location of the vortex core. The open circles are the measured location of the cores determined from the hydrogen bubbles flow visualization.
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From page 484... ...
The reason for this discrepancy is the three-dimensional instability observed in the flow visualization study. Before the instability the outward motion of the core results from the induced velocity field caused by the image vorticity above the free surface.
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From page 485... ...
Bernal and Kwon10 experimented with vortex rings moving parallel 1~ to the surface. These experiments demonstrated that surface dimples result from vortex lines breaking and reconnecting with the free surface.
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From page 486... ...
Observed interaction outcome as a function of vortex ring Reynolds number and incidence angle. For vortex rings formed at conditions outside this region, the vortex lines of the upper core break and reconnect to the surface as in the other cases.
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From page 487... ...
Flow visualization photographs of the interaction of a vortex ring with a contaminated surface at a Reynolds number of 5,000, incidence angle of 20° and surface pressure of 12.7 dynes/cm. Shadowgraph visualizations of the surface are shown on top and Cross-section views at the bottom.
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From page 488... ...
The induced velocity field of the secondary vortex tends to stop the motion of the lower vortex core towards the surface. The surface signature in this case show well defined circular dimples as well as a highly distorted Reynolds ridge.
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From page 489... ...
Yes, Helmholz's theorem requires that vortex lines with a component normal to the free surface continue in the air above the free surface. We have not attempted to measure or in any way characterize the air motion above the free surface during the interaction.
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