Forced vortex dynamics in the surf zone

Forced vortex dynamics in the surf zone.

A relation between the spectra of the forcing and the spectra of the fluid response is a primary goal in all areas of fluid dynamics. In the surf zone, these efforts are made somewhat more difficult, due to the large range of temporal scales that must be resolved as well as the interaction between 2D divergent phenomena (surface gravity waves, infragravity waves, edge waves, tidal waves) and shear flows and eddies, all of which occur simultaneously under strong enough forcing conditions. When topography strongly controls the flow organization, such as on a rip channeled beach, wave and current models with differing forcing mechanisms- either calculating each individual breaking wave crest explicitly, or only calculating the mean wave properties that vary slowly in time and space, are able to reproduce Eulerian and Lagrangian flow statistics close to drifter and current meter observations. Each model agrees with field observations of large, coherent eddies that entrain 10 4 m3 of water in the nearshore. These circulation cells are strongly tied to the topography, and behave like coupled oscillators even when the wave forcing is monochromatic, and exhibit natural oscillations which depend only on a typical eddy length scale and strength of forcing. When wave groups are incident over the same topography, the resulting vorticity field spectra shows a significant peak at the wave group frequency, due to modulation of the breaker line and wave advection. For a narrow band of frequencies about f = 0.004 Hz, resonant edge waves are generated that also modulate the vorticity field at harmonics of the forcing frequency. The natural oscillations are detectable for all ranges of the wave group forcing frequency, but suppressed when the rip current system is forced near resonance.

Author: Joseph D. Geiman
Publication Date: October 18, 2012

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