第85回流体懇話会のご案内
平成15年3月18日
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講師：Frederic MOULIN(東京大学 海洋研究所)
演題：Study of the propagation and breaking of inertia-gravity waves
in a vortex flow
日時：平成15年4月18日(金) １６：３０〜１８：００
場所：電気通信大学 東4号館8階AV会議室
内容：Edges of vortices are examples of dynamical barriers, and prevent
fluid captured inside the vortex from mixing with the ambient. The
existence of a cross-barrier transport material is relevant to large-
scale geophysical flows, in which vortices are redundantly present.
Breaking internal waves at the vortex edge can enhance such a transport.
We have experimentally investigated wave-vortex interactions in a rotat-
ing stratified fluid, and address questions about wave induced mixing in
the vortex.
We consider the interaction of planar waves with baroclinic vortices,
continuously forced by a small rotating disk, and with freely evolving
barotropic vortices initially generated by an elongated sink. Only
cyclonic vortices are considered. The wave pattern observed in the
experiments is well captured by numerical simulations of ray tracing,
based on WKB theory. Waves interacting with a baroclinic vortex are
wrapped around the vortex center and trapped along inclined critical
layers, where their wavelength decreases towards zero without trigger-
ing wave breaking. Waves propagating against a barotropic vortex are
trapped along almost vertical critical layers, leading to wave breaking
in case the viscous damping time-scale is long compared to the typical
time-scale over which the wavelength decreases.
In the experiments with barotropic vortices, the wave breaking is due
to a local inversion of the density field, which drives a buoyancy
induced instability, and leads eventually to a deposit of negative
azimutal momentum in the outer part of the vortex. Meanwhile, mixing is
driven by two distinct processes: vortex streamlines are perturbed by
periodic waves leading to bands where chaotic mixing occurs on a large
time scale; wave breaking leads to turbulent patches of which the
impact depends on the amount of energy provided by the breaking waves.
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