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Internal gravity waves generated by oscillations of a sphere

Published online by Cambridge University Press:  21 April 2006

J. C. Appleby  and
D. G. Crighton
J. C. Appleby
Affiliation:
Department of Engineering Mathematics, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne NE1 7RU, UK
D. G. Crighton
Affiliation:
Department of Applied Mathematics6 & Theoretical Physics, University of Cambridge, Silver Street, Cambridge CB3 9EW, UK
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Abstract

We consider the radiation of internal gravity waves from a spherical body oscillating vertically in a stratified incompressible fluid. A near-field solution (under the Boussinesq approximation) is obtained by separation of variables in an elliptic problem, followed by analytic continuation to the frequencies ω < N of internal wave radiation. Matched expansions are used to relate this solution to a far-field solution in which non-Boussinesq terms are retained. In the outer near field there are parallel conical wavefronts between characteristic cones tangent to the body, but with a wavelength found to be shorter than that for oscillations of a circular cylinder. It is also found that there are caustic pressure singularities above and below the body where the characteristics intersect. Far from the source, non-Boussinesq effects cause a diffraction of energy out of the cones. The far-field wave-fronts are hyperboloidal, with horizontal axes. The case of horizontal oscillations of the sphere is also examined and is shown to give rise to the same basic wave structure.

The related problem of a pulsating sphere is then considered, and it is concluded that certain features of the wave pattern, including the caustic singularities near the source, are common to a more general class of oscillating sources.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 183 , October 1987 , pp. 439 - 450
Copyright
© 1987 Cambridge University Press

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References

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