## Trapping of waves by a submerged elliptical torus

preprint

posted on 24.01.2006 by Maureen McIver, R. Porter#### preprint

Preprints are manuscripts made publicly available before they have been submitted for formal peer review and publication. They might contain new research findings or data. Preprints can be a draft or final version of an author's research but must not have been accepted for publication at the time of submission.

An investigation is made into the trapping of surface gravity waves by totally submerged
three-dimensional obstacles and strong numerical evidence of the existence of trapped modes
is presented. The specific geometry considered is a submerged elliptical torus. The depth
of submergence of the torus and the aspect ratio of its cross-section are held fixed and a
search for a trapped mode is made in the parameter space formed by varying the radius of
the torus and the frequency. A plane wave approximation to the location of the mode in this
plane is derived and an integral equation and a side condition for the exact trapped mode
are obtained. Each of these conditions is satisfied on a different line in the plane and the
point at which the lines cross corresponds to a trapped mode. Although it is not possible to
locate this point exactly, because of numerical error, existence of the mode may be inferred
with confidence as small changes in the numerical results do not alter the fact that the lines
cross.
If the torus makes small vertical oscillations, it is customary to try and express the fluid
velocity as the gradient of the so-called heave potential, which is assumed to have the same
time dependence as the body oscillations. A necessary condition for the existence of this
potential at the trapped mode frequency is derived and numerical evidence is cited which
shows that this condition is not satisfied for an elliptical torus. Calculations of the heave
potential for such a torus are made over a range of frequencies, and it is shown that the
force coefficients behave in a singular fashion in the vicinity of the trapped mode frequency.
An analysis of the time domain problem for a torus which is forced to make small vertical
oscillations at the trapped mode frequency shows that the potential contains a term which
represents a growing oscillation.

### History

#### School

- Science

#### Department

- Mathematical Sciences