Keaney, Iain (2015) Evanescent Wave Reduction Using a Segmented Wavemaker in a Two Dimensional Wave Tank. PhD thesis, National University of Ireland Maynooth.

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Abstract

Evanescent waves are created by the wavemaking process during tank testing. They have long been a nuisance for engineers as they contaminate the wave field in the tank and result in additional inertial force experienced by a wavemaker. Evanescent waves are created by the mismatch between the motion of the wavemaker and the motion of the uid particles in a progressive wave. To avoid contamination of test results, often a considerable distance must be left between the wavemaker and the test area. This space requirement may be costly for small research groups or companies who wish to have a facility to perform some basic proof-of-concept tests in-house, but are restricted for space. The initial aim of this project was to develop a wavemaker which minimised this space requirement over a large range of frequencies. The exploration into the behaviour of evanescent waves from the point of view of the fundamentals of hydrodynamics has been very enlightening. It became clear with the discovery of an interference pattern between the evanescent waves, that this pattern can be optimised to effectively cancel out the evanescent wave fieeld. This interference pattern arises from a phase shift of � radians experienced by some of the evanescent waves, with respect to the others. The significance of this in hydrodynamics is that it explains the existence of negative added mass. The application for this knowledge far out reaches the topic of reducing the distortion in a wave tank. The ability to minimise the added mass of a wavemaker has a great deal of potential in both active absorbing wavemakers and wave energy conversion. For active absorbing wavemakers, the minimisation of added mass may be useful in the absorption of unwanted waves which can be particularly troublesome at high frequencies. The concept of designing the geometry of a wavemaker to simply match the motion of the uid particles has long been proposed; however, the difficulty with designing such a wavemaker is that the ideal geometry is frequency dependent. Hence, a design that eliminates the evanescent waves at one particular frequency will not be able to do so for other frequencies. An investigation into the design of a segmented wavemaker is presented here, as its geometry can easily be adjusted to suit different frequencies. The wavemaker theory for the multi-body problem of the segmented wavemaker is developed, and a new aspect of wavemaker theory that predicts a phase shift of � radians in some of the evanescent waves is presented for the first time. A hypothesis is put forward, and then investigated, proposing that this phase shift can be exploited to create an interference pattern that can effectively cancel out the evanescent waves. The hydrodynamics of the segmented wavemaker were constrained using the Newton-Euler equations of motion with Eliminated Constraints (NE-EC). This approach facilitated a comparison between wavemakers with multiple degrees of freedom and traditional wavemakers with a single degree of freedom. The lengths and strokes of each segment in the wavemaker are optimised to reduce the distortion caused by the evanescent waves using two approaches. Approach one follows the traditional ideas and optimises the lengths and strokes of the segments to best approximate the motion of the uid particles in a progressive wave. Approach two optimises the lengths and strokes of the segments in order to minimise the distance between the wavemaker and the testable area in the tank. Approach two exploits the phase shift in the evanescent waves by finding the optimal interference pattern that effectively cancels out the evanescent waves. A comparison between both approaches shows that effectively eliminating the distortion caused by the evanescent waves is much more achievable by optimising the interference pattern between the evanescent waves, rather than trying to approximate a progressive wave. The results for the segmented wavemakers optimised using approach two predicted that the distortion can be effectively eliminated for a wide range of frequencies using a segment wavemaker consisting of three aps. A sensitivity analysis indicates that the performance of the wavemaker is somewhat effected by errors in the segments strokes, but the overall performance is still better than what has been developed to date.

Item Type:	Thesis (PhD)
Keywords:	Evanescent Wave Reduction; Segmented Wavemaker; Two Dimensional; Wave Tank;
Academic Unit:	Faculty of Science and Engineering > Electronic Engineering
Item ID:	7585
Depositing User:	IR eTheses
Date Deposited:	27 Oct 2016 13:42
URI:
Use Licence:	This item is available under a Creative Commons Attribution Non Commercial Share Alike Licence (CC BY-NC-SA). Details of this licence are available here

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