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Regular wave conditions in a directional wave basin

University of British Columbia

This thesis represents a small step toward improved generation of realistic sea states in laboratory wave basins. Experiments were conducted in an offshore directional wave basin equipped with a segmented wave generator. Regular waves were generated for several periods and propagation directions, and the resulting wave elevations were measured throughout the basin. Motions of the 60 wavemaker segments were based on the snake principle of directional wave generation. Results are summarized and compared with predictions of a boundary element numerical method. Findings encourage further development of this linear diffraction numerical technique. It should be used to correct generator segment motions as prescribed by the snake principle, so as to account for diffraction and reflection effects which affect the wave field. The experiments were conducted at the Hydraulics Laboratory of the National Research Council of Canada, in Ottawa. A six metre square array of wave probes was used to measure wave elevations at discrete points spaced on a 2m grid around the wave basin. Three wave periods were investigated, with waves directed normal to the generator face, as well as in three oblique propagation directions. For all but the most severe propagation direction, tests were run a second time with a large surface piercing cylindrical structure positioned in the test area. Maximum waveheight results were plotted, and compared with numerical model predictions at the same locations around the basin. A linear diffraction computer program based on the boundary element method was used to predict the wave field at the same points around the basin. By this method, generator segment faces and reflecting walls are represented by a distribution of discrete sources around the basin boundaries. Measured elevation time series were analysed using a multiple regression screening program which extracts a prescribed number of sinusoidal components from the signal of interest. The program was modified to accommodate harmonic analysis. Fundamental and second harmonic components were synthesized from each time series. The second harmonic component was generally an order of magnitude smaller than the fundamental component. Discrepancies between these results and the linear numerical model predictions are attributed to nonlinear effects, and to basin resonance. The linear diffraction computer model is seen to predict the wave field to a high degree of accuracy, even though imprecise boundary definition was necessary at the current level of program development.

Text

2010-08-31

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http://hdl.handle.net/2429/28062

Civil Engineering

University of British Columbia

Graduate