UBC Theses and Dissertations
Experiments with withdrawal-induced resonant internal waves Allan, Gary
Control of natural and artificial reservoirs by means of outlet structures (specifically, dams) has been considered by Imberger (1980). Such control has been proposed on Kootenay Lake in eastern British Columbia, as a means to modify the ecology of the lake. This study considers the feasibility of pumped control of a reservoir and the practical problems associated with application on the lake. The generation of internal waves in a rectangular tank by a pump with sinusoidally modulated flow has been investigated via laboratory experiments. Theory appropriate to internal waves in two layer systems has been presented with an extension to deal with the effect of viscosity. The study shows that modulated pumping is a viable method of generating internal waves experimentally and may have applications in controlled reservoirs on which outflow could be modulated. The period of resonant waves increases with interfacial thickness as does wave amplitude: observed periods ranged from 1.01 to 1.05 times greater than theoretically determined periods for two-layer systems. Theoretical determination of the period is based on consideration of the density structure and boundary conditions of the system under study; one would expect real conditions to produce some variation in the observed period versus the theoretical period. The result above is borne out by observation of the amplitude response curves generated by the data and by calculations of the beat frequencies associated with the forcing and generated waves and agrees approximately with the data of Hyden (1974) and Thorpe (1968). There is some indication that as the interface widens the amplitude response curves sharpen, implying that if one hoped to generate a resonant wave in a system with a thick interface one would have to be closer to the resonant period than in a system with a thinner interface to achieve a similar response. Calculation indicates that, for the peak response, the wave with maximum amplitude, 1.7 to 4 % of the energy input per cycle is present in the wave. Energy input per cycle is determined by integrating instantaneous kinetic energy of the water that is pumped over a cycle. Fourier transforms of the data show response, in some cases, in the 6th harmonic modes of the systems, with a decline in energy from the 1st to 2nd mode of 20 to 25 %, typically. Estimates are presented for the damping coefficients for the systems investigated: the average value is 0.0020 s-1. A brief review of problems associated with a practical application of resonant internal waves forced by flow modulation is presented. Several technical problems exist which need further consideration before the application is undertaken, on Kootenay Lake in eastern British Columbia. It may be practicable to try generating internal waves on a smaller lake or reservoir controlled by a dam as a means of testing the feasibility.
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