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Free oscillation transient experiments

Free oscillation transient experiments have been carried out on a system of 2 oscillating water columns (OWC). The system is a model in scale 1:10 with operable air valves. Hydrodynamic parameters characterizing the system have been derived by analysing measured data from the transient oscillations of the water columns, excited by a pressure step applied to 1 of the 2 air chambers. The experimental data may also be used for setting up a time domain mathematical model.

The excitation volume flux coefficient qe, has been derived from the radiated wave, by means of reciprocity relations. However, the measurements of the radiated wave were influenced by crosswaves in the flume. These crosswaves were not measured, so additional experiments have to be done in order to determine qe with sufficient accuracy. Preliminary tests suggest that the simulation model operates as expected.

The fast Fourier transform (FFT) analysis will provide the correct values of a frequency spectrum only in a finite number of points, equally spaced in the frequency domain. Hence, significant information may be lost in regions where a quantity varies rapidly with frequency. This results in an uncertainty in the peak values of the spectrum.

The use of transient experiments has proved to be an easy and powerful approach for retrieving information about the system. The hydrodynamic parameters found in this work show good agreement with results derived by incident wave experiments or by theoretical means. Experiments have shown that the determined hydrodynamic parameters to some degree depend on the initial applied pressure. This phenomenon provides information on nonlinear loss mechanisms in the system.

Reported by

Universitetet i Trondheim
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