Hydrodynamics, control and numerical modelling of absorbing wavemakers

Abstract

This research investigates the effects that geometry and control have on the absorption characteristics of active wavemakers and looks at the feasibility of modelling these wavemakers in commercial computational fluid dynamic software. This thesis presents the hydrodynamic coefficients for four different types of wavemakers. The absorption characteristics of these wavemakers are analysed using different combinations of control impedance coefficients. The effect of combining both geometry and control is then investigated. Results, quantifying the absorption characteristics are then presented. It is shown that the amount of absorption for a given paddle differs greatly depending on the choice of control coefficients used to implement complex conjugate control. Increased absorption can be achieved over a broader bandwidth of frequencies when the geometry of the wavemaker is optimised for one specific frequency and the control impedance is optimised for an alternate frequency. In conjunction to this theoretical study, a numerical investigation is conducted in order to verify and validate two commercial computational fluid dynamic codes' suitability to model the previously discussed absorbing wavemakers. ANSYS CFX and FLOW3D are used to model a physical wavemaker. Both are rigorously verified for discretisation errors and CFX is validated against linear wavemaker theory. Results show good agreement and prediction of the free surface close to the oscillating wavemaker, but problems with wave height attenuation and excessive run times were encountered.