Numerical simulation of Nonlinear water waves using a panel method; domain decomposition and applications
Haas, Paulus Cornelis Antonius de (1997) Numerical simulation of Nonlinear water waves using a panel method; domain decomposition and applications. thesis.

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Abstract:  In studying the influence of water waves on constructions such as dikes, wave
breakers and offshore constructions, on ships but also on natural processes such as sediment transport and changes in bottom topography, more and more use is made of numerical models. An important class of such models consists of models in which the flow is described by potential theory. On the one hand the assumptions made in potential theory are valid in many studies, on the other hand the description  its field equation is Laplace's equation for the velocity potential  offers many possibilities for finding solutions with numerical models. The panel method is a numerical method which makes use of a boundary integral formulation for Laplace's equation, so that only the boundaries of the fluid domain have to be covered with grid points. Moreover this enables a natural description of the movement of the free surface in the time domain, which is determined by nonlinear dynamical and kinematical boundary conditions. Nonlinearity of the freesurface boundary conditions is often of importance in studying the influence of waves in coastal and ocean engineering. In this thesis a twodimensional and threedimensional numerical model are studied, based on a panel method, for the description of nonlinear water waves. The focus is on two important aspects: firstly the dependence of the computational effort on the number of grid points and secondly some specific numerical difficulties which arise when the method is used in applicationlike computations. With respect to the former aspect, a domain decomposition technique is studied. The latter aspect is studied for some examples and the suitability and limitations of some parts of the method for these examples are investigated. In more detail the contents of this thesis is as follows. For the domain decomposition technique, an iterative method is chosen in which the domain is divided in the horizontal direction. The lengthtoheight ratios of the subdomains, among other things, determine the convergence of the iterative method. Because the domains in problems involving water waves generally have large lengthtoheight ratios, relatively many subdomains can be chosen with a limited loss of convergence. As a consequence the panel method can be applied much more efficiently with domain decomposition. In the case of subdomains with fixed lengthtoheight ratios, the computational costs per time step depend at most linearly on the length of the domain. Furthermore the approriateness of the numerical model is studied for three types of problems. Firstly the use of the model for the simulation of waves generated by a translating or rotating wavemaker is studied. An important improvement for such computations is the use of descriptions in the physical domain rather than in the computational domain for extrapolations towards lateral boundaries such as the wavemaker boundary. Secondly the propagation of wave groups is simulated and the use of various formulations of the wave group signal is studied. The domain decomposition technique proves to be a succesful method for these computations. Thirdly the approriateness of the numerical model for the simulation of waves diffracting around a surfacepiercing construction is studied. Because of the large differences in horizontal velocities on the free surface, the use of a mixed EulerianLagrangian description is necessary. 
Item Type:  Thesis 
Faculty:  Electrical Engineering, Mathematics and Computer Science (EEMCS) 
Research Group:  
Link to this item:  http://purl.utwente.nl/publications/29650 
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