University of Twente Student Theses


Numerical modelling of non-linear shallow water waves

Lei, L.H. (2017) Numerical modelling of non-linear shallow water waves.

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Abstract:Marine icing is ice accretion on offshore structures and vessels. Heavy ice accretion can be a severe threat, since it leads to safety problems or damaging of equipment. It can even lead to instability with all its fatal consequences. In SINTEF, fundamental research is done to develop models for forecasting marine icing. Marine icing is mainly caused by sea spray, which is induced by water waves impacting on vessels or offshore structures. In this internship a numerical study is done to verify the possibility of modelling of non-linear water waves using ANSYS Fluent. Before a numerical study was done in ANSYS Fluent, a 1D Boussinesq model had been elaborated. Boussinesq type of systems are mathematical models to describe shallow water waves. One type of shallow water waves are solitary waves, which are characterized by maintaining its shape over a long distance, while it propagates with constant velocity. Solitary waves will never merge. The 1D Boussinesq model was used to gain insight in (numerical) behaviour of these waves, which could be used as a benchmark for the numerical study in ANSYS FLeunt. The Boussinesq model was discretised on a staggered grid using upwinding schemes. The initialization was based on the analytical expression of solitary waves. Results were validated with experiments of solitary wave interactions. In these experiments two cases were considered; head-on and overtaking collisions. For both cases, the 1D Boussinesq model was capable in reproducing the wave interaction with a relatively small error. For overtaking collisions friction was added to come to a better fit. In ANSYS Fluent a 2D geometry was built, which was meshed on a uniform grid. Volume of fluid method was used to model the multi phases of water and air. Initial conditions were set with a user-defined functions. This user-defined function enhanced the standard the features of ANSYS Fluent. The volume fractions, velocity field and pressure distribution were initialized using this function. The results were again validated with experimental data. The data was really well presented by the 2D model. However, numerical dissipation was quite significant. It can be concluded that ANSYS Fluent is a suitable CFD method to handle wave propagation. In the continuation of the project, geometry and user defined functions can be adjusted to make it applicable for this marine icing project at SINTEF.
Item Type:Internship Report (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:Wave propagation, Marine icing, Shallow water waves, Boussinesq approximation, CFD, ANSYS Fluent
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