The water impact of one and twin free-falling wedges is numerically investigated by a Cartesian grid multiphase flow model. The effects of the drop velocity and the gap distance on the hydrodynamic behaviors are parametrically investigated. The numerical model involves a radial basis function ghost cell method (RBFGCM) for treating moving bodies and a gradient-augmented level set method (GALS) method for capturing violent free surfaces. A case of twin wedges entering water is simulated to validate the accuracy of the present method. Good convergences are achieved. Then, the water entry of one and twin wedges in free falling is considered. The interaction mechanisms between twin wedges are discussed by comprehensively examining the variation patterns of the slamming load, the moment, the local pressure, and the fluid field. It is found that the second slamming load and the huge pressure pulse occur at the transition stage at narrow gap distances. The hydrodynamic interaction has more significant effects on the local pressure than that on the global load. In addition, distinct hydrodynamic phenomena for twin wedges entering water are observed such as the connection of the pressure contours, the extremely large jet flow, formation of the cavity, and even the ventilation.

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In reality, the interaction between surface wave-current flow and numerous kinds of man-made maritime structures is local but embedded in a domain with the size of a sea. Detailed simulations of the vast domain are computationally unfeasible. Therefore, articial boundaries are introduced to truncate the large domain so as to obtain a small domain around the structure of interest. These articial boundaries can produce reections which obscure the flow information from true reflectors. To eliminate the reflections from articial boundaries, this thesis proposes absorbing boundary conditions (ABCs). The novelty is that ABCs for free surface waves in a detailed numerical model with a resolved vertical direction are derived in the presence of nonzero mean flow.@en

Wave forces can form a serious threat to offshore platforms and ships. The damage produced by these forces of nature jeopardizes their operability as well as the well-being of their crews. Similar remarks apply to coastal defense systems. To develop the knowledge needed to safely design these constructions, in close cooperation with MARIN and the offshore industry the numerical simulation method ComFLOW is being developed. So far, its development was focussed on predicting wave loads (green water, slamming) on fixed structures, and for those applications the method is already being used successfully by the offshore industry. Often, the investigated object (ship, floating platform) is dynamically moving under the influence of these wave forces, and its hydrodynamic loading depends upon the position of the object with respect to the oncoming waves. Predicting the position (and deformation) of the body is an integral part of the (scientific and engineering) problem. The paper will give an overview of the algorithmic developments necessary to describe the above-mentioned physical phenomena. In particular attention will be paid to fluid-solid body and fluid-structure interaction and non-reflecting outflow boundary conditions. Several illustrations including validation, will demonstrate the prediction capabilities of the simulation method.

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The numerical modeling of wave propagation in offshore applications is a formidable challenge even today when we have highly capable numerical schemes and computational power at our disposal. The waves travel in a huge and unbounded domain while the phenomena of interest are only around the structures. Therefore, the infinite domain is truncated via artificial boundaries. This results in a compact computational domain and a residual infinite domain. One of the bottlenecks is what type of boundary condition should be imposed on these introduced boundaries. It should be chosen such that the solution in the small truncated domain coincides with that in the original infinite domain.@en

The CFD simulation tool ComFLOW is extended to investigate the characteristics of wave motions in the presence of steady uniform currents. Initially, the inflow boundary is the superposition of waves and current. Effect of the latter on the former is resolved by solving Navier-Stokes equations within the domain as a next step. A Generating and Absorbing Boundary Condition (GABC) with currents is introduced that allows the simulation of a combined wave-current environment in truncated domain. This GABC is characterized by a rational function approximation of dispersion relation, based on Sommerfeld condition and irrotational wave model. The artificial boundaries where GABC with current is applied are transparent to incoming and outgoing waves and currents simultaneously. The absorption properties of the GABC for various waves and currents are analysed. The temporal and spatial differences of free surface elevation between the small domain and large domain turn out to be small, i.e. the GABC prevents the reflection from the boundaries well. The large domain here is arranged in such a way that the reflected waves and currents will not reach the outflow boundary of the small domain within the simulation time. The behaviour of GABC in 3D domain is also investigated, where waves and currents are traveling under an angle of incidence colinearly. @en