Simulating the hydrodynamics of deformable, floating structures using a partitioned strategy poses a major challenge when the ratio of the added mass to the structural mass is considerate. Existing computational procedures for fluid-structure interaction become less efficient or even unstable. In these situations, it is advisable to modify the coupling to allow the fluid to respond better to the solid motions. A simultaneous solution of the equations governing fluid and solid-body would be a stable choice but is often not feasible. Usually the numerical problems are taken care of with subiterations between fluid and structure, but their convergence can be slow. In this paper we present a more powerful, quasi-simultaneous approach, which tries to mimic a fully simultaneous coupling in an affordable way. It makes use of a simple approximation of the body dynamics, based on the (6 DOF) solid-body modes and the main elastic modes of the structure. The method will be demonstrated in offshore practice, with a falling life boat, a floating CALM buoy, an elastic membrane and a rubber gate.@en

The physical level of interaction between fluid and structure can be either one-way or two-way depending on the direction of information exchange at the interface of fluid and solid. The former can be solved by a partitioned approach and weak coupling. In problems involving two-way fluid-structure interaction, using a partitioned approach and strong coupling, sometimes stability restriction is encountered. This is an artificial added mass effect, which is independent of the numerical time step. Unfortunately an accurate and efficient method to deal with all the different levels of interaction is scarce. Conventionally, relaxation is applied to remedy this problem. The computational cost is directly related to number of sub-iterations between fluid and structural solver at each time step. In this study, the source of this instability is investigated. A discrete representation of a basic added mass operator is given and instability conditions are assessed. A new method is proposed to relax this restriction, the idea essentially is to remove the instability source from the structure and move it to the fluid and solve it monolithically with the fluid. We call this an interaction law. An estimate of the structural response is derived from structural mode shapes. As a test case, a 2D dam break problem interacting with an elastic vertical flexible beam is selected. The interaction of fluid with the beam undergoes several stages. The breaking waves on the beam can increase the added mass drastically, therefore the added mass ratio increases as well. In such a cases, the asset of interaction law is better elaborated, while the stability condition requires very high relaxation without interaction law, but the relaxation can be lowered by only using first five beam mode shapes. As a consequence, the number of sub-iterations reduces by one order. The numerical observations confirm the reduction in computational time due to utilization of the interaction law.@en

In this paper, the use of an absorbing boundary condition (ABC) is investigated for the numerical simulation of free surface water waves. An enhanced type of an ABC based on the first- and second-order Higdon boundary conditions is presented. The numerical implementation of the ABC using a staggered grid arrangement is explained in detail. Numerical examples are provided to demonstrate the performance of the proposed boundary conditions.@en

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

In this paper the attention is focused on the effect of various VOF methods on efficient and accurate simulation of free surface water waves. For this purpose, we will compare several VOF methods in numerical simulations of propagating waves where strong nonlinear behavior is dominant in the flow. Comparisons and discussions will be provided to underline the significance of free surface modeling on the accuracy of wave propagation.@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