A wave basin experiment has been performed in the MARINTEK laboratories, in one of the largest existing three-dimensional wave tanks in the world. The aim of the experiment is to investigate the effects of directional energy distribution on the statistical properties of surface gravity waves. Different degrees of directionality have been considered, starting from long-crested waves up to directional distributions with a spread of ±30• at the spectral peak. Particular attention is given to the tails of the distribution function of the surface elevation, wave heights and wave crests.Comparison with a simplified model based on second-order theory is reported. The results show that for long-crested, steep and narrow-banded waves, the second-order theory underestimates the probability of occurrence of large waves. As directional effects are included, the departure from second-order theory becomes less accentuated and the surface elevation is characterized by weak deviations from Gaussian statistics.

We present a laboratory experiment in a large directional wave basin to discuss the instability of a plane wave to oblique side band perturbations in finite water depth. Experimental observations, with the support of numerical simulations, confirm that a carrier wave becomes modulationally unstable even for relative water depths k0h < 1.36 (with k the wavenumber of the plane wave and h the water depth), when it is perturbed by appropriate oblique disturbances. Results corroborate that the underlying mechanism is still a plausible explanation for the generation of rogue waves in finite water depth

This file was dowloaded from the institutional repository Brage NIH -brage.bibsys.no/nih Caspersen, C., Berthelsen, A., Eik, M., Pakozdi, C., Kjendlie, P.-L. (2010).Added mass in human swimmers: age and gender differences. Title: Added mass in human swimmers: age and gender differences. AbstractIn unstationary swimming (changing velocity), some of the water around the swimmer is set in motion. This can be thought of as an added mass (Ma) of water. The purpose of this study was to find added mass on human swimmers and investigate the effect of shape and body size. Thirty subjects were connected to a 2.8m long bar with handles, attached with springs (stiffness k=318N/m) and a force cell. By oscillating this system vertically and registering the period of oscillations it was possible to find the added mass of the swimmer, given the known masses of the bar and swimmer.Relative added mass (Ma%) for boys, women and men were, respectively, 26.8±2.9%, 23.6±1.6% and 26.8±2.3% of the subjects total mass. This study reported significantly lower added mass (p<0.001) and relative added mass (p<0.002) for women compared to men, which indicate that the possible body shape differences between genders may be an important factor determining added mass. Boys had significantly lower (p<0.001) added mass than men. When added mass was scaled for body size there were no significant differences (p=0.996) between boys and men, which indicated that body size is an important factor that influences added mass. The added mass in this study seems to be lower and within a smaller range than previously reported (Klauck, 1999;Eik et al., 2008). It is concluded that the added mass in human swimmers, in extended gliding position are approximately ¼ of the subjects' body mass.

In situations where the calculation of ocean wave propagation and impact on structures are required, fast numerical solvers are desired in order to find relevant wave events. Computational fluid dynamics (CFD)-based numerical wave tanks (NWTs) emphasize on the hydrodynamic details such as fluid–structure interaction, which make them less ideal for the event identification due to the large computational resources involved. Therefore, a computationally efficient numerical wave model is needed to identify the events both for offshore deep-water wave fields and coastal wave fields where the bathymetry and coastline variations have strong impact on wave propagation. In the current paper, a new numerical wave model is represented that solves the Laplace equation for the flow potential and the nonlinear kinematic and dynamics free surface boundary conditions. This approach requires reduced computational resources compared to CFD-based NWTs. The resulting fully nonlinear potential flow solver REEF3D::FNPF uses a σ-coordinate grid for the computations. This allows the grid to follow the irregular bottom variation with great flexibility. The free surface boundary conditions are discretized using fifth-order weighted essentially non-oscillatory (WENO) finite difference methods and the third-order total variation diminishing (TVD) Runge–Kutta scheme for time stepping. The Laplace equation for the potential is solved with Hypre’s stabilized bi-conjugated gradient solver preconditioned with geometric multi-grid. REEF3D::FNPF is fully parallelized following the domain decomposition strategy and the message passing interface (MPI) communication protocol. The numerical results agree well with the experimental measurements in all tested cases and the model proves to be efficient and accurate for both offshore and coastal conditions.

Nonlinear wave interactions and superpositions among the different wave components and wave groups in a random sea sometimes produce rogue waves with extremely large wave heights that appear unexpectedly. A good understanding of the generation and evolution of such extreme wave events is of great importance for the analysis of wave forces on marine structures. A fully nonlinear potential flow (FNPF) model is proposed in the presented paper to investigate the different factors that influence the wave focusing location, focusing time and focusing wave height in a numerical wave tank. Those factors include wave steepness, spectrum bandwidth, wave generation method, focused wave spectrum, and wave spreading functions. The proposed model solves the Laplace equation together with the boundary conditions on a σ -coordinate grid using high-order discretisation schemes on a fully parallel computational framework. The model is validated against the focused wave experiments and thereafter used to obtain insights into the effects of the different factors. It is found that the wave steepness contributes to changing the location and time of focus significantly. Spectrum bandwidth and directional spreading affect the focusing wave height and profile, for example, a wider bandwidth and a wider directional spread lead to a lower focusing wave height. A Neumann boundary condition represents the nonlinearity of the wave groups better than a relaxation method for wave generation.