An oscillating water column (OWC) device is a renewable energy device that is used to extract ocean wave energy through the action of waves on a partially submerged chamber consisting of an air and a water column. The operation of an OWC device involves complex hydrodynamic interactions between the waves and the device and a good understanding of these interactions is essential for the design of hydrodynamically efficient and structurally stable devices.\ud \ud In this paper, a two-dimensional numerical wave tank is utilized to simulate the interaction of an OWC device with waves of different wavelengths and steepnesses. The chamber pressure, provided by a turbine in a prototype, is simulated using porous media flow theory in the numerical model. The pressure in the chamber and the velocity of the free surface are calculated to evaluate the efficiency of the device and the model is validated by comparing the numerical results with experimental data. The performance of the device under a range of wavelengths for different wave steepnesses is evaluated. The effect of wave steepness on the device efficiency at a lower wave steepness was found to be low, but a large reduction in performance was found in the presence of steep non-linear waves.(c) 2015 Elsevier Ltd. All rights reserved. This is the authors' accepted and refereed manuscript to the article. Locked until 2017-07-01 due to copyright restrictions. Author's post-print must be released with a Creative Commons Attribution Non-Commercial No Derivatives License

In this paper, the performance of different wave generation and absorption methods in computational fluid dynamics (CFD)-based numerical wave tanks (NWTs) is analyzed. The open-source CFD code REEF3D is used, which solves the Reynolds-averaged Navier–Stokes (RANS) equations to simulate two-phase flow problems. The water surface is computed with the level set method (LSM), and turbulence is modeled with the k-$\omega$ model. The NWT includes different methods to generate and absorb waves: the relaxation method, the Dirichlet-type method and active wave absorption. A sensitivity analysis has been conducted in order to quantify and compare the differences in terms of absorption quality between these methods. A reflection analysis based on an arbitrary number of wave gauges has been adopted to conduct the study. Tests include reflection analysis of linear, second- and fifth-order Stokes waves, solitary waves, cnoidal waves and irregular waves generated in an NWT. Wave breaking over a sloping bed and wave forces on a vertical cylinder are calculated, and the influence of the reflections on the wave breaking location and the wave forces on the cylinder is investigated. In addition, a comparison with another open-source CFD code, OpenFOAM, has been carried out based on published results. Some differences in the calculated quantities depending on the wave generation and absorption method have been observed. The active wave absorption method is seen to be more efficient for long waves, whereas the relaxation method performs better for shorter waves. The relaxation method-based numerical beach generally results in lower reflected waves in the wave tank for most of the cases simulated in this study. The comparably better performance of the relaxation method comes at the cost of larger computational requirements due to the relaxation zones that have to be included in the domain. The reflections in the NWT in REEF3D are generally lower than the published results for reflections using the active wave absorption method in the NWT based on OpenFOAM.

Wave forces and the flow field around cylinders placed in a periodic wave field are investigated with a numerical model using the Reynolds-averaged Navier–Stokes equations. The numerical model is validated by simulating the wave interaction with a single cylinder and comparing the numerical results with experimental data from a large scale experiment. Then, the wave interaction with a single large cylinder and a pair of large cylinders placed in tandem for different incident wave steepnesses is studied. The numerically calculated forces are compared with predictions from potential theory. The numerical results are seen to match the predictions at low incident wave steepness but differ at higher incident wave steepnesses. The wave diffraction pattern around the tandem cylinders for waves of low and high steepness is investigated and the evolution of a strong diffraction pattern is seen in the case of high steepness waves, which results in the difference between the wave forces predicted by potential theory and the numerical model at higher steepnesses.(c) 2015 Elsevier Ltd. All rights reserved. This is the authors' accepted and refereed manuscript to the article. Locked until 2017-11-01 due to copyright restriction. Author's post-print must be released with a Creative Commons Attribution Non-Commercial No Derivatives License