Numerical and physical modelling of extreme waves at Wave Hub

Abstract

With a history of international failures, the survivability of coupled systems of wave energy devices and their moorings, particularly those to be installed at development sites like Wave Hub, is surrounded by uncertainty. Potential design solutions require a better understanding of the hydrodynamics and structural loading experienced during extreme events, like rogue wave impact, in order to mitigate the risk of device and mooring failure. Rogue waves are waves with amplitudes far greater than those expected, given the surrounding sea conditions. Intense study into these events stems from their potential for catastrophic impact on ocean engineering structures. However, little is known about their physical origins and, currently, there is no consensus on their definition or explanation of the mechanism which drives them. This paper concerns the numerical modeling and experimental validation of extreme rogue wave examples at the Wave Hub site. Using hindcast data, the 100 year extreme wave at the Wave Hub site is determined. This extreme wave is replicated in Plymouth University’s new COAST Lab using a NewWave, dispersive focusing input. To simulate and analyse these events, we duplicate these conditions in a numerical wave tank (NWT), solving the fully nonlinear Navier-Stokes equations, with a free surface, using the Volume of Fluid (VoF) method and open source CFD library OpenFOAM®. The comparison shows that the CFD software is capable of simulating focused waves similar to those produced in the physical tank but tends to overestimate the crest heights. It is also noted that nonlinear effects are important when considering the shape and location of focused wave events.