Wave induced motions of point-absorbers: a hierarchical investigation of hydrodynamic models

Paper in proceeding, 2015

We present an initial study of using nonlinear and dispersive Boussinesq-type equations for simulating the response of a point absorber in waves. Boussinesq-type models are ex- pressed in horizontal dimensions only, resulting in computation- ally efficient models with a real possibility to simulate entire sea states in a fully nonlinear sense. The wave-body coupling is achieved using the unified Boussinesq approach of Jiang [15] and the model formulated in one spatial dimension is discretized using a spectral/hp element method. As a first step in our modelling and analysis, we apply the model to a test case of a heaving box subjected to finite-amplitude waves of increasing steepness. The simulations are compared to results obtained by (i) a conventional linear model and (ii) a two-phase volume-of-fluid Reynolds Averaged Navier-Stokes (VOF-RANS) model. We find that there is a significant difference in heave response between the linear and VOF-RANS simulations, were the VOF-RANS give much smaller amplitude response. The Boussinesq model gave smaller response than the linear model but still clearly larger than obtained by VOF-RANS. We believe this shortcoming in the modelling is due to the Boussinesq expansion used below the hull, which needs to be enhanced.