Development of a numerical wave tank with reduced discretization error

Abstract

This paper presents the development of a numerical wave tank (NWT) using commercial Computational Fluid Dynamics (CFD) software, ANSYS Fluent 16.0. NWTs are widely used to analyse and optimise the performance of various wave energy converters, such as the Oscillating Water Column (OWC). A numerical modelling set-up is outlined which utilises Fluents Open Channel Wave Boundary Condition (OCWBC) along with a numerical beach scheme to dampen waves at the far field, ensuring no reflection back into the computational domain occurs. Linear waves are input into the NWT and free surface elevation and horizontal and vertical fluid velocities are compared to theory to quantify errors within the model. Variation in discretization error is observed with changing mesh density and a minimum criterion of 20 cells per wave height and 50 cells per wavelength is defined to ensure an acceptable level of model accuracy is achieved at a moderate computational expense. Further refinements show increased reduction in model error in both free surface elevation and velocity components beneath wave's peak, trough and inflection points, as expected. Further analysis focused on ensuring periodic independence was achieved and it was found that a minimum of six wave periods are required before fully developed waves propagate through the domain. The outlined numerical modelling method has quantified discretization error and outlined mesh requirements to reduce the overall error to acceptable levels for linear waves. Accurate free surface elevation and fluid velocity profiles will ensure that the NWT can now be used in future studies to analyse performance of wave energy converters and optimise device design.