A study on tidal rotors under the combined effects of currents and waves using actuator-line CFD simulations

Abstract

Determining loads and power on a tidal rotor under realistic flow conditions is challenging. Methods used by offshore industries for the design and deployment of marine devices rely on simulations of a significant number of cases that combine different environmental and operational conditions. Design studies of tidal rotors rely on engineering models of low to medium computational cost that often omit important hydrodynamic effects such as blockage, multi-rotor interactions, wave diffraction, etc., and have a questionable accuracy under conditions such as high-thrust regimes or highly unsteady flows. Alternatively, high-fidelity CFD simulations can implicitly capture most of the relevant physics, but their computational cost often limits their application in engineering practice.

This study will present a numerical study of a tidal rotor operating near the free surface and affected by combined currents and surface waves. The simulations will be performed with an intermediate fidelity actuator-line model implemented within OpenFOAM. The AL model is embedded within a simulated tank discretised with the finite volume method, that employs a RANS turbulence model with a Volume of Fluid (VoF) approach for the free surface. Relaxation zones, implemented with the Waves2Foam library, are used for the generation and absorption of surface waves and reflections. A preliminary render of the assembled model simulating a test condition can be seen in fig. 1.

The paper manuscript will present the results of an initial validation of the modelling strategy that will compare simulated cases with towing-tank experiments. The validation will be followed by a quantification of the interactions between the rotor and the free surface, as well as an analysis of the transient loads and power fluctuations for different operating conditions.