Aspects of tidal stream turbine modelling in the natural environment using a coupled BEM–CFD model

Abstract

The problem of designing the optimal array of tidal stream turbines for the generation of marine renewable energy from the ocean, raises a number of questions about the distribution and layout of turbines in relation to the local bathymetry. The computational overhead of modelling such problems may be significant and costly. This paper aims to clarify the effects of particular phenomena associated with modelling tidal stream turbine arrays. To achieve this we use a RANS computational fluid dynamics model with an embedded blade element actuator disk to investigate various aspects of this problem, while maintaining reduced computational overhead.A study of axially aligned turbines, with each in the wake shadow of the previous turbine shows uniform effects for a 20 diameters downstream spacing, but more complex interaction for 10 diameters spacing. Investigation of the significance of inclusion of the nacelle and tower geometry in a CFD model shows that effects are negligible beyond six diameters downstream. An array of transverse contrarotating turbines are considered, where a device is placed close to and in the wake of a pair of upstream devices. Rotational direction has minimal effect on the power generated, but different turbulence is seen in the wake. Finally, marine currents around a headland are modelled and a single row fence of turbines is placed offshore from the headland at various blockage ratios. Power performance estimates and downstream wakes are created and they show increased power per device and improved total power production as the blockage ratio rises from 0.13 to 0.20. Additionally, the authors use stream surface techniques to visualise the flow which can give new insights to the physical processes observed.