Specification of the thrust coefficient when using the momentum sink approach for modelling of tidal turbines

Abstract

A common method for simulating tidal turbines in far-field coastal models is the momentum sink approach, where the thrust induced by a tidal turbine is included as an additional external force in the momentum equations. A value close to 1 is commonly used for the turbine thrust coefficient (CT), but this value is based on a relationship between turbine thrust and undisturbed free-stream flow. Implementation of the momentum sink approach usually means that the thrust is calculated using the flow at the turbine grid cell as opposed to the undisturbed upstream flow speed. This research found that adopting the typical thrust coefficient value in the range of 0.8–1.0 in combination with the local velocity at the turbine can lead to underestimation of the energy extraction and hydrodynamic impacts of a tidal turbine, particularly for finer resolution models. This research implements the momentum sink approach using a local thrust coefficient (C′T) which has appropriate values greater than 1. C′T was shown to be dependent on the turbine blockage (ratio of the turbine area to the grid cell area), and a relationship between the two was determined using a model of an idealised channel. This allows determination of appropriate C′T values for a desired turbine efficiency of 40%. The developed relationship was used to determine appropriate C′T values for four potential tidal turbine deployments in the Shannon estuary. The results show that the use of the appropriate C′T values resulted in the desired energy removal of 40%, while the use of values less than or equal to 1 resulted in lower than desired levels of energy removal.