Numerical modelling of a variable-pitch, vertical axis tidal turbine incorporating flow acceleration

This paper presents details of the numerical modelling of a novel vertical axis tidal turbine that incorporates localised flow acceleration and variable-pitch blades. The focus of this research is to develop a computational fluid dynamics model of a 1:20 scale model of the device using ANSYS® Fluent®. A nested sliding mesh technique has been developed, with an outer sliding mesh being used to model the turbine and additional inner sliding meshes being used for each of the six blades. The turbine sliding mesh is embedded in an outer static domain which includes the flow accelerating bluff body. The variable pitch of the blades is specified in the model using a user-defined function (UDF) which faithfully reproduces the blade pitch during operation of the 1:20 scale model. Modelled power performance and velocity data are compared with experimental results obtained from scale model tests in a recirculating flume. The modelled power curves show good agreement with the measured data; the difference in maximum C_P, for example, is just 5.7 %. The model also accurately reproduces measured flows downstream of the turbine. The high model accuracy means that it can now be used for design optimisation studies.