|The Performance Assessment of Wave and Tidal Array Systems (PerAWaT) project, launched in October 2009 with £8m of ETI investment. The project delivered validated, commercial software tools capable of significantly reducing the levels of uncertainty associated with predicting the energy yield of major wave and tidal stream energy arrays. It also produced information that will help reduce commercial risk of future large scale wave and tidal array developments.
The objective of this deliverable is to extend the existing computational model of a full scale tidal turbine to account for the effect of regular free surface waves. The new model is presented in this report, and its wave modelling capability is demonstrated.
The starting point for this model is that which has been presented in WG3 WP1 D4. In that work, an 18m diameter rotor is simulated in a variety ofsheared and blocked flows. A body-fitted grid is used to capture the rotor geometry. Rotor motion is achieved by defining a sliding mesh interface between the rotor mesh and the surrounding stationary channel mesh. For this model, and preceding models at experimental scale, the effects of the free surface were found to be insignificant, and hence the rigid lid approximation was used.
In this work, the volume-of-fluid (VOF) model is used for free surface tracking. A periodic velocity boundary condition is applied upstream of the turbine which propagates waves downstream, in addition to the mean current. A wave damping zone is defined downstream of the device to prevent reflection of waves back upstream. Model accuracy is quantified through the changes in wave height and length as waves propagate from the inlet to the outlet of the computational domain.
The turbineis simulated in operation in this wave field to predict power, thrust and wake development. The effect of the wave field on the turbine is identified through frequency analysis.
The model is shown to adequately reproduce an open channel flow with free-surface waves, and can be used to study the effect of this class of flows on tidal turbine operation. Wave parameters such as amplitude, wavelength and direction may be altered, enabling future parametric studies in WG3 WP1 D5 and D6.