Modelling a Tidal Turbine in Unsteady Flow

A brief review of the literature is provided on the characteristics of marine currents and the approaches used for simulating tidal turbines. The feasibility of using CFD models to simulate time-dependent turbulent flow around a tidal turbine is then explored. Two different approaches for specifying the structure of the turbulent inflow conditions in CFD models are compared: the von Karman spectral approach and the Synthetic Eddy Method (SEM) of Jarrin et al. (2006). The former model is commonly employed in the wind industry and is coded into Garrad Hassan's Tidal Bladed and NREL's TurbSim. Different approaches are also tested for decomposing the turbulence at the inlet into resolved and modelled components. The results from these tests indicate that the turbulence produced using the SEM inlet conditions is slightly less susceptible to decay with downstream distance than the von Karman approach provided that the modelled turbulent kinetic energy accounts for only a small fraction of the total turbulence energy. Simulations of the unsteady flow around a tidal turbine, represented here as a simple porous disc, provide some insight into the effect of large-scale flow oscillations on the wake of the turbine. The wake structures obtained from unsteady CFD simulations are compared to those obtained using a steady approach. The results indicate that the presence of large coherent turbulent structures in the incident flow field produces a shorter wake than predicted by steady flow simulations. This work represents the first stage in the development of a unified model which will couple meta-scale simulations of flow in an estuary or complete channel to detailed small-scale simulations of the flow around tidal turbine devices.