Numerical Simulation of a Cross Flow Marine Hydrokinetic Turbine

Abstract

In the search for clean, renewable energy, the kinetic energy of water currents in oceans, rivers, and estuaries is being studied as a predictable and environmentally benign source. We investigate the flow past a cross flow hydrokinetic turbine (CFHT) in which a helical blade turns around a shaft perpendicular to the free stream under the hydrodynamic forces exerted by the flow. This type of turbine, while very different from the classical horizontal axis turbine commonly used in the wind energy field, presents advantages in the context of hydrokinetic energy harvesting, such as independence from current direction, including reversibility, stacking, and self-starting without complex pitch mechanisms. This thesis develops a numerical simulation methodology that applies the Reynolds Average Navier Stokes equations and the three-dimensional sliding mesh technique to model CFHTs. The methodology is validated against small scale experiments, available within NNMREC at the University of Washington and is used to investigate the efficiency of the energy capture and the hydrodynamic forces acting on the blades. First, we study the stationary turbine and conclude that the developed methodology accurately models the starting torque of a turbine initially in static conditions; some limitations are found, however, in predicting separated flow. The dynamic performance of the rotating turbine is predicted with reasonable accuracy using the sliding mesh technique. Excellent qualitative agreement with experimental trends is found in the results, and the actual predicted values from the simulations show good agreement with measurements. Though limitations in accurately modeling dynamic stall for the rotating turbine are confirmed, the good qualitative agreement suggests this methodology can be used to support turbine design and performance over a wide range of parameters, minimizing the number of prototypes to build and experiments to run in the pursuit of an optimized turbine. This methodology can also provide a cost-effective way of evaluating detailed full scale effects, such as mooring lines or local bottom bathymetry features, on both turbine performance and environmental assessment.