LARGE-EDDY SIMULATION OF VERTICAL TIDAL AXIS TURBINES: STUDY OF THE BLOCKAGE EFFECT

摘要

The hydrodynamic behaviour of the vertical axis tidal turbines is not fully understood nowadays though the research on this field has grown considerably in the last years. The design of these turbines has been notably improved by experimental testing where different configurations were compared. However, the small amount of data from the experiments do not provide a full understanding of the whole phenomena inside these turbines. This makes necessary the use of complex computational models that reproduce this fluid-structure interaction problem accurately. The high turbulent regime where these vertical axis tidal turbines work makes important the use of turbulent approaches as Large-Eddy Simulation to reproduce the phenomena. Compared to more dissipative turbulent closures as Reynolds Averaged Navier-Stokes, Large-Eddy Simulation reproduces more accurately the vortices behaviour and how they interact with the turbine's blades and the flow field. The large computational cost required in the simulation of moving bodies using LES with mesh moving techniques as sliding-mesh is herein overcome by using a refined Immersed Boundary method. This methodology removes the necessity of re-meshing as the fluid flow is solved on a fixed Eulerian Cartesian mesh and the turbine is modelled by a grid of Lagrangian particles that reproduce the blade shapes and which spins at a constant rotational velocity. In this paper, the blockage effect provoked by the channel width is studied with special focus on the power coefficient and the velocities in the flow field. The aim is to analyse how a concrete turbine design behaves in a confined channel compared to a wider channel. A more complete understanding of this effect helps to understand and predict the final behaviour in a boundless natural environment (river, sea, estuary, etc.) or in a narrow channel of a prototype that it is being tested in a flume. The numerical model setups are analogue to the experiments carried out in the hydraulics laboratory of the School of Engineering at Cardiff. In the present case, a three-bladed Darrieus vertical axis tidal turbine with a tip speed ratio of 2 is compared with both experimental and numerical model results. An extended blockage study is done numerically.