Development and Verification of a Computational Fluid Dynamics Model of a Horizontal-Axis Tidal Current Turbine

Abstract

This paper describes the development of a computational fluid dynamics (CFD) methodology to simulate the hydrodynam ics of horizontal-axis tidal current turbines (HATTs). First, an HATT blade was designed using the blade element momentum method in conjunction with a genetic optimization algorithm. Several unstructured computational grids were generated using this blade geometry and steady CFD simulations were used to perform a grid resolution study. Transient simulations were then performed to determine the effect of time-dependent flow phe nomena and the size of the computational timestep on the numer ical solution. Qualitative measures of the CFD solutions were independent of the grid resolution. Conversely, quantitative com parisons of the results indicated that the use of coarse computa tional grids results in an under prediction of the hydrodynamic forces on the turbine blade in comparison to the forces predicted using more resolved grids. For the turbine operating conditions considered in this study, the effect of the computational timestep on the CFD solution was found to be minimal, and the results from steady and transient simulations were in good agreement. Additionally, the CFD results were compared to corresponding blade element momentum method calculations and reasonable agreement was shown. Nevertheless, we expect that for other turbine operating conditions, where the flow over the blade is separated, transient simulations will be required.

Towards a Low-Cost Modelling System for Optimising the Layout of Tidal Turbine Arrays

“…The finite difference scheme used in the model is based on the alternating direction implicit (ADI) technique where each timestep is sub-divided into two half-timesteps. Values of ζ, q x and φ Energies 2015, 8,[13521][13522][13523][13524][13525][13526][13527][13528][13529][13530][13531][13532][13533][13534][13535][13536][13537][13538][13539] are, therefore, computed for the first half-timestep and ζ, q y and φ are computed for the second half-timestep.…”

“…Corresponding deficit plots for the staggered array and the single turbine of Section 4.1 are included for comparison. For the in-line array, the cumulative effect of energy extraction by each subsequent downstream turbine means the velocity deficit increases through the Energies 2015, 8,[13521][13522][13523][13524][13525][13526][13527][13528][13529][13530][13531][13532][13533][13534][13535][13536][13537][13538][13539] array, reaching a maximum at the fourth row turbine (at 15 RD downstream), and the magnitude of the maximum deficit is much greater than that for the single turbine, 0.27 versus 0.16. On ebbing tides, the first row of turbines (at 0 RD downstream) experience similar reductions in available power to the fourth row turbines.…”

“…As would be expected the more complex of these are used in near-field models. In the sliding mesh model (SMM) approach (e.g., [7,8]) simulation of a rotating turbine is achieved by using a sliding mesh interface to enable a region of cells rotate within a larger, static grid. It is the most computationally expensive approach as it enables detailed simulation of the rotor motion and the resulting complex flows.…”

“…It is the most computationally expensive approach as it enables detailed simulation of the rotor motion and the resulting complex flows. The rotating (or moving) reference frame (RRF) approach (e.g., [8][9][10][11]) enables the simulation of rotating flows and uses detailed blade geometry to provide simulate the hydrodynamics around the turbine rotor including the downstream wake. The governing equations of flow are solved, in a reference frame which rotates at the same speed as the turbine, by the addition of Coriolis and centripetal forcing terms to the momentum equation.…”

“…Near-field models are very highly resolved in three dimensions and employ mesh elements some orders of magnitude smaller than the diameter of the simulated turbine rotor. They are, therefore, ideally suited to the study of flow through and around a turbine (e.g., [5][6][7][8][9][10][11][12][13][14]); however, high computational costs have limited their application to the modelling of single (or very small numbers of) turbines in idealised channels, primarily under conditions of steady flow. Far-field models are typically two-dimensional (depth-averaged) and employ low spatial resolutions, typically orders of magnitude greater than the scale of the simulated turbine rotors.…”