Abstract
This paper documents the predictive capability of rotating blade-resolved unsteady Reynolds averaged Navier-Stokes (URANS) and Improved Delayed Detached Eddy Simulation (IDDES) computations for tidal stream turbine performance and intermediate wake characteristics. Ansys/Fluent and OpenFOAM simulations are performed using mixed-cell, unstructured grids consisting of up to 11 million cells. The thrust, power and intermediate wake predictions compare reasonably well within 10% of the experimental data. For the wake predictions, OpenFOAM performs better than Ansys/Fluent, and IDDES better than URANS when the resolved turbulence is triggered. The primary limitation of the simulations is under prediction of the wake diffusion towards the turbine axis, which in return is related to the prediction of turbulence in the tip-vortex shear layer. The shear-layer involves anisotropic turbulent structures; thus, hybrid RANS/LES models, such as IDDES, are preferred over URANS. Unfortunately, IDDES fails to accurately predict the resolved turbulence in the near-wake region due to the modeled stress depletion issue.
Highlights
Hydroelectric power represents a clean and renewable source of energy that accounts for 16% of all electricity generated in the world [1] and is predominately produced by the impoundment of rivers
Accurate wake modeling research is an essential part of tidal stream turbine (TST) farm design, and both experimental [2,5,6] and computational fluid dynamics (CFD) [7,8,9] studies have contributed to improving the understanding of wind/tidal stream turbine wake characteristics
The analysis of the full-scale experimental data (ReD = 3 × 106 to 107) showed that TST performance is Re independent for the entire λ range (0–7.4), whereas the peak performance characteristics achieved Re independence only above a critical ReD,c > 3 × 105
Summary
Hydroelectric power represents a clean and renewable source of energy that accounts for 16% of all electricity generated in the world [1] and is predominately produced by the impoundment of rivers. Hydrokinetic power, produced by naturally flowing water without impoundment such as river currents, ocean currents, and tidal streams, represents a largely untapped renewable energy source. The flow behind a wind/tidal stream turbine is typically sub-divided into three regions: (1) The near wake region (up to 1D), where D is the rotor diameter. The flow in this region primarily depends on the aerodynamic/hydrodynamic characteristics of the turbine blades. Research in this regime focuses on the performance and the physical process of power extraction [7,10,11]. Vermeer et al [9] noted that the flow physics in the intermediate wake region are not very well understood and are an area of active research
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