Abstract
The accuracy of CFD simulations of vertical axis wind turbines (VAWTs) is known to be significantly associated with the computational parameters, such as azimuthal increment, domain size and number of turbine revolutions before reaching a statistically steady state condition (convergence). A detailed review of the literature, however, indicates that there is a lack of extensive parametric studies investigating the impact of the computational parameters. The current study, therefore, intends to systematically investigate the impact of these parameters, on the simulation results to guide the execution of accurate CFD simulations of VAWTs at different tip speed ratios (λ) and solidities (σ). The evaluation is based on 110 CFD simulations validated with wind-tunnel measurements for two VAWTs. Instantaneous moment coefficient, Cm, and power coefficient, CP, are studied for each case using unsteady Reynolds-averaged Navier-Stokes (URANS) simulations with the 4-equation transition SST turbulence model. The results show that the azimuthal increment dθ is largely dependent on tip speed ratio. For moderate to high λ, the minimum requirement for dθ is 0.5° while this decreases to 0.1° at low to moderate λ. The need for finer time steps is associated to the flow complexities related to dynamic stall on turbine blades and blade-wake interactions at low λ. In addition, the minimum distance from the turbine center to the domain inlet and outlet is 15 and 10 times the turbine diameter, respectively. It is also shown that 20–30 turbine revolutions are required to ensure statistically converged solutions. The current findings can serve as guidelines towards accurate and reliable CFD simulations of VAWTs at different tip speed ratios and solidities.
Highlights
Vertical-axis wind turbines (VAWTs) have recently received great interest for wind energy harvesting both for off-shore applications [1,2] and in the built environment [3,4,5]
Note that these characteristics are the same as those used in the earlier study by Rezaeiha et al [6] in which guidelines for the minimum domain size and azimuthal increment for a low-solidity (σ = 0.12) vertical axis wind turbines (VAWTs) operating at a moderate tip speed ratio of 4.5 were provided
A systematic sensitivity analysis is performed using unsteady Reynolds-averaged Navier-Stokes (URANS) simulations to provide the minimum requirements for azimuthal increment, domain size and convergence criterion for Computational Fluid Dynamics (CFD) simulation of VAWTs at different tip speed ratios and solidities
Summary
Vertical-axis wind turbines (VAWTs) have recently received great interest for wind energy harvesting both for off-shore applications [1,2] and in the built environment [3,4,5]. To the best of our knowledge, there is a lack of sensitivity analysis for minimum requirements for computational settings and parameters for CFD simulations of VAWTs at different tip speed ratios and solidities. The sensitivity of the minimum requirements for azimuthal increment, domain size (distance from the turbine center to the domain inlet and outlet) and convergence criterion to tip speed ratio and solidity is extensively discussed in Sections 4–7, respectively.
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