CFD simulation of a vertical axis wind turbine operating at a moderate tip speed ratio: Guidelines for minimum domain size and azimuthal increment

Abdolrahim Rezaeiha,Ivo Kalkman,Bert Blocken

doi:10.1016/j.renene.2017.02.006

Abdolrahim Rezaeiha, Ivo Kalkman + Show 1 more

Open Access

https://doi.org/10.1016/j.renene.2017.02.006

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Abstract

Accurate prediction of the performance of a vertical-axis wind turbine (VAWT) using Computational Fluid Dynamics (CFD) simulation requires a domain size that is large enough to minimize the effects of blockage and uncertainties in the boundary conditions on the results. It also requires the employment of a sufficiently fine azimuthal increment (dθ) combined with a grid size at which essential flow characteristics can be accurately resolved. The current study systematically investigates the effect of the domain size and azimuthal increment on the performance of a 2-bladed VAWT operating at a moderate tip speed ratio of 4.5 using 2-dimensional and 2.5-dimensional simulations with the unsteady Reynolds-averaged Navier-Stokes (URANS). The grid dependence of the results is studied using three systematically refined grids. The turbine has a low solidity of 0.12 and a swept area of 1 m2. Refining dθ from 10.0° to 0.5° results in a significant (≈43%) increase in the predicted power coefficient (CP) while the effect is negligible (≈0.25%) with further refinement from 0.5° to 0.05° at the given λ. Furthermore, a distance from the turbine center to the domain inlet and outlet of 10D (D: diameter of turbine) each, a domain width of 20D and a diameter of the rotating core of 1.5D are found to be safe choices to minimize the effects of blockage and uncertainty in the boundary conditions on the results.

Highlights

Vertical-axis wind turbines (VAWTs) have received growing interest for wind energy harvesting offshore [1] as well as in the urban environment [2e5]
The current study systematically investigates the effect of the domain size and azimuthal increment on the performance of a 2-bladed vertical-axis wind turbines (VAWTs) operating at a moderate tip speed ratio of 4.5 using 2-dimensional and 2.5-dimensional simulations with the unsteady Reynolds-averaged Navier-Stokes (URANS)
In order to check that this is the case the blade Reynolds number for the given l is calculated based on geometrical relations for VAWTs using Eqns. (1) and (2) [31] and found to be in the range 100,000 < Regeo < 200,000; in this range the static stall angle of the airfoil is approximately 14 [32] while the maximum geometrical angle of attack (calculated using Eqn (3) [31]) is less than 13

Summary

Introduction

Vertical-axis wind turbines (VAWTs) have received growing interest for wind energy harvesting offshore [1] as well as in the urban environment [2e5]. For offshore application this can be attributed to their scalability, reliability and low installation and maintenance costs, while for environments with frequent changes in wind direction such as urban environments their omnidirectional capability is their main advantage. The current renewed interest has resulted in more research and further understanding of VAWT flow complexities. These complexities include dynamic stall [6,7], flow curvature effects [8], blade-wake interactions and unsteady 3D wake dynamics [9]. Increased understanding of the aerodynamics of VAWTs has enabled further optimization of their performance which has been conducted using low-to moderate-fidelity inviscid modeling [10,11], high-fidelity viscous CFD simulations [12,13] and wind tunnel tests [9]

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