Zhenzhou Zhao

Investigation of meshing strategies and turbulence models of computational fluid dynamics simulations of vertical axis wind turbines

Making good use of vertical axis wind turbines (VAWTs) is an attractive and potential way to deal with the energy and environmental issues due to the unique superiorities of it. Computational Fluid Dynamics (CFD) technology is a useful tool for the design process of VAWTs. Various turbulence models have been developed and available for turbulent flow simulations. Currently, there have been few researchers studying on meshing strategies and turbulence model selections of VAWT simulations. In this paper, 2D unsteady models under 4 meshing strategies and 6 turbulence models were established and simulated to investigate the effect of the above two aspects on numerical simulations of VAWTs. The numerical results were compared with the experimental data of Oler et al. (“Dynamic stall regulation of the Darrieus Turbine,” SAND Report No. 83–7029, Sandia National Laboratories, Albuquerque, 1983, pp. 67–96) and the analytical solution of Deglaire et al. (Eur. J. Mech., B: Fluids 28(4), 506–520 (2009)). The results reveal that a mesh of 213 656 grids is sufficient to meet the requirements of grid independence with the help of boundary layer and size function techniques. Besides, the realizable k-e model enables the closest CFD simulation of the experimental data and shows better prediction performance than the analytical model of Deglaire et al. and other turbulence models.

Study on approach of performance improvement of VAWT employing double multiple stream tubes model

On the circular blade path of a Darrieus-type vertical axis wind turbine (VAWT), there are two azimuth positions where torques are small or even negative. These two azimuths have the lowest aerodynamic performance and greatly weaken the overall performance of the VAWT. An approach is proposed to rebuild the flow field of the turbine blades by applying two additional airflows at the two azimuths to enhance the local performance. The aim is to improve the overall power coefficient of the VAWT. A Φ-type VAWT with a diameter of 2 m and a height of 2 m is studied to evaluate the effectiveness of the proposed approach. The effect of additional airflow on the aerodynamic performance of the blade and the turbine is researched using the double multiple streamtube model. The simulation results suggest that the additional airflows are effective in promoting the performance of the rotor blades and the overall rotor; the increase in maximum power coefficient achieved was 6% at an optimum tip speed ratio. Some rules of...

Numerical research on effect of transition on aerodynamic performance of wind turbine blade with vortex generators

In the inner part of a large wind turbine blade, a thick airfoil is commonly used to guarantee structural integrity and assists the blade to run more easily at high angles of attack in variable winds; its local Reynolds number is less than 3.0 × 106. Under these conditions, flow separation and transitions easily occur and exert great impact on blade performance. Vortex generators (VGs) are effective devices used to control flow separation and alter stalling conditions, and hence have been employed in the inner part of the blade in some wind turbines. To clearly understand the aerodynamic performance of a blade with VGs, a blade section employing the thick DU91-W2-250 airfoil and five pairs of VGs is modeled using the transition model. The results for both the coefficient of lift and drag–lift ratio predicted by the transition model are compared with those obtained from the k-ω shear stress transport model and experiment. The comparison demonstrates that the transition model has much higher accuracy. The e...

Application of a turbulent vortex core model in the free vortex wake scheme to predict wind turbine aerodynamics

Developing suitably generalized models of rotor blade vortices that accurately predict their evolution continues to be a challenge for wind turbine analysis. The choice of vortex core model in a free vortex wake (FVW) method is essential to accurately predict rotor aerodynamic performance. In this work, a turbulent vortex core model (β-Vatistas model) was considered and improved. A formula was developed that approximates the key parameter β, which represents the degree of turbulence in the β-Vatistas model. The improved β-Vatistas model was used in the FVW model, and the β value was calculated based on the tip vortex circulation in each iteration. The wake flow characteristic and low speed shaft torque were calculated using the FVW model coupled with the turbulent vortex core model or two laminar vortex core models. The use of the improved β-Vatistas model provided smaller maximum vorticities at the rolled-up angle and a slower dissipation speed of the tip vortex than those calculated by two laminar vorte...

Comparison of two vortex models of wind turbines using a free vortex wake scheme

Developing suitably generalized models for rotor blade vortices that accurately predict their evolution continues to be a challenge for wind turbine analysts. During the past few decades, several vortex models have been developed according to the theoretical analysis and the experimental research. A comparison of two different vortex models is made for predicting wind turbine aerodynamic performance using a free vortex wake (FVW) model. The two models are the Lamb-Oseen vortex model for laminar vortices and the β-Vatistas model for turbulent vortices. A new formula that approximates parameter β, which represents the degree of turbulence in the β-Vatistas model, is proposed. The formula of parameter β is validated by comparison of simulated and measured aerodynamic performances of wind turbines of different blade tip vortex Reynolds numbers. Then, the induced velocity streamlines and the distribution of the axial velocity in the rotational plane are simulated. Also, the differences due to the vortex models are discussed.

Trailing-Edge Flap Control for Mitigating Rotor Power Fluctuations of a Large-Scale Offshore Floating Wind Turbine under the Turbulent Wind Condition

A trailing-edge flap control strategy for mitigating rotor power fluctuations of a 5 MW offshore floating wind turbine is developed under turbulent wind inflow. The wind shear must be considered because of the large rotor diameter. The trailing-edge flap control strategy is based on the turbulent wind speed, the blade azimuth angle, and the platform motions. The rotor power is predicted using the free vortex wake method, coupled with the control strategy. The effect of the trailing-edge flap control on the rotor power is determined by a comparison with the rotor power of a turbine without a trailing-edge flap control. The optimal values of the three control factors are obtained. The results show that the trailing-edge flap control strategy is effective for improving the stability of the output rotor power of the floating wind turbine under the turbulent wind condition.

An analysis of influence of braces on lift type vertical axis wind rotor performance

A vertical axis and lift type wind rotor with 900mm height and 900mm rotational diameter was built. Then the wind rotor was located in a low velocity wind tunnel with a 1000mm×1000mm test exit, to investigation the maintain braces influence to the performance of wind rotor. Three kinds of brace were used; these were 10mm diameter cylider, airfoil and 3mm lamina. The research result show that the brace with cylinder structure is greatly infect on performance of the rotor, the part of useful work which can not be neglected is lost. So airfoil or thin plad is suggested to be used.