Vertical Axis Wind Turbine Airfoil Research Articles

Effects of fluctuating velocity on dynamic stall of vertical axis wind turbine airfoil

In contrast to the classical dynamic stall with steady velocity, the dynamic stall phenomenon observed in vertical axis wind turbines encompasses not only periodic variations in the angle of attack but also periodic fluctuation in velocity. Therefore, this study performs a research to deepen understanding of the dynamic stall characteristics of an airfoil used in a vertical axis wind turbine when subjected to fluctuating velocities. The formation of the leading-edge vortex is brought forward for a larger resultant velocity and postponed for a smaller one, according to the comparing the dynamic stall characteristics under different fluctuating velocities and phase angles. Meanwhile, the convection and dissipation of the leading-edge vortex is also diminished as a result of the smaller resultant velocity. As a result, the aerodynamic loads, particularly the drag coefficient, present a nonsymmetrical form. Finally, the tangential force coefficient of an airfoil under fluctuating velocity is shown to have a larger peak in the upwind region and smaller peak in the downwind region, indicating that the blade of a vertical axis wind turbine would suffer more obviously aerodynamic imbalance under a lower tip speed ratio condition.

Aerodynamic shape optimization of H-VAWT blade airfoils considering a wide range of angles of attack

Abstract The current H-type vertical axis wind turbine (VAWT) airfoils are from horizontal axis wind turbine airfoils or symmetry airfoils that are designed at one angle of attack (such as α = 6°) rather than different angles of attack. As a consequence, it cannot, to a certain extent, increase wind power efficiency. Therefore, an optimal method of H-type VAWT blade airfoils in different ranges of angles of attack is presented. It can be expressed by airfoil integrated function. Then, an optimized mathematical model in which the objective function is the average of tangential force coefficients is established. The particle swarm optimization algorithm coupled with RFOIL program is introduced to optimize the H-type VAWT airfoil profiles with high aerodynamic performance. The optimized results show that the new HVAWT-00153 airfoil is more suitable to VAWTs than the other two new airfoils and NACA-0015 airfoil. Besides, by using computational fluid dynamics technology, the superiority of HVAWT-00153 airfoil over NCAC-0015 airfoil is reviewed. The results indicate that the H-type VAWT with new HVAWT-00153 airfoils could exhibit larger torque coefficients and higher power coefficients than that of the original H-type VAWT with NACA-0015 airfoils. The maximum power coefficient can reach 0.362, increased by 8.45% compared with that of the original one. This study has a good guidance to how to design the H-type VAWT airfoils with high wind energy power.

Effect of airfoil shape on power performance of vertical axis wind turbines in dynamic stall: Symmetric Airfoils

The current design of vertical axis wind turbines (VAWTs) suffers from inevitable change in tip speed ratio, λ, in variant wind conditions due to fixed rotor speed. At relatively high wind speeds, which are promising due to high wind power potential, VAWTs operate at low λ with poor power coefficient. Morphing airfoils can be a potential solution by modifying the airfoil shape to optimal at each λ. The optimal airfoil shape for VAWTs at low λ, where dynamic stall is present, has not yet been studied in the literature, therefore, the present study addresses this gap by focusing on this regime to serve as a step towards designing morphing airfoils for VAWTs by identifying the optimal airfoil shape at low λ. The present study performs a combined analysis of three shape defining parameters, namely the airfoil maximum thickness and its position as well as the leading-edge radius, to reveal the overall design space. The analysis is based on 252 high-fidelity transient CFD simulations of 126 identical airfoil shapes. The simulations are verified and validated with three experiments. The results show that the three shape defining parameters have a fully coupled impact on the turbine power and thrust coefficients. When λ reduces from 3.0 to 2.5, the optimal airfoil changes from NACA0018–4.5/2.75 to NACA0024–4.5/3.5, that is increasing the maximum thickness from 18%c to 24%c and shifting its position from 27.5%c to 35%c, while the leading-edge radius index, I, remains 4.5. In general, reducing I from the default value of 6.0 to 4.5 is found to increase the turbine CP.

Surrogate-based aerodynamic shape optimization for delaying airfoil dynamic stall using Kriging regression and infill criteria

The dynamic stall phenomenon is characterized by the formation of a leading-edge vortex, which is responsible for adverse aerodynamic forces and moments adversely impacting the structural strength and life of a system. Aerodynamic shape optimization (ASO) provides a cost-effective approach to delay or mitigate the dynamic stall characteristics. Unfortunately, ASO requires multiple evaluations of accurate but time-consuming computational fluid dynamics (CFD) simulations to produce optimum designs rendering the optimization process computationally costly. The current work proposes a surrogate-based optimization (SBO) technique to alleviate the computational burden of ASO to delay and mitigate the deep dynamic stall characteristics of airfoils. In particular, the Kriging regression surrogate model is used for approximating the objective and constraint functions. The airfoil geometry is parametrized using six PARSEC parameters. The objective and constraint functions are evaluated with the unsteady Reynolds-averaged Navier-Stokes equations with a C-grid mesh topology and Menter's shear stress transport turbulence model. The approach is demonstrated on a vertical axis wind turbine airfoil at a Reynolds number of 135,000 and a Mach number of 0.1 undergoing a sinusoidal oscillation with a reduced frequency of 0.05. The surrogate model is constructed with 60 initial samples and further refined with 20 infill samples using expected improvement. The generated surrogate model is validated with the normalized root mean square error based on 20 test data samples. The refined surrogate model is utilized for finding the optimal design using multi-start gradient-based search. The optimal airfoil has a higher thickness, larger leading-edge radius, and an aft camber compared to the baseline. These geometric shape changes delay the dynamic stall angle by over 3∘ and reduces the severity of the pitching moment coefficient fluctuation. Finally, global sensitivity analysis is conducted on the optimal design using Sobol' indices revealing the most influential shape variables and their interaction effects impacting the airfoil dynamic stall characteristics.

Experimental and numerical investigation of the influence of roughness and turbulence on LUT airfoil performance

Vertical-axis wind turbines (VAWTs) have been widely used in urban environments, which contain dust and experience strong turbulence. However, airfoils for VAWTs in urban environments have received considerably less research attention than those for horizontal-axis wind turbines (HAWTs). In this study, the sensitivity of a new VAWT airfoil developed at the Lanzhou University of Technology (LUT) to roughness was investigated via a wind tunnel experiment. The results show that the LUT airfoil is less sensitive to roughness at a roughness height of < 0.35 mm. Moreover, the drag bucket of the LUT airfoil decreases with increasing roughness height. Furthermore, the loads on the LUT airfoil during dynamic stall were studied at different turbulence intensities using a numerical method at a tip-speed ratio of 2. Before the stall, the turbulence intensity did not considerably affect the normal or tangential force coefficients of the LUT airfoil. However, after the stall, the normal force coefficient varied obviously at low turbulence intensity. Moreover, as the turbulence intensity increased, the normal and tangential force coefficients decreased rapidly, particularly in the downwind region of the VAWT.

Optimization Research on Lift-Type Vertical Axis Wind Turbine Airfoil by CFD

The blade airfoil has an important effect on the performance of a lift-type vertical axis wind turbine (VAWT). In this paper, a research has been carried out based on CFD numerical technology to compare the effects of 4 kinds of S series airfoil (S1012, S1016, S1046, S1048) and traditional airfoil NACA0015 on lift-type VAWT. The turbulence model is used k-ω SST model. The simulation results indicate that: The airfoil NACA0015 has the highest torque coefficient and power coefficient. In terms of S series airfoils, the airfoil S1012 has the best performance and airfoil S1016 performs worst. The airfoil S1016 has the largest static torque coefficient. And the airfoil which has higher power coefficient would have a lower static torque coefficient for S series airfoil. The static torque coefficient of airfoil NACA0015 is lower than airfoil S1016 and S1046.

Airfoil optimization to improve power performance of a high-solidity vertical axis wind turbine at a moderate tip speed ratio

The relatively low power coefficient restricts the wide application of vertical axis wind turbines (VAWTs). An effective solution to this problem is to design specific airfoil profiles which directly influence the capture ratio of wind power. The main aim of the present study is to develop an automatic airfoil profile optimization system to improve the power performance of a VAWT. A three-bladed high-solidity VAWT is adopted as the research object with its chord length, blade span and rotor diameter being 0.2 m, 0.8 m and 0.8 m, respectively. The optimization is conducted at a moderate tip speed ratio (TSR) with a value of 1.0 and the method of coupled CFD simulations with genetic algorithms is employed. The following points make this paper different from previous studies: (a) introducing Multi-Island Genetic Algorithm to optimize airfoils for VAWTs; (b) investigating the airfoil as part of the VAWT rather than as a single isolated body with 3D simulations. The results show that the power coefficient of the VAWT equipped with the optimized blades sensibly improves at all TSRs from 0.4 to 1.5 and the maximum growth rate of it occurs at TSR = 0.9 with a value of 26.82%. The integrated optimization system used in this paper provides an effective way to generate suitable airfoil profiles for given VAWTs with the goal to achieve higher power efficiency.

Investigation of vertical axis wind turbine airfoil performance in rain

Rainfall is a common meteorological condition that wind turbines may encounter and by which their performance may be affected. This paper comprehensively investigates the effects of rainfall on a NACA 0015 airfoil which is commonly used in vertical axis wind turbines. A CFD-based Eulerian–Lagrangian multiphase approach is proposed to study the static, rotating, and oscillating performances of the NACA 0015 airfoil in rainy conditions. It is found that for the different airfoil movements, the airfoil performance can seriously be deteriorated in the rain condition. Rain also causes premature boundary layer separations and more severe flow recirculations than in the dry condition. These findings seem to be the first open reports on rain effects on wind turbine performance and should be of some significance to practical design.

Numerical simulation of a vertical axis wind turbine airfoil experiencing dynamic stall at high Reynolds numbers

Abstract Multi-megawatt floating vertical axis wind turbines (VAWTs) are a promising solution to exploit offshore wind energy resources in deep water sites. At this large-scale, the VAWT’s blades will operate at high Reynolds numbers and encounter dynamic stall at low tip-speed ratios. In particular, the selection of an accurate turbulence modeling approach is still a challenging undertaking in the prediction of transient blade forces during this complex unsteady event. In the present paper, the performance of Unsteady Reynolds-Averaged Navier–Stokes (U-RANS) and Detached Eddy Simulation (DES) modeling methods are compared in simulating the aerodynamics of an isolated NACA0018 airfoil experiencing Darrieus pitching motion. The U-RANS turbulence models employed were the Spalart–Allmaras (S-A) model and the k − ω SST model. Investigations were conducted to ensure satisfactory independency of the solution for both spatial and temporal discretisations, respectively. A quantitative assessment identified the S-A model as the most applicable for a VAWT design study as it showed the most desirable compromise between model fidelity and computational requirement. A qualitative analysis revealed that the thick VAWT airfoil creates a dynamic stall vortex topology highly concentrated at the trailing edge region. Finally, increasing the Reynolds number showed to be beneficial to the airfoil’s aerodynamic performance as a higher maximum tangential coefficient is attained, owing to the delay in flow separation to much higher angles of attack.

Study on stall behavior of a straight-bladed vertical axis wind turbine with numerical and experimental investigations

This paper presents a straight-bladed vertical axis wind turbine (VAWT) model for the evaluation of the stall phenomenon associated with unsteady flow around the airfoil surface with numerical and experimental investigations. In wind tunnel experiments, in order to investigate flow visualization, light-weight tufts attached to the inner surface of blade are used to gain insight on the flow characteristics of VAWT during rotation in the low Reynolds numbers. In numerical analysis, a 2D computational investigation on the stall phenomenon and aerodynamic characteristics at the VAWT airfoil is associated with the standard κ−ε model and Shear Stress Transport κ−ω (SST κ−ω) turbulence model. And then, the stall behavior is validated according to compare the results of wind tunnel experiment and numerical analysis. From this study, it is concluded that the numerical simulation captures well the vortex-shedding predominated flow structure which is experimentally obtained and the results quantitatively agree well with the wind tunnel experimental data. Moreover, the flow separation behavior of the airfoil shows the importance of stall phenomenon and the interaction of the separated vortex with the blade as mechanisms in lift and drag coefficients.

Numerical Simulation of Vertical Axis Wind Turbine Blade Airfoil Performance

Research of blade airfoil aerodynamic characteristics is an important foundation for the vertical axis wind turbine aerodynamic design and performance analysis. CFD simulation software has been applied in this paper. Representative lift-type vertical axis wind turbine airfoil NACA0014, NACA2414, NACA4414, NACA6414, NACA8414 's aerodynamic simulation have been studied. Camber airfoil relative with the change in to the flow velocity is analyzed. At different angles of attack effect on the aerodynamic performance of wind turbines, variation of parameters for airfoil aerodynamic had been analyzed. It will help the optimal design of airfoils for vertical axis wind turbines.

Analysis for 2D Flow Field around H-Vertical Axis Wind Turbine with Different Types of Airfoil

In this study, moving mesh technique is used to construct an outside flow CFD model around H-vertical axis wind turbine with different types of airfoil. RNG turbulent models and the implicit Couple arithmetic based on pressure are selected to solve the transient equation. From the results of the calculation, the author obtains the velocity field, pressure field distribution of H-type vertical axis wind turbine airfoil at different moments and analyze the wind wheel blade torque variation. The conclusion provides the theoretical basis and a method to optimize aerodynamic performance for the further study of vertical axis wind turbine in the future.

The Performance of a High-Lift Airfoil in Turbulent Wind

The performance of a prospective candidate vertical axis wind turbine airfoil S1223 has been investigated at Reynolds numbers of 55,000, 75,000, and 100,000. The airfoil was tested at angles of attack from −5 to 25 deg in a quasi-isotropic turbulent flow generated using orificed plates. The independent effects of the turbulence intensity were examined with a constant integral length scale Λ/ c=0.14 (where c is the chord length). By increasing the turbulence intensity independently from 4.1 to 9.5%, the airfoil exhibits strong stall characteristics at low turbulence, whereas the stall behaviour becomes less obvious at high turbulence. The independent roles of integral length scale were examined by varying integral length Λ/ c from 0.14 to 0.23. The less turbulent free-stream with smaller length scales appears to delay the stall of the airfoil based on the observation of the lift and drag curve, implying that the smaller integral length under low turbulence intensity (4.1%) delays the boundary layer separation on the suction surface. At a higher turbulence of intensity 9.5%, the effect of varying Λ/ c from 0.08 to 0.15 only leads to subtle changes in the lift and drag.