Horizontal Axis Wind Turbine Aerodynamics: Three- Dimensional, Unsteady, and Separated Flow Influences (original) (raw)
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The 8th BSME International Conference on Thermal Engineering, 2018
Vertical axis wind turbines (VAWTs) are a type of wind turbines, mainly useful for urban and residential areas to produce electricity. It has some advantages over Horizontal axis wind turbines in terms of costs and maintenances. Dynamic Stalling is a common feature of these VAWTs in unsteady flow conditions. In fact, dynamic stalling is regarded as one of the prior obstructions for the improved aerodynamic features of VAWTs. Thus, it is important to understand the effects of dynamic stalling on it. This paper aims to present the dynamic stall investigation of a two-dimensional VAWT blade, i.e. NACA 0012 at the low-speed condition. The phenomenon was simulated using computational fluid dynamics (CFD) techniques to capture the leading-edge vortex (LEV) and trailing edge vortex on the airfoil due to unsteady flow conditions. ANSYS FLUENT with manually hooked UDF subroutine was used to simulate the numerical results which were later compared to experimental data. Unsteady Reynold Average Navier Stokes (URANS) SST í µí± − í µí¼ modeling was used to capture the dynamic stalling in a more detailed fashion.
Unsteady Flow Simulation and Dynamic Stall around Vertical Axis Wind Turbine Blades
This paper presents a computational study of a rooftop size vertical axis wind turbine with straight blades (Htype). The computational model solves for the two dimensional and three dimensional unsteady flow fields around the turbine using the sliding mesh technique. Interesting features about the dynamic stall around the blades and the interaction of the blade wakes with the following blades are illuminated. Comparison of the 2D and 3D simulations highlight the strong 3D effects, including the blade tip losses and the effects of the blade supporting shaft and arms.
Dynamic Stall for a Vertical Axis Wind Turbine in a Two-Dimensional Study
Proceedings of the World Renewable Energy Congress – Sweden, 8–13 May, 2011, Linköping, Sweden, 2011
The last few years have proved that Vertical Axis Wind Turbines (VAWTs) are more suitable for urban areas than Horizontal Axis Wind Turbines (HAWTs). To date, very little has been published in this area to assess good performance and lifetime of VAWTs either in open or urban areas. At low tip speed ratios (TSRs<5), VAWTs are subjected to a phenomenon called 'dynamic stall'. This can really affect the fatigue life of a VAWT if it is not well understood. The purpose of this paper is to investigate how CFD is able to simulate the dynamic stall for 2-D flow around VAWT blades. During the numerical simulations different turbulence models were used and compared with the data available on the subject. In this numerical analysis the Shear Stress Transport (SST) turbulence model seems to predict the dynamic stall better than the other turbulence models available. The limitations of the study are that the simulations are based on a 2-D case with constant wind and rotational speeds instead of considering a 3-D case with variable wind speeds. This approach was necessary for having a numerical analysis at low computational cost and time. Consequently, in the future it is strongly suggested to develop a more sophisticated model that is a more realistic simulation of a dynamic stall in a threedimensional VAWT.
Dynamic stall analysis of horizontal-axis-wind-turbine blades using computational fluid dynamics
2012
Dynamic stall has been widely known to significantly affect the performance of the wind turbines. In this paper, aerodynamic simulation of the unsteady low-speed flow past two-dimensional wind turbine blade profiles, developed by the National Renewable Energy Laboratory (NREL), will be performed. The aerodynamic simulation will be performed using Computational Fluid Dynamics (CFD). The governing equations used in the simulations are the Unsteady-Reynolds-Averaged-Navier-Stokes (URANS) equations. The unsteady separated turbulent flow around an oscillating airfoil pitching in a sinusoidal pattern in the regime of low Reynolds number is investigated numerically. The investigation employs the URANS approach with the most suitable turbulence model. The development of the light dynamic stall of the blades under consideration is studied. The S809 blade profile is simulated at different mean wind speeds. Moreover, the S826 blade profile is also considered for analysis of wind turbine blade which is the most suitable blade profile for the wind conditions in Egypt over the site of Gulf of El-Zayt. In order to find the best oscillating frequency, different oscillating frequencies are studied. The best frequency can then be used for the blade pitch controller. The comparisons with the experimental results showed that the used CFD code can accurately predict the blade profile unsteady aerodynamic loads.
2017
Unsteady flow physics of two types of vertical axis wind turbines (VAWTs), namely, a modified Savonius turbine and a hybrid Darrieus-modified-Savonius (HDMS) turbine, are numerically studied using a fluid-structure interaction approach. As a first step, a numerical solver is developed by coupling the wind turbine dynamics with a high-fidelity flow solver, thus, synchronizing the flow-turbine interaction. The solver is then used to study unsteady aerodynamics of wind-driven modified Savonius and HDMS VAWTs under different loading conditions. The relationship between the power efficiency and tip speed ratio (TSR) is invested for both types of wind turbines. It is found that there exists large disparity on energy harvesting performance between the two types of VAWTs under wind-driven flow conditions. The modified Savonius VAWT achieves relatively high power efficiency (~30% or ~51% of the Betz's limit) at small TSRs; the HDMS VAWT achieves high power efficiency (~40% or ~67% of the Betz's limit) at large TSRs. At the same incoming wind speed, the maximum power efficiency of the HDMS VAWT is about 10% higher than that of the Savonius VAWT. It is also observed that the external loading can affect dynamic stall over Darrieus blades of the HDMS VAWT, thus significantly varying the TSR and the corresponding energy harvesting efficiency.
Wind Energy
Growing horizontal axis wind turbines are increasingly exposed to significant sources of unsteadiness, such as tower shadowing, yawed or waked conditions and environmental effects. Due to increased dimensions, the use of steady tabulated airfoil coefficients to determine the airloads along long blades can be questioned in those numerical fluid models that do not have the sufficient resolution to solve explicitly and dynamically the flow close to the blade. Various models exist to describe unsteady aerodynamics (UA). However, they have been mainly implemented in engineering models, which lack the complete capability of describing the unsteady and multiscale nature of wind energy. To improve the description of the blades' aerodynamic response, a 2D unsteady aerodynamics model is used in this work to estimate the airloads of the actuator line model in our fluid-structure interaction (FSI) solver, based on 3D large eddy simulation. At each section along the actuator lines, a semiempirical Beddoes-Leishman model includes the effects of noncirculatory terms, unsteady trailing edge separation, and dynamic stall in the dynamic evaluation of the airfoils' aerodynamic coefficients. The aeroelastic response of a utility-scale wind turbine under uniform, laminar and turbulent, sheared inflows is examined with oneand two-way FSI coupling between the blades' structural dynamics and local airloads, with and without the enhanced aerodynamics' description. The results show that the external half of the blade is dominated by aeroelastic effects, whereas the internal one is dominated by significant UA phenomena, which was possible to represent only thanks to the additional model implemented.
Viscous Flow and Dynamic Stall Effects on Vertical-Axis Wind Turbines
International Journal of Rotating Machinery, 1995
The present paper describes a numerical method, aimed to simulate the flow field of vertical-axis wind turbines, based on the solution of the steady, incompressible, laminar Navier-Stokes equations in cylindrical coordinates. The flow equations, written in conservation law form, are discretized using a control volume approach on a staggered grid. The effect of the spinning blades is simulated by distributing a time-averaged source terms in the ring of control volumes that lie in the path of turbine blades. The numerical procedure used here, based on the control volume approach, is the widely known “SIMPLER” algorithm. The resulting algebraic equations are solved by the TriDiagonal Matrix Algorithm (TDMA) in the r- and z-directions and the Cyclic TDMA in the 0-direction. The indicial model is used to simulate the effect of dynamic stall at low tip-speed ratio values. The viscous model, developed here, is used to predict aerodynamic loads and performance for the Sandia 17-m wind turbi...
Journal of Solar Energy Engineering-transactions of The Asme, 2005
The aerodynamic performance of the National Renewable Energy Laboratory (NREL) Phase VI horizontal axis wind turbine (HAWT) under yawed flow conditions is studied using a three-dimensional unsteady viscous flow analysis. Simulations have been performed for upwind cases at several wind speeds and yaw angles. Results presented include radial distribution of the normal and tangential forces, shaft torque, root flap moment, and surface pressure distributions at selected radial locations. The results are compared with the experimental data for the NREL Phase VI rotor. At low wind speeds ͑ϳ7 m/s͒ where the flow is fully attached, even an algebraic turbulence model based simulation gives good agreement with measurements. When the flow is massively separated (wind speed of 20 m/s or above), many of the computed quantities become insensitive to turbulence and transition model effects, and the calculations show overall agreement with experiments. When the flow is partially separated at wind speed above 15 m/s, encouraging results were obtained with a combination of the Spalart-Allmaras turbulence model and Eppler's transition model only at high enough wind speeds.
Advances in engineering research, 2015
Vertical axis turbines have many attractive features and are well suited for extraction of wind and hydrokinetic energy. At low angular velocities, the rotor blades momentarily experience high angles of attack and move in and out of stall. This gives rise to unsteady aerodynamic loads that may cause vibration and structural fatigue. The high pressure drag and loss of lift during dynamic stall also leads to a lower efficiency compared to horizontal axis systems. In this work, a three-pronged approach with increasing complexity is used to understand and model this complex phenomenon, for a representative system. Empirical curve fits are first used to map the efficiency of the turbine as a function of tip speed ratio. An existing double multiple streamtube methodology (DMST) is next used, with a variety of empirical methods for modeling dynamic stall. Finally, computational fluid dynamics (CFD) results are presented and compared with test data and the DMST approach. It is concluded that DMST approaches are well suited for screening and designing vertical axis wind turbine configurations, followed by detailed modeling of the physical phenomena using well validated CFD tools.