VAWT Aerodynamic Modeling

Aerodynamic modeling of floating vertical axis wind turbines using the actuator cylinder flow method

Zhengshun Chenga, Helge Aagaard Madsend, Zhen Gaoa, Torgeir Moan, Energy Procedia, January 2016

Recently the interest in developing vertical axis wind turbines (VAWTs) for offshore application has been increasing. Among the aerodynamic models of VAWTs, double multi-streamtube (DMST) and actuator cylinder (AC) models are two favorable methods for fully coupled modeling and dynamic analysis of floating VAWTs in view of accuracy and computational cost. This paper deals with the development of an aerodynamic code to model floating VAWTs using the AC method developed by Madsen. It includes the tangential load term when calculating induced velocities, addresses two different approaches to calculate the normal and tangential loads acting on the rotor, and proposes a new modified linear solution to correct the linear solution.

Recent work by Rolin & Porte-Agel (2015) has examined the far wake of a VAWT, up to approximately 7 rotor diameters downstream of the turbine. They noted that the effect of the boundary layer in the core of the wake was to re-energize the region with downward entrained momentum. The near wake is dominated by the periodic shedding of vortices from the turbine blades, and the far wake exhibits low-frequency oscillations characteristic of bluff bodies. In between these two regions of the flow, there is a transition, where it is likely that a shear-layer instability develops; this is thought to be the underlying mechanism that leads to the far-wake oscillations in VAWTs just as it does for any other bluff body. A key conclusion drawn from the results is that there are two primary competing fluid mechanisms within a VAWT array that contribute to the overall performance. This includes turbine blockage, which can locally accelerate the  flow adjacent to a turbine and thereby increase the performance of neighboring turbines above their performance in isolation, and the turbine wake, which locally decelerates the flow and leads to a decrease in performance for downstream turbines.

The fixed-pitch straight-bladed vertical axis wind turbine (SB-VAWT) is one of the simplest types of wind turbine. The overall cost of the SB-VAWT will mainly depend on judicious choice of multiple design parameters. An attempt has been made in this paper to analyze the influence of three design parameters related to smaller-capacity fixed-pitch SB-VAWT with the help of a computational scheme. The three design parameters are: (i) solidity, (ii) aspect ratio and (iii) blade pitching. Affect of these parameters are analyzed for a SB-VAWT equipped with a special purpose airfoil "MIVAWT1" which has been designed for a smaller-capacity fixedpitch SB-VAWT. It has been demonstrated in this paper that proper selections of these three parameters are important for a cost-effective smaller-capacity SB-VAWT which can be considered as a candidate for urban and off-grid rural applications.

A Low-Reynolds-Number, High-Angle-of-Attack Investigation of Wind Turbine Aerofoils

S Worasinchai, G Ingram and R Dominy, SAGE and Institution of Mechanical Engineers, 18 July 2011

This article describes an experimental, aerodynamic investigation of four aerofoils intended for small wind turbine applications. The aerofoils of these small machines (both horizontal and vertical axes) normally experience conditions that are quite different from large-scale machines due to smaller chord length and lower wind speed, resulting in significantly lower Reynolds numbers. They also operate with an unusually wide range of incidence angles (0° to 90° for horizontal axis and 0° to 360° for vertical axis). Four appropriate aerofoils were chosen for testing at three Reynolds numbers (65000, 90000, and 150000) through 360° incidence to cover almost all possible conditions that might be encountered by both types of turbines...

A Model for the Response of Vertical Axis Wind Turbines to Turbulent Flow Parts 1 and 2

David R. Malcolm, Sandia National Laboratories, July 1988

This report describes a project intended to incorporate the effects of atmospheric turbulence into the structural .response of Darrieus rotor, vertical axis wind turbines. The basis of the technique is the generation of a suitable time series of wind velocities, which are passed through a double multiple streamtube aerodynamic representation of the rotor. The aerodynamic loads are decomposed into components of the real eigenvectors of the rotor and subsequently into full-power and cross-spectral densities. These modal spectra are submitted as input to a modified NASTRAN random load analysis and the power spectra of selected responses are obtained. This procedure appears to be successful. Results at zero...

Multi-megawatt Vertical Axis Wind Turbines (VAWTs) are experiencing an increased interest for floating off-shore applications. However, VAWT development is hindered by the lack of fast, accurate and validated simulation models. This work compares six different numerical models for VAWTS: a multiple streamtube model, a double-multiple streamtube
model, the actuator cylinder model, a 2D potential ow panel model, a 3D unsteady lifting line model, and a 2D conformal mapping unsteady vortex model. The comparison covers rotor configurations with two NACA0015 blades, for several tip speed ratios, rotor solidity and fi xed pitch angle, included heavily loaded rotors, in inviscid flow.

Wind alone can fulfill most of the energy requirement of the world by its efficient conversion in to energy. Though Horizontal Axis Wind Turbine (HAWT) is more popular but needs high wind speed to generate energy. On the other hand Vertical Axis Wind Turbine (VAWT) needs low wind speed and can be installed anywhere which are some of the reasons for this research. The main objective of this research is to improve the design and performance of VAWT to make it more attractive, efficient, durable and sustainable. For a VAWT the blades perform the main role to extract energy from the wind. Airfoil is considered as the blade for this new design of VAWT. Airfoil has some good aerodynamic characteristics, match with...

This paper investigates the effects of optimized airfoil on VAWT (vertical axis wind turbine) aerodynamic performance. The thickness and camber of the airfoil are selected as the constraints, the value of the maximum tangential force coefficient is chosen as the objective function, optimizing NACA0015 airfoil to enhance the wind energy utilization efficiency of the VAWT, a 3D CFD simulation is used to get the flow characteristics of the VAWT under variable tip speed ratio (TSR) conditions. To ensure the accuracy of the numerical simulation, the power coefficient calculated by CFD is validated against previous experimental result. The optimized airfoil is shown to improve the aerodynamic performance of the wind turbine. Through investigating the effects of optimized airfoil on the rotor flow field, this paper proposes measures to improve the VAWT aerodynamic performance under variable TSRs: measures should be made to avoid or delay the flow separation of blades and the stall vortex shedding and reduce the stall vortex scale at low TSR, while at high TSR measures should be made to shorten the wake length of the blades and reduce its diffusion range.

Aspect Ratio (AR) is one of the main design parameters of straight-bladed vertical axis turbines. This paper will examine whether a high AR, with long blades and low tip losses, or a low AR, with a higher diameter and higher losses, is more suitable to achieve the maximum power output given a fixed cross-sectional area. Traditional Double-Multiple Stream-Tube (DMST) approaches are limited by a lack of tip loss formulations specifically conceived for vertical axis turbines. Therefore, a CFD-3D investigation covering a power range from micro-generation to MW has been done. Results show that both Reynolds number and tip losses strongly influence the aerodynamic performance of the rotor. More advantages seem to be achieved by limiting tip losses rather than increasing chord-based Reynolds number (Rec), addressing towards high AR at least for medium and large-size turbines. However, as turbine size and wind speed decrease, this difference narrows considerably. For micro turbines, tip losses are balanced by the effects of Rec, thus a variation of AR does not imply a variation of CP. For all the cases that have been analysed, turbine size and therefore Rec does not appreciably affect the normalized CP distribution along the blade, which only depends on AR.

Effect Of The Shaft On The Aerodynamic Performance Of Urban Vertical Axis Wind Turbines

Abdolrahim Rezaelha, Ivo Kalkman, Hamid Montazeri, Bert Blocken, Elsevier Ltd, October 2017

The central shaft is an inseparable part of a vertical axis wind turbine (VAWT). For small turbines such as those typically used in urban environments, the shaft could operate in the subcritical regime, resulting in large drag and considerable aerodynamic power loss. The current study aims to (i) quantify the turbine power loss due to the presence of the shaft for different shaft-to-turbine diameter ratios δ from 0 to 16%, (ii) investigate the impact of different operational and geometrical parameters on the quantified power loss and (iii) evaluate the impact of the addition of surface roughness on turbine performance improvement. Unsteady Reynolds-averaged Navier-Stokes (URANS) calculations are...

Fish schooling as a basis for vertical axis wind turbine farm design*

Robert Whittlesey, Sebastian Liska, and John Dabiri, California Institute of Technology, February 11, 2010

Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the high power coefficient (mechanical power output divided by the power of the free-stream air through the turbine cross-sectional area) of an isolated turbine. However when in close proximity to neighbouring turbines, HAWTs suffer from a reduced power coeffi- cient. In contrast, previous research on vertical axis wind turbines (VAWTs) suggests that closely-spaced VAWTs may experience only small decreases (or even increases) in an individual turbine’s power coefficient when placed in close proximity to neighbours, thus yielding much higher power outputs for a given area of land...

Measurements and Calculations of Aerodynamic Torques for a Vertical Axis Wind Turbine

Robert E. Akins, Dale E. Berg, W. Tait Cyrus, Sandia National Labs, October 1987

This report describes measurements of aerodynamic torque on a vertical-axis wind turbine. Accelerometers mounted at the equator of the rotor and a torque meter mounted at the base of the rotor were used to compute the net aerodynamic torque acting on the rotor. Assumptions concerning blade- response symmetry were required to achieve blade torque as a function of rotor position on each half of a revolution for a two-bladed rotor. Results are presented for tip-speed ratios from 2.5 to 8.0 for two turbine rotational speeds. Evidence of dynamic stall is observed at low tip-speed ratios.

MESH CONVERGENCE STUDY FOR 2-D STRAIGHT-BLADE VERTICAL AXIS WIND TURBINE SIMULATIONS AND ESTIMATION FOR 3-D SIMULATIONS

Saman Naghib Zadeh, Matin Komeili and Marius Paraschivoiu
Mechanical and Industrial Engineering, Concordia University, September 2014

Mesh resolution requirements are investigated for 2-D and 3-D simulations of the complex flow around a straight-blade vertical axis wind turbine (VAWT). The resulting flow, which may include large separation flows over the blades, dynamic stall, and wake-blade interaction, is simulated by an Unsteady Reynolds- Averaged Navier–Stokes analysis, based on the Spalart–Allmaras (S–A) turbulence model. A grid resolution study is conducted on 2-D grids to examine the convergence of the CFD model. Hence, an averaged grid residual of y+ > 30 is employed, along with a wall treatment, to capture the near-wall region’s flow structures. Furthermore a 3-D simulation on a coarse grid of the VAWT model is performed in order to explore the influence of the 3-D effects on the aerodynamic performance of the turbine. Finally, based on the 2-D grid convergence study and the 3-D results, the required computational time and mesh to simulate 3-D VAWT accurately is proposed.

Modeling Stochastic Wind Loads on Vertical Axis Wind Turbines

Paul S. Veers, Sandia National Laboratories, September 1984

The Vertical AXIS Wind Turbine (VAWT) is a machine which extracts energy from the wind. Since random turbulence is always present, the effect of this turbulence on the wind turbine fatigue life must be evaluated. This problem is approached by numerically simulating the turbulence and calculating, In the time domain, the aerodynamic loads on the turbine blades. These loads are reduced to the form of power and cross spectral densities which can be used in standard linear structural analysis codes. The relative importance of the turbulence on blade loads is determined.

Horizontal axis wind turbines (HAWTs) are the more common form of wind energy production, but attention is increasingly given to vertical axis wind turbines (VAWTs), which have their main rotor shaft positioned transverse the wind. VAWTs can accept wind coming from any direction and operate in harsher wind conditions. However, VAWT usage grows slower than that of HAWTs, especially in large scale, because they also have shortcomings, like trouble self-starting and lower power efficiency. To increase power efficiency, researchers from universities in China and Denmark proposed and evaluated a new blade design for H-type VAWTs. They report their findings in the Journal of Renewable and Sustainable Energy.

Numerical Investigations of the Effects of the Rotating Shaft and Optimization of Urban Vertical Axis Wind Turbines

Lidong Zhang, Kaiqi Zhu, Junwei Zhong, Ling Zhang, Tieliu Jiang, Shaohua Li and Zhongbin Zhang, Energies - Open Access Journal of Energy Research, Engineering and Policy, 18, July 2018

The central shaft is an important and indispensable part of a small scale urban vertical axis wind turbines (VAWTs). Normally, it is often operated at the same angular velocity as the wind turbine. The shedding vortices released by the rotating shaft have a negative effect on the blades passing the wake of the wind shaft. The objective of this study is to explore the influence of the wake of rotating shaft on the performance of the VAWT under different operational and physical parameters. The results show that when the ratio of the shaft diameter to the wind turbine diameter ( ) is 9%, the power loss of the wind turbine in one revolution increases from 0% to 25% relative to that of no-shaft wind turbine (this is a numerical experiment for which the shaft of the VAWT is removed in order to study the interactions between the shaft and blade). When the downstream blades pass through the wake of the shaft, the pressure gradient of the suction side and pressure side is changed, and an adverse effect is also exerted on the lift generation in the blades. In addition, = 5% is a critical value for the rotating shaft wind turbine (the lift-drag ratio trend of the shaft changes differently). In order to figure out the impacts of four factors; namely, tip speed ratios (TSRs) , turbulence intensity (TI), and the relative surface roughness value (ks/ds) on the performance of a VAWT system, the Taguchi method is employed in this study. The influence strength order of these factors is featured by TSRs > ks/ds > > TI. Furthermore, within the range we have analyzed in this study, the optimal power coefficient (Cp) occurred under the condition of TSR = 4, = 5%, ks/ds = 1  10􀀀2, and TI = 8%.

Blade fatigue life is an important element in determining the economic viability of the VerticalAxis Wind Turbine (VAWT). A principal source of blade fatigue is thought to be the stochastic (i.e., random) aerodynamic loads created by atmospheric turbulence. This report describes the theoretical background of the VAWT Stochastic Aerodynamic Loads (VAWT-SAL) computer code, whose purpose is to numerically simulate these random loads, given the rotor geometry, operating conditions, and assumed turbulence properties. A Double-Multiple-StreamTube (DMST) analysis is employed to model the rotor’s aerodynamic response. The analysis includes the effects of Reynolds number variations, different airfoil sections and...

Parametrical evaluation of the aerodynamic performance of vertical axis wind turbines for the proposal of optimized designs

Andrés Meana-Fernández, Irene Solís-Gallego, Jesús Manuel Fernández Oro, Katia María Argüelles Díaz, Sandra Velarde-Suárez, March 2018

Many studies have tried to give insight into the optimal values of solidity and the airfoil geometry that maximize the performance and self-starting capability of vertical axis wind turbines, but there is still no consensus. In addition, most studies focus on one particular airfoil or airfoil family, which makes the generalization of the results difficult. In this work, these research gaps are intended to be assessed. An exhaustive analysis of the influence of solidity, blade Reynolds number and airfoil geometry on the performance of a straight-bladed vertical axis wind turbine has been performed using a methodology based on streamtube models. An airfoil database of 34 airfoils has been generated, developing a practical and cost-effective tool for the quick comparison of turbine designs (70 different configurations were analyzed). This tool, validated with results from the literature and computational fluid dynamics simulations performed by the authors, has allowed to propose an optimal solidity range from 0.25 to 0.5 and the use of almost symmetrical airfoils (camber 3%). Finally, this tool has been applied to design two vertical axis wind turbines optimized for low and medium wind speeds.

Passive Stall Control Systems of Power Limitation Modes for Vertical Axis Wind Turbines (VAWT)

Ihor Shchur, Andrii Lozinskyi, Bohdan Kopchak, Yurii Biletskyi, Vsevolod Shchur, October 2017

Vertical axis wind turbines (VAWT) with direct drive permanent magnet synchronous generator operate with the greatest energy efficiency and reliability in low-power wind energy conversion systems (WECS). This article offers a classification of optimal control methods of such WECS. Special attention is also given to an unexplored area—the development of control systems of power limitation mode when VAWT work at high wind speeds—passive stall and feathering control. In particular, the structures of control systems were developed, the parameters of power regulators were obtained, and these regimes were compared by means of computer simulation. The fractional order control method was also used for this mode and the parameters of fractional order PID power regulator were found by the method of Particle Swarm Optimization (PSO). The article also demonstrates how to realize the mode of passive stall control in the energy-shaping control system (ESCS) previously developed by the authors.

Performance Evaluation of a Vertical Axis Wind Turbine Using Real-Time Measuring Wind Data

Choon-Man Jang, Chul-Kyu Kim, Sang-Moon Lee, Sajid Ali, January 2018

Recently, small vertical axis wind turbine has been in the limelight as an essential component of hybrid renewable energy system, based on many advantages, such as low noise and cut-in wind speed, cost and site flexibility, etc. Turbine performance is analyzed by introducing the effect of different time averaging steps using real-time measuring wind data. All measured data through wind master is stored in the computer every second using a data acquisition system. The test turbine is installed at the island, which is located near Seoul, South Korea. Vertical axis wind turbine (VAWT) having the rated power of 1.5 kW is installed. The performance of the VAWT is compared and analyzed using 10-, 20-, and 30-min averaged data. Numerical simulation has also been performed to compare the turbine performance with experimental results. From the comparisons between experimental and numerical simulation, it is found that performance of the tested wind turbine is slightly different according to the different time-averaged data. Turbine performance analyzed by using 10-min time-averaged data, which has relatively lower standard deviation, has a good agreement to the result of numerical simulation.

The Influence Of Blade Roughness On The Performance Of A Vertical Axis Tidal Turbine

Luis Priegue, Thorsten Stoesser Elsevier Ltd, April 2017

This paper reports the findings of an experimental study investigating the influence of blade roughness on the performance of a vertical axis tidal turbine. Due to their design, vertical axis turbines undergo periods of stall, i.e. flow separation from the blade, during each revolution. It is hypothesised that roughening turbine blades delays flow separation (in analogy to flows over rough bluff bodies) and hence diminishes turbine stall which in turn should result in an increase in turbine performance. Laboratory experiments were undertaken in Cardiff University’s hydraulics laboratory, testing vertical axis turbines with rotors comprising smooth and rough blades.

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 revolu- tions 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)...

Unsteady Flow Numerical Simulation of Vertical Axis Wind Turbine

Du Gang, Wu Chun Kau, Elsevier Ltd, 2015 

This paper sets up model of the flow field for vertical axis wind turbine based on airfoil DU93-W 210. FLUENT is used to solve the two dimensional unsteady incompressible N-S equations with RNG κ ε turbulence model. COUPLE algorithm and sliding mesh is used to simulate the 2-D unsteady flow field of the wind turbine. The rotor power coefficient of wind energy and the variation of the wind turbine’s total torque are analyzed under the variation of varied blade installation angle and chord length. As the result shows, the power coefficient of wind energy at the best installation angle is increased by 2%, and the power coefficient of wind energy in the best solidity is increased by 15%.

The vertical axis wind turbine was “re-invented” in 1970 by Peter South and Raj Rangi [1] of the Canadian National Research Council, Ottawa, where much development was carried out.  Considerable development was also carried out at Sandia National Laboratories, New Mexico, especially into the structural dynamics of the curved Darrieus  rotor [2, 3].  It was determined that the rotating frame effects of Coriolis action, rotational softening, and pretensioning played important roles.

Vertical axis wind turbine design load cases investigation and comparison with horizontal axis wind turbine – (Slide Deck)

C. Galinos, T.J. Larsen, H.A. Madsen, U.S. Paulsen, Department of Wind Energy, 2016

Wind Tunnel testing of small Vertical-Axis Wind Turbines in Turbulent Flows

Andreu Carbó Molina, Gianni Bartoli, Tim de Troyer, Elsevier Ltd, 2017

This study presents an innovatinve wind tunnel approach to evluate the efficiency of Vertical-Axis Wind Turbines (VAWT) in turbulent flows, to study their integration in urban environments. The first part of the research is devoted to obtaining highly-turbulent wind profiles in the wind tunnel, with the use of different configurations of square grids. A careful study and validation of this technique is done, in order to obtain uniform wind conditions with adequate values of turbulence intensity and length scales to model the urban flows. The set-up is used to test a H-Darrieus VAWT under values of turbulence over 5%, in comparison with the operation of the turbine under free stream...

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