**A Numerical Study on a Vertical-Axis Wind Turbine with Inclined Arms**

* Agostino De Marco, Domenico P. Coiro Domenico Cucco and Fabrizio Nicolosi*, University of Naples Federico II, 30 June 2014

This work focuses on a particular type of vertical-axis wind turbine, in which a number of inclined arms with airfoil-shaped cross-sections are mounted to connect the principal blades to their hub. While the majority of the known studies on vertical axis turbines is devoted to the role of principal blades, in most of the cases without taking into account other parts of the wind turbine, the objective of this work is to investigate the effect of uncommon arm geometries, such as the inclined arms. The inclined arms are known to have a potentially beneficial role in the power extraction from the wind current but, due to the complexity of the phenomena, the investigation on aerodynamics of this type of turbine is often impossible...

**Analytical methods in Vertical Axis Wind turbines**

*Hong-Hieu Le, October 2017*

Horizontal and vertical axis wind turbines (HAWTs and VAWTs) are two main kinds of wind turbines, which are the most popular way to receive energy from wind. By comparison, VAWTs have a number of advantages, but it is also complex in aerodynamics that research is needed. A Code is developed based on Double multiple stream-tube and corrections of dynamic stall for Darrieus VAWTs. It is capable of estimating the output power versus different operation condition. The code is also validated with experimental data of many SANDIA Darrieus VAWT turbines.

**CFD-based Performance Analysis on Design Factors of Vertical Axis Wind Turbines at Low Wind Speeds**

*Chaianant Sranpat, Suchaya Unsakul, Premchai Choljararux, Thananchai Leephakpreeda, October 2017*

This paper presents effects of design factors on mechanical performances of Vertical Axis Wind Turbines (VAWTs), which are suitable to low wind speeds conditions in Thailand. Potential VAWTs models are numerically analyzed within virtual wind tunnels at low wind speeds by utilizing X-Flow^{TM} Computational Fluid Dynamics (CFD) software. Design factors include types/patterns, numbers of blades, types of materials, height-to-radius ratios and design modifications in this study. The performance curves of each VAWTs are represented by plots of power coefficients against tip speed ratios. It is found that the types/patterns, numbers of blades, and height-to-radius ratios have significant effects on mechanical performances whereas types of materials result in shifts of operating speeds of VAWTs. The proposed methodology can be used in designing VAWTs to improve mechanical performance before physical fabrication.

**Channel geometry optimization for vertical axis wind turbines in skyscrapers**

*Seifeddine Kefi, Ajay Joneja, Tim K.T. Tse, Sunwei Li, October 2017 (Purchase Only)*

The desire for sustainability and improved air quality has led architects to explore integrating *vertical axis wind turbines* (VAWT) in urban skyscrapers. However, the efficiency of such solutions is sensitive to the geometry of the wind channel. In this paper, we present a general technique for optimization of the wind channel geometry. Using parametric curves to define the profile of the channel, and by quantizing the location of the control points, we propose an experimental design approach to determine near-optimal channel geometry. The solution is further improved by interpolating the performance function so obtained via a statistical tool called kriging. The approach is tested by an experimental study, in which the parameters of the fluid dynamic model are determined by a series of wind tunnel tests.

**Design of a vertical-axis wind turbine: how the aspect ratio affects the turbine's peformance**

*S. Brusca, R. Lanzafame, M. Messina, April 2014*

This work analyses the link between the aspect ratio of a vertical-axis straight-bladed (H-Rotor) wind turbine and its performance (power coefficient). The aspect ratio of this particular wind turbine is defined as the ratio between blade length and rotor radius. Since the aspect ratio variations of a vertical-axis wind turbine cause Reynolds number variations, any changes in the power coefficient can also be studied to derive how aspect ratio variations affect turbine performance. Using a calculation code based on the Multiple Stream Tube Model, symmetrical straight-bladed wind turbine performance was evaluated as aspect ratio varied. This numerical analysis highlighted how turbine performance is strongly influenced by the Reynolds number of the rotor blade. From a geometrical point of view, as aspect ratio falls, the Reynolds number rises which improves wind turbine performance.

**Designing of Vertical Axis Wind Turbines for Low Speed, Low Altitude Regions of Central India**

*Sonali Mitra, Abhineet Singh, Pragyan Jain, S. V. H. Nagendra*

This work analyses the link between the aspect ratio of a vertical-axis straight-bladed (H-Rotor) wind turbine and its performance (power coefficient). The aspect ratio of this particular wind turbine is defined as the ratio between blade length and rotor radius. Since the aspect ratio variations of a vertical-axis wind turbine cause Reynolds number variations, any changes in the power coefficient can also be studied to derive how aspect ratio variations affect turbine performance. Using a calculation code based on the Multiple Stream Tube Model, symmetrical straight-bladed wind turbine performance was evaluated as aspect ratio varied. This numerical analysis highlighted how turbine performance is strongly influenced by the Reynolds number of the rotor blade. From a geometrical point of view, as aspect ratio falls, the Reynolds number rises which improves wind turbine performance.

**Effect of some design parameters on the performance of a Giromill vertical axis wind turbine**

*M. El-Samanoudy, A.A.E Ghorab, Sh.Z. Youssef, Ain Shams Engineering Journal, November 2010*

This paper describes the effect of some design parameters on the performance of a Giromill vertical axis wind turbine. A Giromill wind turbine has been designed, manufactured and tested. The turbine performance has been investigated with varying the design parameters such as, pitch angle, number of blades, airfoil type, turbine radius and its chord length. Then, the results were used for the comparison between the performance achieved while changing the design parameters.

**Excitation Methods for a 60 kW Vertical Axis Wind Turbine**

*Todd Griffith, Randy Mayes, Patrick Hunter, Society for Experimental Mechanics Inc., February 2010*

A simple modal test to determine the first tower bending mode of a 60 kW (82 feet tall) vertical axis wind turbine was performed. The minimal response instrumentation included accelerometers mounted only at easily accessible locations part way up the tower and strain gages near the tower base. The turbine was excited in the parked condition with step relaxation, random human excitation, and wind excitation. The resulting modal parameters from the various excitation methods are compared.

**Finite Element Analysis and Modal Testing of a Rotating Wind Turbine**

*Thomas Carne, Donald Lobitz, Arlo Nord, Robert Watson, Sandia National Laboratories, October 1982*

A finite element procedure, which includes geometric stiffening, and centrifugal and Coriolis terms resulting from the use of a rotating coordinate system, has been developed to compute the mode shapes and frequencies of rotating structures. Special applications of this capability has been made to Darrieus, vertical axis wind turbines. In a parallel development effort, a technique for the modal testing of a rotating vertical axis wind turbine has been established to measure modal parameters directly. Results from the predictive and experimental techniques for modal frequencies and mode sja[es are compared over a wide range of rotational speeds.

**Modal Testing in the Design Evaluation of Wind Turbines**

*James Lauffer, Thomas Carne, Thomas Ashwill, Sandia National Laboratories, April 1988*

In the design of large, flexible wind turbines subjected to dynamic loads, knowledge of the modal frequencies and mode shapes is essential in predicting structural response and fatigue life. During design, analytical models must be depended upon for estimating modal parameters. When turbine hardware becomes available for testing, actual modal parameters can be measured and used to update the analytical predictions or modify the model. The modified model can then be used to reevaluate the adequacy of the structural design. Because of problems in providing low-frequency excitation (0.1 to 5.0 Hz), modal testing of large turbines can be difficult. This report reviews several techniques of low-frequency excitation used successfully to measure modal parameters for wind turbines, including impact, wind, step relaxation, and human input. As one application of these techniques, a prototype turbine was tested and two modal frequencies were found to be close to integral multiples of the operating speed, which caused a

resonant condition. The design was modified to shift these frequencies, and the turbine was retested to confirm expected changes in modal frequencies.

**Modal Testing of a Rotating Wind Turbine**

*Thomas Cazarne, Arlo Nord, Sandia National Laboratories, November 1982*

A testing technique has been developed to measure the modes of vibration of a rotating vertical axis wind turbine. This technique has been applied to the Sandia 2-m turbine, where the changes in individual modal frequencies as a function of the rotational speed have been tracked from 0rpm (parked) to 600 rpm. During rotational testing, the structural response was measured using a combination of strain gages and accelerometers, passing the signals through slip rings. Excitation of the turbine structure was provided by a scheme that suddenly released a pre-tensioned cable, thus plucking the turbine as it was rotating at a set speed. In addition to calculating the real modes of the parked turbine, the modes of the rotating turbine were also determined at several rotational speeds. The modes of the rotating system proved to be complex because of centrifugal and Coriolis effects. The modal data for the parked turbine were used to update a finite element model. Also, the measured modal parameters for the rotating turbine were compared

to the analytical results, thus verifying the analytical procedures used to incorporate the effects of the rotating coordinate system.

**Parametric analysis of resistance type vertical axis wind turbines**

*Baolin Li*, Zhixin Bian, Kedi Chen, Boletín Técnico, Vol.55, Issue 12, 2017, pp.453-458*

The performance of wind turbines is usually evaluated by proprietary parameters, but these parameters are used to analyze the dynamic performances of the blades qualitatively. Thus, it cannot exactly express the actual working performances of wind turbines. In this paper, a comparative analysis on physical meanings is made between resistance type vertical axis wind turbines and horizontal axis wind turbines. And the actual meanings of the parameters to be expressed was also discussed. The result shows that analysis theory of blades in different type of wind turbines are different. And then a design method was put forward to calculate and analyze the resistance type vertical axis wind turbines. It is concluded that wind turbines with different structures and blades have its own analysis theory and method.

The desirable performance attributes of a vertical axis wind turbine (VAWT) include high starting torque, high peak efficiency, broad operating grange and a reasonable insensitivity to the parameters that define its operation. The theoretical performances of three variable pitch mechanisms for VAWT are compared. Cycloturbines use cam devices or gears to impose a sinusoidal pitch regime. In the mass-stabilised system, pitch is determined by the interplay of two opposing moment on the blades. These two mechanisms are compared with “Aeropitch”, a hypothetical pitch control system in which stabilising moments are related to the blade relative velocity.

*Sam Kanner, Per-Olof Persson, January 2018*

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 compu- tational 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). The evaluation is based on 110 CFD simulations validated with wind- tunnel measurements for two VAWTs. Instantaneous moment coefficient, Cm, and power coefficient, CP, are studied for each case using unsteady Reynolds-averaged Navier-Stokes (URANS) simulations with the 4-equation transition SST turbulence model. The results show that the azimuthal increment dθ is largely dependent on tip speed ratio. For moderate to high λ, the minimum requirement for dθ is 0.5° while this decreases to 0.1° at low to moderate λ. The need for finer time steps is associated to the flow complexities related to dynamic stall on turbine blades and blade-wake interactions at low λ. In addition, the minimum distance from the turbine center to the domain inlet and outlet is 15 and 10 times the turbine diameter, respectively. It is also shown that 20–30 turbine revolutions are required to ensure statistically converged solutions. The current findings can serve as guidelines towards accurate and reliable CFD simulations of VAWTs at different tip speed ratios and solidities.

**Validation of High-Order Wall-Resolved Large-Eddy Simulation of Vertical-Axis Wind Turbines**

*Sam Kanner, Per-Olof Persson*

*Christos Galinos, Torben Larsen, Helge Madsen, Uwe Paulsen, January 2016*

The paper studies the applicability of the IEC 61400-1 ed.3, 2005 International Standard of wind turbine minimum design requirements in the case of an onshore Darrieus VAWT and compares the results of basic Design Load Cases (DLCs) with those of a 3-bladed HAWT. The study is based on aeroelastic computations using the HAWC2 aero-servo-elastic code A 2-bladed 5 MW VAWT rotor is used based on a modified version of the DeepWind rotor For the HAWT simulations the NREL 3-bladed 5 MW reference wind turbine model is utilized Various DLCs are examined including normal power production, emergency shut down and parked situations, from cut-in to cut-out and extreme wind conditions. The ultimate and 1 Hz equivalent fatigue loads of the blade root and turbine base bottom are extracted and compared in order to give an insight of the load levels between the two concepts. According to the analysis the IEC 61400-1 ed.3 can be used to a large extent with proper interpretation of the DLCs and choice of parameters such as the hub-height. In addition, the design drivers for the VAWT appear to vier from the ones of the HAWT. Normal operation results in the highest tower bottom and blade root loads for the VAWT, where parked under storm situation (DLC 6.2) and extreme operating gust (DLC 2.3) are more severe for the HAWT. Turbine base bottom and blade root edgewise fatigue loads are much higher for the VAWT compared to the HAWT. The interpretation and simulation of DLC 6.2 for the VAWT lead to blade instabilities, while extreme wind shear and extreme wind direction change are not critical in terms of loading of the VAWT structure. Finally, the extreme operating gust wind condition simulations revealed that the emerging loads depend on the combination of the rotor orientation and the time stamp that the frontal passage of gust goes through the rotor plane.

**Vertical Axis Wind Turbine Evaluation and Design**

*Lucas Desadze, Drew Digeser, Christopher Dunn, Dillon Shoikat, 25 April 2013*

This project studied the potential for installing roof-mounted vertical axis wind turbine (VAWT) systems on house roofs. The project designed several types of VAWT blades with the goal of maximizing the efficiency of a shrouded turbine. The project also used a wind simulation software program, WASP, to analyze existing wind data measured on the roofs of various WPI buildings. Scale-model tests were performed in the WPI closed-circuit wind tunnel. An RPM meter and a 12-volt step generator were used to measure turbine rotation speeds and power output at different wind speeds...