*D.W. Lobitz, W.N. Sullivan, Sandia National Laboratories, 1984*

A specialized finite element capability has been developed to predict dynamic structural response of the vertical axis wind turbine (VAWT). This report is concerned with evaluating this finite element analysis technique. To achieve this, several types of experimental data taken from the DOE 100kW rotor are compared with predictions. These data include parked rotor natural frequencies, very low wind centrifugal and gravitational load response, and vibratory response from wind loads covering the rotor operational spectrum. Generally, the agreement between theory and experiment is very satisfactory. It is concluded that the analysis package is suitable for engineering design. Shortcomings observed in modeling accuracy are believed to be due primarily to inadequacies in blade aerodynamic load calculations.

**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.

**Avoidance of resonances in a semi-guy-wired vertical axis wind turbine**

*Erik Mollerstrom, Fredric Ottermo, Jonny Hylander, Hans Bernhoff, March 2014*

Resonance analysis for a vertical axis wind turbine is performed. The turbine may be described as semi-guy wired, being bolted stiffly to the ground as well as supported by guy wires. The influence of the first mode eigen frequency of the guy wires and how it is affected by wind load is examined. Using beam theory, an analytical model for calculating the first mode eigen frequency of the guy wire for different wind loads is derived. The analytical model is verified with FEM-simulations and then used to assemble a diagram showing how to combine the wire size, inclination angle and pre-tension for an eigen frequency range over the 3P load for nominal rotational speed and for a certain effective spring force acting on the tower. This diagram, here called an EA-T diagram, may be used as a quick tool for comparing wire setups and a similar diagram can be used for other guy wired structures.

**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.

**Computational and Experimental Study on Vertical Axis Wind Turbine in Search for an Efficient Design**

*Mohammad Mohibbul Bashar, Georgia Southern University Thesis, Fall 2014*

**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.

**Design, performance and economics of the DAF Indal 50 kW and 375 kW Vertical Axis Wind Turbine**

*LA Schiehbein, DA Malcolm, March 1982*

A review of the development and performance of the DAF Indal 50 kW vertical axis Darrieus wind turbines shows that a high level of technical development and reliability has been achieved. Features of the drive train, braking and control systems are discussed and performance details are presented.

**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, International Journal of Energy and Environmental Engineering, August 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.

**Dynamic response of a Darrieus rotor wind turbine subject to turbulent flow**

*D.J. Malcolm, Engineering Structures, April 1988*

A method is presented for the frequency response analysis of a Darrieus rotor wind turbine subject to turbulent flow. A number of time series vectors of longitudinal and lateral turbulent velocities are generated and the interaction with a two-bladed rotor is carried out with a double multiple streamtube model. The resulting time domain loads are transformed into the frequency domain in terms of components of a real set of eigenvectors of the rotor. The structural analysis is carried out using a modal frequency response solution of MSC NASTRAN and a random loading routine. Some DMAP modifications are necessary in order to present power spectra in terms of modal rather than physical degrees of freedom. The spectra of some of the modal loads applicable to the Indal 6400 rotor are examined. Some numerical results are also presented for that rotor and a comparison with field data is made. The model correctly predicts the observed response of the fundamental in-plane blade bending modes to stochastic loading. The procedure appears to be both economic in computing resources and able to simulate observed behaviour. It promises to be a useful tool in the design of Darrieus rotors against fatigue damage.

**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.

*Erik Mollerstrom, Fredric Ottermo, Jonny Hylander, Hans Bernhoff, June 2014*

Eigen frequencies of a vertical axis wind turbine tower made out of laminated wood which are both bolted to the ground and supported by guy wires are studied and compared. Using beam theory, an analytical model taking the guy wires into account for calculating the first mode eigen frequency of the tower has been derived. The analytical model is then evaluated by comparing with FEM-simulations and measurements performed on the actual tower. The model is found to be reasonably accurate keeping in mind that the estimated masses and second moments of area are somewhat rough. Furthermore the model can be used to give an indication of the magnitude of change in eigen frequency when modifying a tower or guy wire property.

**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 Identification of a Rotating Blade System**

*T.G. Carne, D.R. Martinez, S.R. Ibrahim, Sandia National Laboratories, 1983*

A new testing technique and the Ibrahim time domain (ITD) modal identification algorithm have been combined, resulting in a capability to estimate modal parameters for rotating blade systems. This capability has been evaluated on the Sandia two-meter, vertical-axis wind turbine. Variation in modal frequencies as a function of rotation speed has been experimentally determined from 0 rpm to 800 rpm. Excitation of the rotating turbine was provided by a scheme which suddenly released a pretensioned cable, thus plucking the turbine as it rotated. The structural response was obtained by passing the signals through slip rings. Using the measured free-decay responses as input data for the ITD algorithm, the modes of the rotating turbine were determined at seven rotation speeds. The measured modal parameters were compared with analytical results obtained from a finite element analysis and with experimental results obtained from a complex exponential identification algorithm.

**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.

**Noise Emission of a 200 kW Vertical Axis Wind Turbine**

*Erik Mollerstrom, Fredric Ottermo, Jonny Hylander, Hans Bernhoff, Energies, December 2015*

The noise emission from a vertical axis wind turbine (VAWT) has been investigated. A noise measurement campaign on a 200 kW straight-bladed VAWT has been conducted, and the result has been compared to a semi-empirical model for turbulent-boundary-layer trailing edge (TBL-TE) noise. The noise emission from the wind turbine was measured, at wind speed 8 m/s, 10 m above ground, to 96.2 dBA. At this wind speed, the turbine was stalling as it was run at a tip speed lower than optimal due to constructional constraints. The noise emission at a wind speed of 6 m/s, 10 m above ground was measured while operating at optimum tip speed and was found to be 94.1 dBA. A comparison with similar size horizontal axis wind turbines (HAWTs) indicates a noise emission at the absolute bottom of the range. Furthermore, it is clear from the analysis that the turbulent-boundary-layer trailing-edge noise, as modeled here, is much lower than the measured levels, which suggests that other mechanisms are likely to be important, such as inflow turbulence.

**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.

**Self-similarity and flow characteristics of vertical-axis wind turbine wakes: an LES study**

*Mahdi Abkar and John Dabiri, A Journal of Turbulence, 31 Jan 2017*

Large eddy simulation (LES) is coupled with a turbine model to study the structure of the wake behind a vertical-axis wind turbine (VAWT). In the simulations, a tuning-free anisotropic minimum dissipation model is used to parameterise the subfilter stress tensor, while the turbine-induced forces are modelled with an actuator line technique. The LES framework is first validated in the simulation of the wake behind a model straight-bladed VAWT placed in the water channel and then used to study the wake structure downwind of a full-scale VAWT sited in the atmospheric boundary layer. In particular, the self-similarity of the wake is examined, and it is found that the wake velocity deficit can be well characterised by a two-dimensional multivariate Gaussian distribution. By assuming a self-similar Gaussian distribution of the velocity deficit, and applying mass and momentum conservation, an analytical model is developed and tested to predict the maximum velocity deficit downwind of the turbine. Also, a simple parameterisation of VAWTs for LES with very coarse grid resolutions is proposed, in which the turbine is modelled as a rectangular porous plate with the same thrust coefficient. The simulation results show that, after some downwind distance (*x*/*D* ≈ 6), both actuator line and rectangular porous plate models have similar predictions for the mean velocity deficit. These results are of particular importance in simulations of large wind farms where, due to the coarse spatial resolution, the flow around individual VAWTs is not resolved.

*Arti Tirkey, Yamini Sarthi, Khemraj Patel, Ritesh Sharma, Prakash Kumar Sen, International Journal of Science, Engineering and Technology Research, December 2014*

This paper present the effect of blade profile, number of blade, surface roughness of blade, aspect ratio and Reynolds number on the performance of vertical axis wind turbine. A numerical analysis, adopting the multiple stream tube method, is carried out to evaluate the performance depending on the parameters. The numerical result shows that the variation of blade profile directly affected the influence power production. An enhancement of the power production is observed with increasing the Reynolds number on the whole tested blade speed ratio range. Aspect ratio of wind turbine is the ratio between blade length and rotor radius. Since the aspect ratio variations of a vertical-axis wind turbine cause Reynold number variations, and changes in the power coefficient can also be studied to derive how aspect ratio variations affect the turbine performance. It is shown experimentally that the surface roughness on the turbine blade has a significant effect on the performance of turbine.

**Three Pitch Control Systems for Vertical Axis Wind Turbines Compared**

*L. Lazauskas, Wind Engineering, January 1992*

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.

*Tristen Charles Hohman, Princeton University, ProQuest Dissertations Publishing, 2017*

**Turbulence influence on optimum tip speed ratio for a 200 KW wind turbine**

*Erik Mollerstrom, Fredric Ottermo, Anders Goude, Sandra Eriksson, Jonny Hylander*

The influence of turbulence intensity (TI) on the tip speed ratio for maximum power coefficient, here called λCp_max, is studied for a 200 kW VAWT H-rotor using logged data from a 14 month period with the H-rotor operating in wind speeds up to 9 m/s. The TI - λCp_max relation is examined by dividing 10 min mean values in different turbulence intensity ranges and producing multiple CP(λ) curves. A clear positive relation between TI and λCp_max is shown and is further strengthened as possible secondary effects are examined and deemed non-essential. The established relation makes it possible to tune the control strategy to enhance the total efficiency of the turbine.

**Turbulence influence on wind energy extraction for a medium size vertical axis wind turbine**

*Erik Mollerstrom, Fredric Ottermo, Anders Goude, Sandra Eriksson, Jonny Hylander, Hans Bernhoff, Wind Energy, February 2016*

The relation between power performance and turbulence intensity for a VAWT H-rotor is studied using logged data from a 14 month (discontinuous) period with the H-rotor operating in wind speeds up to 9 m/s. The turbine, designed originally for a nominal power of 200 kW, operated during this period mostly in a restricted mode due to mechanical concerns, reaching power levels up to about 80 kW. Two different approaches are used for presenting results, one that can be compared to power curves consistent with the International Electrotechnical Commission (IEC) standard and one that allows isolating the effect of turbulence from the cubic variation of power with wind speed. Accounting for this effect, the turbine still shows slightly higher efficiency at higher turbulence, proposing that the H-rotor is well suited for wind sites with turbulent winds. The operational data are also used to create a Cp(λ) curve, showing slightly lower Cp compared with a curve simulated by a double multiple streamtube model. Copyright

**Turbulence in vertical axis wind turbine canopies**

*Matthias Kinzel, Daniel Araya and John Dabiri, The Physics of Fluids, November 2015*

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, University of California Berkeley, 2018*

*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...

**Vertical Axis Wind Turbine Experiments at Full Dynamic Similarity**

*Miller MA, Duvvuri S, Brownstein ID, Lee M, Dabiri JO, Hultmark M, Journal of Fluid Dynamics, June 2018 (Purchase only)*

Laboratory experiments were performed on a geometrically scaled vertical-axis wind turbine model over an unprecedented range of Reynolds numbers, including and exceeding those of the full-scale turbine. The study was performed in the high-pressure environment of the Princeton High Reynolds number Test Facility (HRTF). Utilizing highly compressed air as the working fluid enabled extremely high Reynolds numbers while still maintaining dynamic similarity by matching the tip speed ratio (defined as the ratio of tip velocity to free stream, 𝜆=𝜔R/U ) and Mach number (defined at the turbine tip, Ma=𝜔R/a ). Preliminary comparisons are made with measurements from the full-scale field turbine. Peak power for both the field data and experiments resides around 𝜆=1 . In addition, a systematic investigation of trends with Reynolds number was performed in the laboratory, which revealed details about the asymptotic behaviour. It was shown that the parameter that characterizes invariance in the power coefficient was the Reynolds number based on blade chord conditions ( Rec ). The power coefficient reaches its asymptotic value when Rec>1.5×106 , which is higher than what the field turbine experiences. The asymptotic power curve is found, which is invariant to further increases in Reynolds number.

**Vertical Axis Wind Turbine Farms: Modeling and Optimization**

*Elie Bou-Zeid and Alexander Smits, Princeton Environmental Institute, 2013*

Vertical axis wind turbines (VAWTs) are proposed as an alternative to the more commonly used horizontal axis wind turbines (HAWTs) due to the potential increase in packing density (reduction in distance between turbines) that is possible with VAWTs. But for VAWTs to be more widely adopted and for their advantages to be maximized, more work is needed in the turbine and farm designs. This project aims at improving the design of VAWTS and their configurations in large farms.

**Wind tunnel and numerical study of a small vertical axis wind turbine**

*Robert Howell, Ning Qin, Jonathan Edwards, Naveed Durrani, Renewable Energy, February 2010*

This paper presents a combined experimental and computational study into the aerodynamics and performance of a small scale vertical axis wind turbine (VAWT). Wind tunnel tests were carried out to ascertain overall performance of the turbine and two- and three-dimensional unsteady computational fluid dynamics (CFD) models were generated to help understand the aerodynamics of this performance.

**Wind Turbine Design: With Emphasis on Darrieus Concept**

*Ion Paraschivoiu, Polytecnic International Press, 2002 (Book available for Purchase)*

The depletion of global fossil fuel reserves combined with mounting environmental concerns has served to focus attention on the development of ecologically compatible and renewable alternative sources of energy. Wind energy, with its impressive growth rate of 40% over the last five years, is the fastest growing alternate source of energy in the world since its purely economic potential is complemented by its great positive environmental impact. The wind turbine, whether it may be a Horizontal Axis Wind Turbine (HAWT) or a Vertical Axis Wind Turbine (VAWT), offers a practical way to convert the wind energy into electrical or mechanical energy. Although this book focuses on the aerodynamic design and performance of VAWTs based on the Darrieus concept, it also discusses the comparison between HAWTs and VAWTs, future trends in design and the inherent socio-economic and environmental friendly aspects of wind energy as an alternate source of energy.