**A Comparison of Wake Measurements in Motor-Drien and Flow-Drien Turbine Experiments**

* Daniel B. Araya, John o. Dabiri, Springer-Verlag Berlin Heidelberg, July 2015*

We present experimental data to compare and contrast the wake characteristics of a turbine whose rotation is either driven by the oncoming flow or prescribed by a motor. Velocity measurements are collected using two-dimensional particle image velocimetry in the nearwake region of a lift-based, vertical-axis turbine. The wake of this turbine is characterized by a spanwise asymmetric velocity profile which is found to be strongly dependent on the turbine tip speed ratio (TSR), while only weakly dependent on Reynolds number (Re). For a given Re, the TSR is controlled.

**Aerodynamic Interference of Vertical Axis Wind Turbines**

*Rajagopalan, R. Ganesh, Rickerl, Ted L. & Klimas, Paul C., J. Propul. Power 6 (5), 645, 1990*

The laminal flowflied and performance of clusters of two-dimensional vertical axis wind turbines are analyzed by idealizing the rotors as momentum sources. The flowfield dominated by the pressure field of the operating turbines is determined by solving the incompressible Navier-Stokes equations and mutual interference is observed to be elliptic in nature. Physical positioning of the turbines with respect to each other significantly affects the aerodynamic performance of the turbines. Several cases are examined starting with...

**Aerodynamic Simulation of Vertical-Axis Wind Turbines**

*Korobenko, A., Hsu, M. C., Akkerman, I. & Bazilevs, Y. J. Appl. Mech., 2014*

Full-scale, 3D, time-dependent aerodynamics modeling and simulation of a Darrieus-type vertical-axis wind turbine (VAWT) is presented. The simulations are performed using a moving-domain finite-element-based ALE-VMS technique augmented with a sliding-interface formulation to handle the rotor-stator interactions present. We simulate a single VAWT using a sequence of meshes with increased resolution to assess the computational requirements for this class of problems. The computational results are in good agreement with experimental data. We also perform a computation of two side-by-side counterrotating VAWTs to illustrate how the ALE-VMS technique may be used for the simulation of multiple...

**Aerodynamics of vertical-axis wind turbines in full-scale andlaboratory-scale experiments**

* Daniel Borsodi Araya, Caltech, California Institute of Technology, 2016*

Within a wind farm, multiple turbine wakes can interact and have a substantial effect on the overall power production. This makes an understanding of the wake recovery process crit- ically important to optimizing wind farm efficiency. Vertical-axis wind turbines (VAWTs) exhibit features that are amenable to dramatically improving this efficiency. However, the physics of the flow around VAWTs is not well understood, especially as it pertains to wake interactions, and it is the goal of this thesis to partially fill this void. This objective is approached from two broadly different perspectives: a low-order view of wind farm aerody- namics, and a detailed experimental analysis of the VAWT wake.

**A Free Wake Method For Vertical-Axis Wind Turbine Performance Prediction**

* Horia Dumitrescu, Vladimir Cardos, ResearchGate, 21 November 2014*

The fatigue analysis of a wind turbine component typically uses representative samples of cyclic loads to determine lifetime loads. In this paper, several techniques currently in use are compared to one another based on fatigue life analyses. The generalized Weibull fitting technique is used to remove the artificial truncation of large-amplitude cycles that is inherent in relatively short data sets. Using data from the Sandia/DOE 34-m Test Bed, the generalized Weibull fitting technique is shown to be excellent for matching the body of the distribution of cyclic loads and for extrapolating the tail of the distribution. However, the data also illustrate that the fitting technique is not a substitute for an adequate data base.

* John O. Dabiri, California Institue of Technology*

This document is a slideshow put together by Dr. Dabiri that summarizes his work on small vertical axis wind turbines and their impacts on "planform kinetic flux", energy density in wind farms and related issues. This is an excellent resources with plenty of figures and graphics that reviews much of his work up to the publication date.

**Benefits of Collocating Vertical-Axis and Horizontal-Axis Wind Turbines in Large Wind Farms**

* Shengbail Xie, Cristina L. Archer, Niranjan Ghaisas and Charles Meneveau, Wiley Online Library, 2016*

In this study, we address the beneﬁts of a vertically staggered (VS) wind farm, in which vertical-axis and horizontal-axis wind turbines are collocated in a large wind farm. The case study consists of 20 small vertical-axis turbines added around each large horizontal-axis turbine. Large-eddy simulation is used to compare power extraction and ﬂow properties of the VS wind farm versus a traditional wind farm with only large turbines. The VS wind farm produces up to 32% more power than the traditional one, and the power extracted by the large turbines alone is increased by 10%, caused by faster wake recovery from enhanced turbulence due to the presence of the small turbines. A theoretical analysis based on a...

**Dynamic Stall: The Case of the Vertical Axis Wind Turbine**

*Laneville, A. and Vittecoq, P., Sol Energ. Eng. 108, May 1986*

This paper presents the results of an experimental investigation on a driven Darrieus turbine rotating at different tip speed ratios. For a Reynolds number of 3.8 × 104 , the results indicate the presence of dynamic stall at tip speed ratio less than 4, and that helicopter blade aerodynamics can be used in order to explain some aspects of the phenomenon. It was observed that in deep stall conditions, a vortex is formed at the leading edge; this vortex moves over the airfoil surface with 1/3 of the airfoil speed and then is shed at the trailing edge. After its shedding, the vortex can interact with the airfoil surface as the blade passes downstream.

**Emergent Aerodynamics in Wind Farms**

*John O. Dabiri, Physics Today, October 2014*

The defining element of modern wind farms is the propeller like structure known as a horizontal-axis wind turbine. A marvel of engineering, the HAWT typically comprises more than 8000 parts, and its blades reach more than 200 m above the ground.

**Energy Exchange in an Array of Vertical-Axis Wind Turbines**

* Matthias Kinzel*, Quinn Mulligan and John O. Dabiri, Journal of Turbulence, Vol. 13, No.38, 1-13, 2012*

We analyze the ﬂow ﬁeld within an array of 18 counter-rotating, vertical-axis wind turbines (VAWTs), with an emphasis on the ﬂuxes of mean and turbulence kinetic energy. The turbine wakes and the recovery of the mean wind speed between the turbine rows are derived from measurements of the velocity ﬁeld using a portable meteorological tower with seven, vertically-staggered, three-component ultrasonic anemometers. The data provide insight to the blockage effect of both the individual turbine pairs within the array and the turbine array as a whole. The horizontal and planform kinetic energy ﬂuxes into the turbine array are analyzed, and various models for the roughness length of the turbine array are compared...

**[Erratum] Energy Exchange in an Array of Vertical-Axis Wind Turbines**

* Matthias Kinzel*, Quinn Mulligan and John O. Dabiri, Journal of Turbulence, Vol. 13, No.38, 1-13, 2012*

The calculation of the planform kinetic energy flux in this paper contains an error. The equation stated in the manuscript, Pvert ≈ −ρAplanu < u′w′ >, is correct. However, a typographical error in the data processing code had the effect of calculating the planform kinetic energy flux using u2 instead of u. This error caused a quantitative change in the planform kinetic energy flux as can be seen in the revised version of Figure 7.

**Fish Schooling As A Basis for Vertical Axis Wind Turbine Farm Design**

* Robert W Whittlesey, Sebastian Liska, and John O. Dabiri, 11 February 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 coefficient. 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. A potential flow model of inter-VAWT interactions is...

Tuyen Quang Le, Kwang-Soo Lee, Jin-Soon Park and Jin Hwan Ko, Ocean Eng., 2014

In this study, flow-driven rotor simulations with a given load are conducted to analyze the operational characteristics of a vertical-axis Darrieus turbine, specifically its self-starting capability and fluctuations in its torque as well as the RPM. These characteristics are typically observed in experiments, though they cannot be acquired in simula- tions with a given tip speed ratio (TSR). First, it is shown that a flow-driven rotor simulation with a two-dimensional (2D) turbine model obtains power coefficients with curves similar to those obtained in a simulation with a given TSR. 3D flow- driven rotor simulations with an optimal geometry then show that...

* Sanjay Mittal and Bhaskar, Indian Institute of Technology, 23 August 2002*

The fatigue analysis of a wind turbine component typically uses representative samples of cyclic loads to determine lifetime loads. In this paper, several techniques currently in use are compared to one another based on fatigue life analyses. The generalized Weibull fitting technique is used to remove the artificial truncation of large-amplitude cycles that is inherent in relatively short data sets. Using data from the Sandia/DOE 34-m Test Bed, the generalized Weibull fitting technique is shown to be excellent for matching the body of the distribution of cyclic loads and for extrapolating the tail of the distribution. However, the data also illustrate that the fitting technique is not a substitute for an adequate data base.

**Fluid Dynamics Theory and Computation**

* Dan S. Henningson, Martin Berggren, 24 August 2005*

These lecture notes has evolved from a CFD course (5C1212) and a Fluid Mechanics course (5C1214) at the department of Mechanics and the department of Numerical Analysis and Computer Science (NADA) at KTH. Erik Stalberg and Ori Levin has typed most of the LATEX formulas and has created the electronic versions of most figures. In the latest version of the lecture notes study questions for the CFD course 5C1212 and recitation material for the Fluid Mechanics course 5C1214 has been added.

**Fluid-Structure Interaction Modeling of Vertical-Axis Wind Turbines**

*Y. Bazilevs, A. Korobenko, X. Deng, J. Yan, M. Kinzel, J. O. Dabiri, Journal of Applied Mechanics, August 2014*

Full-scale, 3D, time-dependent aerodynamics and ﬂuid–structure interaction (FSI) simulations of a Darrieus-type vertical-axis wind turbine (VAWT) are presented. A structural model of the Windspire VAWT (Windspire energy, http://www.windspireenergy.com/) is developed, which makes use of the recently proposed rotation-free Kirchhoff–Love shell and beam/cable formulations. A moving-domain ﬁnite-element-based ALE-VMS (arbitrary Lagrangian–Eulerian-variational-multiscale) formulation is employed for the aerodynamics in combination with the sliding-interface formulation to handle the VAWT mechanical components in relative motion. The sliding-interface formulation is augmented to handle nonstationary cylindrical sliding...

**Low-Order Modeling of Wind Farm Aerodynamics Using Leaky Rankine Bodies**

* Daniel B. Araya, Anna E. Craig, Matthias Kinzel, and John O. Dabiri, Journal of Renewable and Sustainable Energy, 2014*

We develop and characterize a low-order model of the mean ﬂow through an array of vertical-axis wind turbines (VAWTs), consisting of a uniform ﬂow and pairs of potential sources and sinks to represent each VAWT. The source and sink in each pair are of unequal strength, thereby forming a “leaky Rankine body” (LRB). In contrast to a classical Rankine body, which forms closed streamlines around a bluff body in potential ﬂow, the LRB streamlines have a qualitatively similar appearance to a separated bluff body wake; hence, the LRB concept is used presently to model the VAWT wake. The relative strengths of the source and sink are determined from ﬁrst principles analysis of an actuator disk model of the VAWTs. The LRB...

**Near wake flow analysis of a vertical axis wind turbine by stereoscopic particle image velocimetry**

*Tescione, G., Ragni, D., He, C., Ferreira, C. J. Sima ̃o & van Bussel, G.J.W., Renew. Energ. 70, 47–61, 2014*

The development of the near wake of a vertical axis wind turbine is investigated by stereoscopic particle image velocimetry. The experiments are conducted in an open-jet wind tunnel on an H-shaped rotor, operated at a tip speed ratio of 4.5 and at an average chord-based Reynolds number of 1.7 × 105. Phase-locked measurements are acquired at the turbine mid span in order to study the horizontal wake dynamics at the symmetry plane. Results show the evolution of the vorticity shed by the blade, how it organizes in large scale vortical structures at the edges of the wake and the resulting asymmetric induction field in the wake. The evolution of the blade tip vortices and the 3D wake geometry are detailed by a second...

**Nested Contour Dynamics Models for Axisymmetric Vortex Rings and Vortex Wakes**

*Clara O’Farrell and John O. Dabiri, Cambridge University Press, 2014*

Inviscid models for vortex rings and dipoles are constructed using nested patches of vorticity. These models constitute more realistic approximations to experimental vortex rings and dipoles than the single contour models of Norbury and Pierrehumbert, and nested contour dynamics algorithms allow their simulation with low computational cost. In two dimensions, nested-contour models for the analytical Lamb dipole are constructed. In the axisymmetric case, a family of models for vortex rings generated by a piston–cylinder apparatus at different stroke ratios is constructed from experimental data. The perturbation response of this family is considered by the introduction of a small region of vorticity at the rear of the vortex,...

*Akiyoshi Iida, Keiichi Kato and Akisato Mizuno, 16th Australasian Fluid Mechanics Conference Crown Plaza, Gold Coast, Australia, December 2007*

The goal of this investigation is to develop high performance Vertical Axis Wind Turbines (hereafter VAWT) for clean energy supply systems. For this purpose, we attempted to simulate flow around a VAWT with Large Eddy Simulation (LES). Since the angles of attack of VAWT are widely changed during the rotor rotation, large scale separation and interaction between the turbulent wakes are occurred [1]-[3]. Therefore, unsteady and high accuracy simulation is necessary to simulate flow around a VAWT. LES with a sliding mesh technique was utilised to solve the complicated flow around the VAWT. The numerical results show the large separation occurred and unsteady aerodynamic forces were observed in the wake of VAWT.

* Matthieu Duponcheel, Denis-Gabriel Caprace, Gregoire Winckelmans, Philippe Chatelain, Universite Catholique de Louvain, 2016*

The fatigue analysis of a wind turbine component typically uses representative samples of cyclic loads to determine lifetime loads. In this paper, several techniques currently in use are compared to one another based on fatigue life analyses. The generalized Weibull fitting technique is used to remove the artificial truncation of large-amplitude cycles that is inherent in relatively short data sets. Using data from the Sandia/DOE 34-m Test Bed, the generalized Weibull fitting technique is shown to be excellent for matching the body of the distribution of cyclic loads and for extrapolating the tail of the distribution. However, the data also illustrate that the fitting technique is not a substitute for an adequate data base.

**Perturbation Response and Pinch-off of Vortex Rings and Dipoles**

* Clara O’Farrell and John O. Dabiri, Cambridge University Press, 2012*

The nonlinear perturbation response of two families of vortices, the Norbury family of axisymmetric vortex rings and the Pierrehumbert family of two-dimensional vortex pairs, is considered. Members of both families are subjected to prolate shape perturbations similar to those previously introduced to Hill’s spherical vortex, and their response is computed using contour dynamics algorithms. The response of the entire Norbury family to this class of perturbations is considered, in order to bridge the gap between past observations of the behaviour of thin-cored members of the family and that of Hill’s spherical vortex. The behaviour of the Norbury family is contrasted with the response of the analogous...

**Pinch-off of Non-Axisymmetric Vortex Rings**

* Clara O’Farrell and John O. Dabiri, Cambridge University Press, 2014*

The formation and pinch-off of non-axisymmetric vortex rings is considered experimentally. Vortex rings are generated using a non-circular piston–cylinder arrangement, and the resulting velocity fields are measured using digital particle image velocimetry. Three different nozzle geometries are considered: an elliptical nozzle with an aspect ratio of two, an elliptical nozzle with an aspect ratio of four and an oval nozzle constructed from tangent circular arcs. The formation of vortices from the three nozzles is analysed by means of the vorticity and circulation, as well by investigation of the Lagrangian coherent structures in the flow. The results indicate that, in all three nozzles, the maximum circulation the vortex can attain...

* John O. Dabiri, Journal of Renewable and Sustainable Energy, 2011*

Modern wind farms comprised of horizontal-axis wind turbines (HAWTs) require significant land resources to separate each wind turbine from the adjacent turbine wakes. This aerodynamic constraint limits the amount of power that can be extracted from a given wind farm footprint. The resulting inefficiency of HAWT farms is currently compensated by using taller wind turbines to access greater wind resources at high altitudes, but this solution comes at the expense of higher-engineering costs and greater visual, acoustic, radar, and environmental impacts. We investigated the use of counter-rotating vertical-axis wind turbines (VAWTs) in order to achieve higher power output per unit land area than existing...

**Renewable Fluid Dynamic Energy Deried from Aquatic Animal Locomotion**

* John O. Dabiri, IOPScience, 2007*

Aquatic animals swimming in isolation and in groups are known to extract energy from the vortices in environmental ﬂows, signiﬁcantly reducing muscle activity required for locomotion. A model for the vortex dynamics associated with this phenomenon is developed, showing that the energy extraction mechanism can be described by simple criteria governing the kinematics of the vortices relative to the body in the ﬂow. In this way, we need not make direct appeal to the ﬂuid dynamics, which can be more difﬁcult to evaluate than the kinematics. Examples of these principles as exhibited in swimming ﬁsh and existing energy conversion devices are described. A beneﬁt of the developed framework is that the potentially...

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**The Near Wake Structure of a Vertical Axis Wind Turbine**

*K.R. Dixon, Delft University of Technology, April 24, 2008*

A 3D unsteady multi-body panel method is developed that can model arbitrary geometries. The method has been specifically designed so that it can handle the blade-wake interactions and viscous wake effects that are important for modelling a vertical axis wind turbine (VAWT). A systematic validation and verification of the model is presented against theoretical and experimental results. Validation of VAWT wake shapes is given in 2D and 3D by comparison with experimental results using several different techniques (smoke-trail, PIV, and water-tank dye injection). A generalized wake structure for a straight bladed VAWT is explained, and how this structure deforms in time and changes with tip speed ratio and height-diameter ratio.

**Transition to bluff-body dynamics in the wake of vertical-axis wind turbines**

*Araya, Daniel B. and Colonius, Tim and Dabiri, John O., Journal of Fluid Mechanics, 2017*

We present experimental data to demonstrate that the far wake of a vertical-axis wind turbine (VAWT) exhibits features that are quantitatively similar to that of a circular cylinder with the same aspect ratio. For a fixed Reynolds number ( ) and variable tip-speed ratio, two-dimensional particle image velocimetry (PIV) is used to measure the velocity field in the wake of four different laboratory-scale models: a 2-bladed, 3-bladed and 5-bladed VAWT, as well as a circular cylinder. With these measurements, we use spectral analysis and proper orthogonal decomposition (POD) to evaluate statistics of the velocity field and investigate the large-scale coherent motions of the wake. In all cases, we observe three distinct regions in the VAWT wake...

**Turbulence in Vertical Axis Wind Turbine Canopies**

* Matthias Kinzel, Daniel B. Araya, and John O. Dabiri, AIP, 9 November 2015*

Experimental results from three different full scale arrays of vertical-axis wind turbines (VAWTs) under natural wind conditions are presented. The wind velocities throughout the turbine arrays are measured using a portable meteorological tower with seven, vertically staggered, three-component ultrasonic anemometers. The power output of each turbine is recorded simultaneously. The comparison between the horizontal and vertical energy transport for the different turbine array sizes shows the importance of vertical transport for large array conﬁgurations. Quadrant-hole analysis is employed to gain a better understanding of the vertical energy transport at the top of the VAWT arrays. The results show a striking...

* Lydia A. Ruiz, Robert W. Whittlesey, John O. Dabiri, Cambridge University Press, 2010*

It has been previously suggested that the generation of coherent vortical structures in the near-wake of a self-propelled vehicle can improve its propulsive eﬃciency by manipulating the local pressure ﬁeld and entrainment kinematics. This paper investigates these unsteady mechanisms analytically and in experiments. A self-propelled underwater vehicle is designed with the capability to operate using either steady-jet propulsion or a pulsed-jet mode that features the roll-up of large-scale vortex rings in the near-wake. The ﬂow ﬁeld is characterized by using a combination of planar laser-induced ﬂuorescence, laser Doppler velocimetry and digital particle-image velocimetry. These tools enable measurement of vortex dynamics and...

**Vortex Shedding From A Spinning Cylinder**

*F. Diaz, J. Gavalda, J. G Kawall, J.F. Keffer, and F. Giralt, Universitat de Barcelona, 30 August 1983*

Wind turbines must withstand harsh environments that induce many stress cycles into their components. A numerical analysis package is used to illustrate the sobering variability in predicted fatigue life with relatively small changes in inputs. The variability of the input parameters is modeled to obtain estimates of the fatigue reliability of the turbine blades.

**Vortex Suppression And Drag Reduction in the Wake of Counter-Rotating Cylinders**

*Andre S. Chang, Peter A. Dewey, Antony Jameson, Chunlei Liang, and Alexander J. Smits, Stanford University, 12 May 2011*

The ﬂow over a pair of counter-rotating cylinders is investigated numerically and experimentally. It is demonstrated that it is possible to suppress unsteady vortex shedding for gap sizes from one to ﬁve cylinder diameters, at Reynolds numbers from 100 to 200, expanding on the more limited work by Chan & Jameson (Intl J. Numer. Meth. Fluids, vol. 63, 2010, p. 22). The degree of unsteady wake suppression is proportional to the speed and the direction of rotation, and there is a critical rotation rate where a complete suppression of ﬂow unsteadiness can be achieved. In the doublet-like conﬁguration at higher rotational speeds, a virtual elliptic body that resembles a potential doublet is formed, and the drag is reduced to...

**Wake Flow Simulation Of A Vertical Axis Wind Turbine Under The Influence Of Wind Shear**

*Victor Mendoza and Anders Goude, Journal Of Physics: Conference Series 2017*

The current trend of the wind energy industry aims for large scale turbines installed in wind farms. This brings a renewed interest in vertical axis wind turbines (VAWTs) since they have several advantages over the traditional Horizontal Axis Wind Tubines (HAWTs) for mitigating the new challenges. However, operating VAWTs are characterized by complex aerodynamics phenomena, presenting considerable challenges for modeling tools. An accurate and reliable simulation tool for predicting the interaction between the obtained wake of an operating VAWT and the flow in atmospheric open sites is fundamental for optimizing the design and location of wind energy facility projects.