Wake & Vertical Mixing

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.

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.

A New Approach to Wind Energy

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...

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...

Flow Past a Rotating Cylinder

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...

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,...

Performance And Wake Development of Vertical Axis Wind Turbine A LES Study Using a Vortex Particle-Mesh Method

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...

 

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...

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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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...

Vortex-Enhanced Propulsion

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...

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