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Aerodynamics of Vertical Axis Wind Turbines: Development of Simulation Tools and Experiments
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis combines measurements with the development of simulation tools for vertical axis wind turbines (VAWT). Numerical models of aerodynamic blade forces are developed and validated against experiments. The studies were made on VAWTs which were operated at open sites. Significant progress within the modeling of aerodynamics of VAWTs has been achieved by the development of new simulation tools and by conducting experimental studies.        

An existing dynamic stall model was investigated and further modified for the conditions of the VAWT operation. This model was coupled with a streamtube model and assessed against blade force measurements from a VAWT with curved blades, operated by Sandia National Laboratories. The comparison has shown that the accuracy of the streamtube model has been improved compared to its previous versions. The dynamic stall model was further modified by coupling it with a free vortex model. The new model has become less dependent on empirical constants and has shown an improved accuracy.    

Unique blade force measurements on a 12 kW VAWT were conducted. The turbine was operated north of Uppsala. Load cells were used to measure the forces on the turbine. A comprehensive analysis of the measurement accuracy has been performed and the major error sources have been identified.

The measured aerodynamic normal force has been presented and analyzed for a wide range of operational conditions including dynamic stall, nominal operation and the region of high flow expansion. The improved vortex model has been validated against the data from the new measurements. The model agrees quite well with the experiments for the regions of nominal operation and high flow expansion. Although it does not reproduce all measurements in great detail, it is suggested that the presented vortex model can be used for preliminary estimations of blade forces due to its high computational speed and reasonable accuracy.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 86 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1274
Keyword [en]
wind power, vertical axis turbine, H-rotor, simulations, streamtube model, vortex model, dynamic stall, measurements, blade, force
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-260573ISBN: 978-91-554-9307-3 (print)OAI: oai:DiVA.org:uu-260573DiVA: diva2:847672
Public defence
2015-10-09, Polhemssalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2015-09-17 Created: 2015-08-20 Last updated: 2015-10-01
List of papers
1. Dynamic Stall Modeling for the Conditions of Vertical Axis Wind Turbines
Open this publication in new window or tab >>Dynamic Stall Modeling for the Conditions of Vertical Axis Wind Turbines
2014 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 52, no 1, 72-81 p.Article in journal (Refereed) Published
Abstract [en]

Unsteady aerodynamics imposes additional demands on the modeling of vertical axis wind turbines. Large variations in the angles of attack of the blades cause force oscillations, which can lead to the fatigue-associated problems. Therefore, an accurate estimation of the dynamic loads is essential for the vertical axis wind turbines design. Dynamic stall modeling is in focus because it represents complex phenomena associated with the unsteady flow conditions. The purpose of the study is to find a suitable dynamic stall model for the vertical axis wind turbine conditions. Three versions of the Leishman-Beddoes model are explicitly presented. Additional modifications are implemented for the model to work when the angles of attack change sign and have high amplitudes. All the model parameters are presented. The model is assessed against measured data. The conditions for the simulation tests are close to the vertical axis wind turbine operational conditions. Aversion of the model, originally designed for low Mach numbers, is the most accurate throughout a number of tests.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-229324 (URN)10.2514/1.J052633 (DOI)000337803700007 ()
Available from: 2014-08-06 Created: 2014-08-05 Last updated: 2017-12-05Bibliographically approved
2. Simulating Dynamic Stall Effects for Vertical Axis Wind Turbines Applying a Double Multiple Streamtube Model
Open this publication in new window or tab >>Simulating Dynamic Stall Effects for Vertical Axis Wind Turbines Applying a Double Multiple Streamtube Model
2015 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 8, no 2, 1353-1372 p.Article in journal (Refereed) Published
Abstract [en]

The complex unsteady aerodynamics of vertical axis wind turbines (VAWT) poses significant challenges to the simulation tools. Dynamic stall is one of the phenomena associated with the unsteady conditions for VAWTs, and it is in the focus of the study. Two dynamic stall models are compared: the widely-used Gormont model and a Leishman-Beddoes-type model. The models are included in a double multiple streamtube model. The effects of flow curvature and flow expansion are also considered. The model results are assessed against the measured data on a Darrieus turbine with curved blades. To study the dynamic stall effects, the comparison of force coefficients between the simulations and experiments is done at low tip speed ratios. Simulations show that the Leishman-Beddoes model outperforms the Gormont model for all tested conditions.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-253091 (URN)10.3390/en8021353 (DOI)000353561900001 ()
Funder
StandUpSwedish Energy Agency
Available from: 2015-05-27 Created: 2015-05-20 Last updated: 2017-12-04Bibliographically approved
3. Simulating Pitching Blade With Free Vortex Model Coupled With Dynamic Stall Model for Conditions of Straight Bladed Vertical Axis Turbines
Open this publication in new window or tab >>Simulating Pitching Blade With Free Vortex Model Coupled With Dynamic Stall Model for Conditions of Straight Bladed Vertical Axis Turbines
2015 (English)In: Journal of solar energy engineering, ISSN 0199-6231, E-ISSN 1528-8986, Vol. 137, no 4, 041008Article in journal (Refereed) Published
Abstract [en]

This study is on the straight bladed vertical axis turbines (VATs), which can be utilized for both wind and marine current energy. VATs have the potential of lower installation and maintenance cost. However, complex unsteady fluid mechanics of these turbines imposes significant challenges to the simulation tools. Dynamic stall is one of the phenomena associated with the unsteady conditions, and it is in the focus of the study. The dynamic stall effects are very important for VATs, since they are usually passively controlled through the dynamic stall. A free vortex model is used to calculated unsteady attached flow, while the separated flow is handled by the dynamic stall model. This is compared to the model based solely on the Leishman-Beddoes algorithm. The results are assessed against the measured data on pitching airfoils. A comparison of force coefficients between the simulations and experiments is done at the conditions similar to the conditions of H-rotor type VATs.

National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-258739 (URN)10.1115/1.4030674 (DOI)000356966700008 ()
Funder
VINNOVA
Available from: 2015-07-21 Created: 2015-07-20 Last updated: 2017-12-04Bibliographically approved
4. Evaluation of a Blade Force Measurement System for a Vertical Axis Wind Turbine Using Load Cells
Open this publication in new window or tab >>Evaluation of a Blade Force Measurement System for a Vertical Axis Wind Turbine Using Load Cells
Show others...
2015 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 8, no 6, 5973-5996 p.Article in journal (Refereed) Published
Abstract [en]

Unique blade force measurements on an open site straight-bladed vertical axis wind turbine have been performed. This paper presents a method for measuring the tangential and normal forces on a 12-kW vertical axis wind turbine prototype with a three-bladed H-rotor. Four single-axis load cells were installed in-between the hub and the support arms on one of the blades. The experimental setup, the measurement principle, together with the necessary control and measurement system are described. The maximum errors of the forces and accompanying weather data that can be obtained with the system are carefully estimated. Measured forces from the four load cells are presented, as well as the normal and tangential forces derived from them and a comparison with theoretical data. The measured torque and bending moment are also provided. The influence of the load cells on the turbine dynamics has also been evaluated. For the aerodynamic normal force, the system provides periodic data in agreement with simulations. Unexpected mechanical oscillations are present in the tangential force, introduced by the turbine dynamics. The measurement errors are of an acceptable size and often depend on the measured variable. Equations are presented for the calculation of measurement errors.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-260148 (URN)10.3390/en8065973 (DOI)000357489700067 ()
Funder
StandUpStandUp for Wind
Available from: 2015-08-17 Created: 2015-08-17 Last updated: 2017-12-04
5. Measurements of the Aerodynamic Normal Forces on a 12-kW Straight-Bladed Vertical Axis Wind Turbine
Open this publication in new window or tab >>Measurements of the Aerodynamic Normal Forces on a 12-kW Straight-Bladed Vertical Axis Wind Turbine
2015 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 8, no 8Article in journal (Refereed) Published
Abstract [en]

The knowledge of unsteady forces is necessary when designing vertical axis wind turbines (VAWTs). Measurement data for turbines operating at an open site are still very limited. The data obtained from wind tunnels or towing tanks can be used, but have limited applicability when designing large-scale VAWTs. This study presents experimental data on the normal forces of a 12-kW straight-bladed VAWT operated at an open site north of Uppsala, Sweden. The normal forces are measured with four single-axis load cells. The data are obtained for a wide range of tip speed ratios: from 1.7 to 4.6. The behavior of the normal forces is analyzed. The presented data can be used in validations of aerodynamic models and the mechanical design for VAWTs.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-260569 (URN)10.3390/en8088482 (DOI)000360586600052 ()
Available from: 2015-08-20 Created: 2015-08-20 Last updated: 2017-12-04Bibliographically approved
6. Numerical Validation of a Vortex Model Against Experimental Data on a Straight-Bladed Vertical Axis Wind Turbine
Open this publication in new window or tab >>Numerical Validation of a Vortex Model Against Experimental Data on a Straight-Bladed Vertical Axis Wind Turbine
2015 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 8, no 10, 11800-11820 p.Article in journal (Refereed) Published
Abstract [en]

Cyclic blade motion during operation of vertical axis wind turbines (VAWTs) imposes challenges on the simulations models of the aerodynamics of VAWTs. A two-dimensional vortex model is validated against the new experimental data on a 12-kW straight-bladed VAWT, which is operated at an open site. The results on the normal force on one blade are analyzed. The model is assessed against the measured data in the wide range of tip speed ratios: from 1.8 to 4.6. The predicted results within one revolution have a similar shape and magnitude as the measured data, though the model does not reproduce every detail of the experimental data. The present model can be used when dimensioning the turbine for maximum loads.

Place, publisher, year, edition, pages
Basel: MDPI, 2015
Keyword
wind turbine; vertical axis turbine; force; measurement; open site; simulation; vortex model; dynamic stall
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-260572 (URN)10.3390/en81011800 (DOI)000364230500059 ()
Funder
StandUp
Available from: 2015-08-20 Created: 2015-08-20 Last updated: 2017-12-04Bibliographically approved

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