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Numerical computations of wind turbine wakes
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Numerical simulations of the Navier-Stokes equations are performed to achieve a better understanding of the behaviour of wakes generated by wind turbines. The simulations are performed by combining the in-house developed computer code EllipSys3D with the actuator line and disc methodologies. In the actuator line and disc methods the blades are represented by a line or a disc on which body forces representing the loading are introduced. The body forces are determined by computing local angles of attack and using tabulated aerofoil coefficients. The advantage of using the actuator disc technique is that it is not necessary to resolve blade boundary layers. Instead the computational resources are devoted to simulating the dynamics of the flow structures. In the present study both the actuator line and disc methods are used. Between approximately six to fourteen million mesh points are used to resolve the wake structure in a range from a single turbine wake to wake interaction in a farm containing 80 turbines. These 80 turbines are however represented by 20 actuator discs due to periodicity because of numerical limitations. In step one of this project the objective was to find a numerical method suitable to study both the flow structures in the wake behind a single wind turbine and to simulate complicated interaction between a number of turbines. The study resulted in an increased comprehension of basic flow features in the wake, but more importantly in the use of a numerical method very suitable for the upcoming purpose. The second objective of the project was to study the basic mechanisms controlling the length of the wake to obtain better understanding of the stability properties of wakes generated by wind turbine rotors. The numerical model was based on large eddy simulations of the Navier-Stokes equations using the actuator line method to generate the wake and the tip vortices. To determine critical frequencies the flow is disturbed by inserting a harmonic perturbation. The results showed that instability is dispersive and that growth occurs only for specific frequencies and mode types. The study also provides evidence of a relationship between the turbulence intensity and the length of the wake. The relationship however needs to be calibrated with measurements. In the last project objective, full wake interaction in large wind turbine farms was studied and verified to measurements. Large eddy simulations of the Navier-Stokes equations are performed to simulate the Horns Rev off-shore wind farm 15 km outside the Danish west coast. The aim is to achieve a better understanding of the wake interaction inside the farm. The simulations are performed by using the actuator disc methodology. Approximately 13.6 million mesh points are used to resolve the wake structure in the park containing 80 turbines. Since it is not possible to simulate all turbines, the 2 central columns of turbines have been simulated with periodic boundary conditions. This corresponds to an infinitely wide farm with 10 turbines in downstream direction. Simulations were performed within plus/minus 15 degrees of the turbine alignment. The infinitely wide farm approximation is thus reasonable. The results from the CFD simulations are evaluated and the downstream evolution of the velocity field is depicted. Special interest is given to what extent production is dependent on the inflow angle and turbulence level. The study shows that the applied method captures the main production variation within the wind farm. The result further demonstrates that levels of production correlate well with measurements. However, in some cases the variation of the measurement data is caused by the different measurement conditions during different inflow angles.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , xx, 77 p.
Series
Trita-MEK, ISSN 0348-467X ; 2009:01
Keyword [en]
Actuator Line Model, Actuator Disc Model, Wind Turbine Wake, CFD, LES, EllipSys3D
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-9916ISBN: 978-91-7415-216-6 (print)OAI: oai:DiVA.org:kth-9916DiVA: diva2:158181
Public defence
2009-02-19, Sal F3, KTH, Lindstedtsvägen 26, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20100720Available from: 2009-01-30 Created: 2009-01-30 Last updated: 2012-03-20Bibliographically approved
List of papers
1. Analysis of numerically generated wake structures
Open this publication in new window or tab >>Analysis of numerically generated wake structures
2009 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 1, 63-80 p.Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulations of the Navier-Stokes equations are performed to achieve a better understanding of the behaviour of wakes generated by wind turbines. The simulations are performed by combining the in-house developed computer code EllipSys3D with the actuator-line methodology. In the actuator-line method, the blades are represented by lines along which body forces representing the loading are introduced. The body forces are determined by computing local angles of attack and using tabulated aerofoil coefficients. The advantage of using the actuator-line technique is that it is not needed to resolve blade boundary layers and instead the computational resources are devoted to simulating the dynamics of the flow structures. In the present study, approximately 5 million mesh points are used to resolve the wake structure in a 120-degree domain behind the turbine. The results from the computational fluid dynamics (CFD) simulations are evaluated and the downstream evolution of the velocity field is depicted. Special interest is given to the structure and position of the tip vortices. Further, the circulation from the wake flow field is computed and compared to the distribution of circulation on the blades.

Keyword
wind wake; EllipSys3D; actuator line; circulation
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-9910 (URN)10.1002/we.285 (DOI)000262814100004 ()2-s2.0-77249107659 (Scopus ID)
Note
QC 20100720Available from: 2009-01-30 Created: 2009-01-30 Last updated: 2017-12-14Bibliographically approved
2. Stability analysis of the tip vortices of a wind turbine
Open this publication in new window or tab >>Stability analysis of the tip vortices of a wind turbine
2010 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 13, no 8, 705-715 p.Article in journal (Refereed) Published
Abstract [en]

 

The aim of the present project is to obtain a better understanding of thestability properties of wakes generated by wind turbine rotors. To accomplishthis a numerical study on the stability of the tip vortices of the Tjaereborgwind turbine has been carried out. The numerical model is based on large eddysimulations of the Navier-Stokes equations using the actuator line method togenerate the wake and the tip vortices. To determine critical frequencies theflow is disturbed by inserting harmonic perturbations.The results show that the instability is dispersive and that growth arisesonly for some specific frequencies and type of modes. The study also providesevidence of a relationship between the turbulence intensity and the length ofthe near wake. The relationship however needs to be calibrated against measurements.

Keyword
wakes, CFD, EllipSys3D, wake length, Horns Rev, Actuator line, Harmonic perturbations, Wind turbines
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-9911 (URN)10.1002/we.391 (DOI)000285314400002 ()2-s2.0-78649637517 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

Uppdaterad från submitted till published 20100720 QC 20100720

Available from: 2009-01-30 Created: 2009-01-30 Last updated: 2017-06-19Bibliographically approved
3. Three dimensional actuator disc modelling ofwind farm wake interaction
Open this publication in new window or tab >>Three dimensional actuator disc modelling ofwind farm wake interaction
2008 (English)In: European Wind Energy Conference and Exhibition, 2008, 1-10 p.Conference paper, Published paper (Refereed)
Abstract [en]

Large eddy simulations of the Navier-Stokes equations are performed to achieve a better under-standing of the behaviour of wakes generated by wind turbines. The simulations are performed bycombining the in-house developed computer code EllipSys3D with the actuator-disc methodology.In the actuator-disc method the blades are represented by a disc at which body forces representingthe loading are introduced. The body forces are determined by computing local angles of attack andusing tabulated aerofoil coefficients. The advantage of using the actuator-disc technique is that itis not needed to resolve blade boundary layers and instead the computational resources are devotedto simulating the dynamics of the flow structures. In the present study approximately six millionsmesh points are used to resolve the wake structure in a park containing up to 9 turbines. Theresults from the CFD simulations are evaluated and the downstream evolution of the velocity fieldis depicted. Special interest is given to what extent the production is dependent of inflow angle.The result clearly indicates that turbulent inflow has a strong impact on the result and leads to amore complex flow structure.

Keyword
Actuator disc, Angles of attack, Body forces, CFD simulations, Complex flow structures, Computational resources, In-house developed computer, Mesh points, Velocity field, Wake interactions, Wake structures, Wind farm, Computational fluid dynamics, Electric utilities, Exhibitions, Flow structure, Loading, Navier Stokes equations, Velocity, Wakes, Wind power, Actuator disks
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-9912 (URN)2-s2.0-84870035629 (Scopus ID)
Conference
European Wind Energy Conference and Exhibition 2008, EWEC 2008; Brussels; Belgium
Note

QC 20100720

Available from: 2009-01-30 Created: 2009-01-30 Last updated: 2015-01-30Bibliographically approved
4. Actuator disc modelling of wake interaction in Horns Rev wind farm
Open this publication in new window or tab >>Actuator disc modelling of wake interaction in Horns Rev wind farm
(English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824Article in journal (Other academic) Submitted
Abstract [en]

Large eddy simulations of the Navier-Stokes equations are performed tosimulate the Horns Rev off shore wind farm 15 km outside the Danish westcoast. The aim is to achieve a better understanding of the wake interactioninside the farm. The simulations are performed by combining the in-housedeveloped computer code EllipSys3D with the actuator-disc methodology. Inthe actuator-disc method the blades are represented by a disc at which bodyforces representing the aerodynamic loading are introduced. The body forcesare determined by computing local angles of attack and tabulated aerofoil coefficients.The advantage of using the actuator-disc technique is that it is notnecessary to resolve blade boundary layers since the computational resourcesare devoted to simulating the dynamics of the flow structures.In the present study approximately 13.6 million mesh points are used toresolve the wake structure in the park. The park contains 80 wind turbinesdistributed over an area of about 20km2. Since it is not possible to simulate allturbines, the 2 central columns of turbines have been simulated with periodicboundary conditions. This corresponds to an infinitely wide farm with 10turbines in downstream direction. Simulations were performed within plusminus 15 degrees of the turbine alignment, making the wide farm approximationreasonable.The results from the CFD simulations are evaluated and the downstreamevolution of the velocity field is depicted. Special interest is given to whatextent the production is dependent on the inflow angle and turbulence level.The study shows that the applied method captures the main productionvariation within the wind farm. The result further demonstrates that levelsof production correlate well with measurements. However, in some cases thevariation of the measurement data is caused by variation of measurement conditionswith inflow angles.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-9913 (URN)
Note
QS 20120327Available from: 2009-01-30 Created: 2009-01-30 Last updated: 2017-12-14Bibliographically approved
5. Validation of methods using EllipSys3D
Open this publication in new window or tab >>Validation of methods using EllipSys3D
2008 (English)Report (Other academic)
Abstract [en]

Numerical simulations of the Navier-Stokes equations are performed toachieve a better understanding of the behaviour of wakes generated by windturbines. The simulations are performed by combining the in-house developedcomputer code EllipSys3D with the actuator line and actuator disc methodologies.In the actuator methods the blades are represented by lines or a discat which body forces representing the loading are introduced. The body forcesare determined by computing local angles of attack and using tabulated aerofoilcoefficients. The advantage of using the actuator techniques is that theyare not needed to resolve blade boundary layers. Instead the computationalresources are devoted to simulating the dynamics of the flow structures. Inthe present study we run simulations using both the actuator line and theactuator disc methodologies. The influence from changing a number of parametersis evaluated. The study serves as a validation and sensitivity study toboth the actuator line and the actuator disc methods. The result shows thatboth methods produce realistic results. There is however a sensitivity to someparameters.

Series
Trita-MMK, ISSN 1400-1179 ; 2008:12
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-9914 (URN)
Note
QC 20100720Available from: 2009-01-30 Created: 2009-01-30 Last updated: 2010-07-20Bibliographically approved

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Citation style
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Output format
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