Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Numerical study on instability and interaction of wind turbine wakes
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Numerical simulations of the Navier-Stokes equations are conducted to achieve a better understanding of the behavior of wakes generated by the wind turbines. The simulations are performed by combining the in-house developed code EllipSys3D with the actuator line technique.

In step one of the project, a numerical study is carried out focusing on the instability onset of the trailing tip vortices shed from a 3-bladed wind turbine. To determine the critical frequency, the wake is perturbed using low-amplitude excitations located near the tip spirals. Two basic flow cases are studied; symmetric and asymmetric setups. In the symmetric setup a 120 degree flow symmetry condition is dictated due to the confining the polar computational grid to 120 degree or introducing identical excitations. In the asymmetric setup, uncorrelated excitations are imposed near the tip of the blades. Both setups are analyzed using proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). By analysing the dominant modes, it was found that in the symmetric setup the amplification of specific waves (traveling structures) traveling along the tip vortex spirals is responsible for triggering the instability leading to wake breakdown, while by breaking the symmetry almost all the modes are involved in the tip vortex destabilization. The presence of unstable modes in the wake is related to the mutual inductance (vortex pairing) instability where there is an out-of-phase displacement of successive helix turns. Furthermore, using the non-dimensional growth rate, it is found that the mutual inductance instability has a universal growth rate equal to Π/2. Using this relationship, and the assumption that breakdown to turbulence occurs once a vortex has experienced sufficient growth, an analytical relationship is provided for determining the length of the stable wake. This expression shows that the stable wake length is inversely proportional to thrust, tip speed ratio and the logarithmic of the turbulence intensity.

In second study, large eddy simulations of the Navier-Stokes equations are also performed to investigate the wake interaction. Previous actuator line simulations on the single model wind turbine show that the accuracy of the results is directly related to the quality of the input airfoil characteristics. Therefore, a series of experiments on a 2D wing are conducted to obtain high quality airfoil characteristics for the NREL S826 at low Reynolds numbers. The new measured data are used to compute the rotor performance. The results show that the power performance as well as the wake development behind the rotor are well-captured. There are, however, some difficulties in prediction of the thrust coefficients. The continuation of this work considers the wake interaction investigations of two turbines inline (full-wake interaction) and two turbines with spanwise offset (half wake interaction). It is demonstrated that the numerical computations are able to predict the rotor performances as well as the flow field around the model rotors, and it can be a suitable tool for investigation of the wind turbine wakes.

In the last study, an evaluation of the performance and near-wake structure of an analytical vortex model is presented. The vortex model is based on the constant circulation along the blades (Joukowsky rotor) and it is able to determine the geometry of the tip vortex filament in the rotor wake, allowing the free wake expansion and changing the local tip vortex pitch. Two different wind turbines have been simulated: a wind turbine with constant circulation along the blade and the other setup with a realistic circulation distribution, to compare the outcomes of the vortex model with real operative wind turbine conditions. The vortex model is compared with the actuator line approach and the presented comparisons show that the vortex method is able to approximate the single rotor performance and qualitatively describe the flow field around the wind turbine but with a negligible computational effort. This suggests that the vortex model can be a substitute of more computationally-demanding methods like actuator line technique to study the near-wake behavior.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , xi, 44 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014:23
Keyword [en]
Wind turbine wakes, Stability, interaction, POD, DMD
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-153961ISBN: 978-91-7595-298-7 (print)OAI: oai:DiVA.org:kth-153961DiVA: diva2:754535
Public defence
2014-10-31, F3, Lindstedsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20141010

Available from: 2014-10-10 Created: 2014-10-10 Last updated: 2014-10-10Bibliographically approved
List of papers
1. Mutual inductance instability of the tip vortices behind a wind turbine
Open this publication in new window or tab >>Mutual inductance instability of the tip vortices behind a wind turbine
Show others...
2014 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, 705-731 p.Article in journal (Refereed) Published
Abstract [en]

Two modal decomposition techniques are employed to analyse the stability of wind turbine wakes. A numerical study on a single wind turbine wake is carried out focusing on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations using the actuator line (ACL) method to simulate the wake behind the Tj ae reborg wind turbine. The wake is perturbed by low-amplitude excitation sources located in the neighbourhood of the tip spirals. The amplification of the waves travelling along the spiral triggers instabilities, leading to breakdown of the wake. Based on the grid configurations and the type of excitations, two basic flow cases, symmetric and asymmetric, are identified. In the symmetric setup, we impose a 120 degrees symmetry condition in the dynamics of the flow and in the asymmetric setup we calculate the full 360 degrees wake. Different cases are subsequently analysed using dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD). The results reveal that the main instability mechanism is dispersive and that the modal growth in the symmetric setup arises only for some specific frequencies and spatial structures, e.g. two dominant groups of modes with positive growth (spatial structures) are identified, while breaking the symmetry reveals that almost all the modes have positive growth rate. In both setups, the most unstable modes have a non-dimensional spatial growth rate close to pi/2 and they are characterized by an out-of-phase displacement of successive helix turns leading to local vortex pairing. The present results indicate that the asymmetric case is crucial to study, as the stability characteristics of the flow change significantly compared to the symmetric configurations. Based on the constant non-dimensional growth rate of disturbances, we derive a new analytical relationship between the length of the wake up to the turbulent breakdown and the operating conditions of a wind turbine.

Keyword
instability, vortex interaction, wakes
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-145662 (URN)10.1017/jfm.2014.326 (DOI)000341128600036 ()
Funder
Swedish e‐Science Research Center
Note

QC 20140930. Updated from manuscript to article in journal.

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2017-12-05Bibliographically approved
2. Simulation of wind turbine wakes using the actuator line technique
Open this publication in new window or tab >>Simulation of wind turbine wakes using the actuator line technique
Show others...
2015 (English)In: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, ISSN 1364-503X, Vol. 373, 2035- p.Article in journal (Refereed) Published
Abstract [en]

The actuator line technique was introduced as a numerical tool to be employed in combination with large eddy simulations to enable the study of wakes and wake interaction in wind farms. The technique is today largely used for studying basic features of wakes as well as for making performance predictions of wind farms. In this paper, we give a short introduction to the wake problem and the actuator line methodology and present a study in which the technique is employed to determine the near-wake properties of wind turbines. The presented results include a comparison of experimental results of the wake characteristics of the flow around a three-bladed model wind turbine, the development of a simple analytical formula for determining the near-wake length behind a wind turbine and a detailed investigation of wake structures based on proper orthogonal decomposition analysis of numerically generated snapshots of the wake.

Keyword
wind turbines, wakes, actuator line, large eddy simulation
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-154023 (URN)10.1098/rsta.2014.0071 (DOI)000347845700003 ()2-s2.0-84921323545 (Scopus ID)
Note

Updated from "Pre-print" to "Article in journal". QC 20150227

Available from: 2014-10-10 Created: 2014-10-10 Last updated: 2017-01-10Bibliographically approved
3. The experimental results of the NREL S826 airfoil at low Reynolds numbers
Open this publication in new window or tab >>The experimental results of the NREL S826 airfoil at low Reynolds numbers
2012 (English)Report (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-120583 (URN)
Note

This report presents the performance characteristics of NREL S826 airfoil forReynolds numbers in a range of about 40,000 to 120,000. QC 20130412

Available from: 2013-04-12 Created: 2013-04-12 Last updated: 2014-10-10Bibliographically approved
4. Numerical study on the performance and the wake development of single andtwo in-line model wind turbines
Open this publication in new window or tab >>Numerical study on the performance and the wake development of single andtwo in-line model wind turbines
2014 (English)Report (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-153967 (URN)
Note

QC 20141010

Available from: 2014-10-10 Created: 2014-10-10 Last updated: 2014-10-10Bibliographically approved
5. Numerical investigation of the wake interaction between two model wind turbines with span-wise offset
Open this publication in new window or tab >>Numerical investigation of the wake interaction between two model wind turbines with span-wise offset
2014 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 524, no 1Article in journal (Refereed) Published
Abstract [en]

Wake interaction between two model scale wind turbines with span-wise offset is investigated numerically using Large Eddy Simulation (LES) and the results are validated against the experimental data. An actuator line technique is used for modeling the rotor. The investigated setup refers to a series of experimental measurements of two model scale turbines conducted by NTNU in low speed wind tunnel in which the two wind turbines are aligned with a span-wise offset resulting in half wake interaction. Two levels of free-stream turbulence are tested, the minimum undisturbed level of about Ti 0.23% and a high level of about Ti = 10% using a passive upstream grid. The results show that the rotor characteristics for both rotors are well captured numerically even if the downstream rotor operates into stall regimes. There are however some difficulties in correct prediction of the thrust level. The interacting wake development is captured in great details in terms of wake deficit and streamwise turbulence kinetic energy. The present work is done in connection with Blind test 3 workshops organized jointly by NOWITECH and NORCOWE.

National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-153969 (URN)10.1088/1742-6596/524/1/012137 (DOI)000344193600137 ()2-s2.0-84903714784 (Scopus ID)
Conference
5th Science of Making Torque from Wind Conference, TORQUE 2014; Copenhagen; Denmark
Note

QC 20141010

Available from: 2014-10-10 Created: 2014-10-10 Last updated: 2017-12-05Bibliographically approved
6. A comparison between a simplified vortex model and actuator line simulations of a horizontal axis wind turbine
Open this publication in new window or tab >>A comparison between a simplified vortex model and actuator line simulations of a horizontal axis wind turbine
(English)Manuscript (preprint) (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-120588 (URN)
Funder
StandUp for Wind
Note

QS 2013

Available from: 2013-04-12 Created: 2013-04-12 Last updated: 2017-01-18Bibliographically approved

Open Access in DiVA

PhD thesis(4353 kB)627 downloads
File information
File name FULLTEXT01.pdfFile size 4353 kBChecksum SHA-512
88f121c1cb260fb477f75d44bd7ff84b717bfacf6f3939653d2a0e150326ae628e7e395e22b75ff74032ee23123a91e7df0d60e3438a0e8230d20a90fae08a3a
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Sarmast, Sasan
By organisation
Stability, Transition and ControlLinné Flow Center, FLOW
Fluid Mechanics and Acoustics

Search outside of DiVA

GoogleGoogle Scholar
Total: 627 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 842 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf