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Wind-turbine wakes - Effects of yaw, shear and turbine interaction
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Vindturbinsvakar –Effekten av girning, skjuvning och turbininteraktion (Swedish)
Abstract [en]

The actuator-line method is used together with the incompressible Navier–Stokes equations to investigate the flow development behind wind turbines. Initial investigations focus on providing a thorough validation of the implementation in the spectral-element flow solver Nek5000 against existing numerical and experimental datasets. It is shown that the current implementation gives an accurate representation of the flow field for different turbine geometries, inflow conditions, yaw misalignment, and when considering multiple turbines. This enables an in-depth study of the wake physics in these configurations.

The yawed wind-turbine wake development is shown to depend on the tip-speed ratio, both in terms of the wake deficit and the generation of the counter-rotating vortices known to occur in yawed turbine wakes. For lower tip-speed ratios the wake deficit exhibited significant asymmetries with respect to the horizontal plane due to the advancing/retreating effect. At high tip-speed ratios this effect became negligible compared to the skewed wake effect, which affects the symmetry with respect to the vertical plane. These inhomogeneities in the averaged wake development also affect the tip-vortex breakdown, leading to different locations of the tip-vortex breakdown along the wake azimuth due to the significant azimuthal variations of the tip-vortex strength and convection velocity. An analysis of the interaction of a yawed wind-turbine wake with a sheared inflow exposed a dependency of the wake deflection and recovery on the yaw orientation, which then resulted in significant differences in the combined power output of a two-turbine setup. More detailed studies of the tip-vortex breakdown in sheared flows using single-frequency perturbations revealed that a sheared inflow changes the spatial growth rate of the tip vortices along the vertical axis, due to the varying tip-vortex convection velocity. However, by applying a scaling based on local vortex parameters, the growth rates collapse to the canonical case of an infinite row of point vortices. Finally, an idealized scenario of two in-line turbines with a steady tip-vortex development is investigated. By applying a range of controlled perturbations, modes were excited, which exhibited in-phase or out-of-phase displacement between the vortex system of the upstream and the downstream turbine for certain frequencies.

Abstract [sv]

Den så kallade actuator line-metoden används tillsammans med inkompressibla Navier–Stokes ekvationer för att undersöka strömningens utveckling bakom vindturbiner. Inledande studier syftar till att utförligt validera implementationen i spektralelementkoden Nek5000 mot befintliga numeriska och experimentella datamängder. Det visas att den nuvarande implementationen ger en noggrann representation av strömningsfältet för alla undersökta turbingeometrier. Vidare fångas utvecklingen hos vaken väl för en rad olika inflödesvillkor, förturbingirning, och under interaktion mellan flera turbiner.

Vakutvecklingen för en girad turbin visas bero signifikant på kvoten mellan vingspetsens och friströmmens hastighet, både när det gäller hastighetsunderskottet i vaken och bildningen av de motroterande vakvirvlarna. För låga hastighetskvoter mellan vingspetsen och friströmmen uppvisar vakens hastighetsunderskott en betydande asymmetri med avseende på horisontalplanet genom en så kallad avancerande/retirerande effekt. För höga hastighetskvoter blir denna effekt däremot försumbar i jämförelse med vakens skevhet som påverkar symmetrin med avseende på vertikalplanet. Dessa inhomogeniteter i den medelvärdesbildade vakutvecklingen påverkar också det turbulenta nedbrottet hos vingspetsvirvlarna, vilket inträffar vid olika positioner i vinkelled på grund av signifikanta vinkelvariationer hos virvelstyrkan och konvektionshastigheten. En analys of interaktionen mellan en girad turbinvak och en inkommande skjuvströmning avslöjar ett beroende hos vakens förskjutning och återhämtning på girningens riktning, vilket resulterar i betydande skillnader i den sammantagna effekten hos två turbiner. Mer detaljerade studier av spetsvirvlarnas nedbrott i skjuvströmningar med enfrekvensstörningar visar att ett skjuvat inflöde förändrar den spatiella tillväxtgraden längs den vertikala axeln på grund av varierande konvektionshastighet hos spetsvirvlarna.Tillväxtgraderna sammanfaller dock med motsvarande värde för det klassiska fallet med två oändliga virvelrader, om de skalas med lokala virvelparametrar. Slutligen studeras en stationär virvelutveckling för ett idealiserat fall bestående av två turbiner i linje med varandra. Genom att applicera en rad kontrollerade störningar, exciteras moder som beroende på frekvens uppvisar förskjutningar i eller ur fas med virvelsystemen från turbinen uppströms och nedströms.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. , p. 58
Series
TRITA-SCI-FOU ; 2019:29
Keywords [en]
wind-turbine wakes, yaw, tip-vortex breakdown, shear, computational fluid dynamics, actuator-line method, spectral-element method
Keywords [sv]
Vindturbinsvakar, girning, turbulent nedbrott hos spetsvirvlar, skjuvning, vakinteraktion, beräkningsströmningsdynamik, actuator line-metod, spektralelementmetod
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-251450OAI: oai:DiVA.org:kth-251450DiVA, id: diva2:1315683
Public defence
2019-06-04, H1, Teknikringen 33, Stockholm, 13:50 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency
Note

QC20190514

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-05-14Bibliographically approved
List of papers
1. Actuator line simulations of a Joukowsky and Tjæreborg rotor using spectral element and finite volume methods
Open this publication in new window or tab >>Actuator line simulations of a Joukowsky and Tjæreborg rotor using spectral element and finite volume methods
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2016 (English)In: SCIENCE OF MAKING TORQUE FROM WIND (TORQUE 2016), Institute of Physics (IOP), 2016, Vol. 753, no 8, article id 082011Conference paper, Published paper (Refereed)
Abstract [en]

The wake structure behind a wind turbine, generated by the spectral element code Nek5000, is compared with that from the finite volume code EllipSys3D. The wind turbine blades are modeled using the actuator line method. We conduct the comparison on two different setups. One is based on an idealized rotor approximation with constant circulation imposed along the blades corresponding to Glauert's optimal operating condition, and the other is the Tjffireborg wind turbine. The focus lies on analyzing the differences in the wake structures entailed by the different codes and corresponding setups. The comparisons show good agreement for the defining parameters of the wake such as the wake expansion, helix pitch and circulation of the helical vortices. Differences can be related to the lower numerical dissipation in Nek5000 and to the domain differences at the rotor center. At comparable resolution Nek5000 yields more accurate results. It is observed that in the spectral element method the helical vortices, both at the tip and root of the actuator lines, retain their initial swirl velocity distribution for a longer distance in the near wake. This results in a lower vortex core growth and larger maximum vorticity along the wake. Additionally, it is observed that the break down process of the spiral tip vortices is significantly different between the two methods, with vortex merging occurring immediately after the onset of instability in the finite volume code, while Nek5000 simulations exhibit a 2-3 radii period of vortex pairing before merging.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2016
Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 753
Keywords
Actuators, Codes (symbols), Finite volume method, Merging, Torque, Turbine components, Turbomachine blades, Wakes, Wind turbines, Domain differences, Finite volume code, Helical vortices, Numerical dissipation, Onset of instabilities, Optimal operating conditions, Spectral element method, Wind turbine blades, Vortex flow
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-201777 (URN)10.1088/1742-6596/753/8/082011 (DOI)000436325702068 ()2-s2.0-84995394418 (Scopus ID)
Conference
Science of Making Torque from Wind, TORQUE 2016; Munich; Germany; 5 October 2016 through 7 October 2016
Note

QC 20170217

Available from: 2017-02-17 Created: 2017-02-17 Last updated: 2019-09-20Bibliographically approved
2. Parametric study of the actuator-line method in high-order codes
Open this publication in new window or tab >>Parametric study of the actuator-line method in high-order codes
2019 (English)Report (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-251417 (URN)
Note

QC 20190619

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-06-19Bibliographically approved
3. High-Order Numerical Simulations of Wind Turbine Wakes
Open this publication in new window or tab >>High-Order Numerical Simulations of Wind Turbine Wakes
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2017 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 854, no 1, article id 012025Article in journal (Refereed) Published
Abstract [en]

Previous attempts to describe the structure of wind turbine wakes and their mutual interaction were mostly limited to large-eddy and Reynolds-averaged Navier-Stokes simulations using finite-volume solvers. We employ the higher-order spectral-element code Nek5000 to study the influence of numerical aspects on the prediction of the wind turbine wake structure and the wake interaction between two turbines. The spectral-element method enables an accurate representation of the vortical structures, with lower numerical dissipation than the more commonly used finite-volume codes. The wind-turbine blades are modeled as body forces using the actuator-line method (ACL) in the incompressible Navier-Stokes equations. Both tower and nacelle are represented with appropriate body forces. An inflow boundary condition is used which emulates homogeneous isotropic turbulence of wind-tunnel flows. We validate the implementation with results from experimental campaigns undertaken at the Norwegian University of Science and Technology (NTNU Blind Tests), investigate parametric influences and compare computational aspects with existing numerical simulations. In general the results show good agreement between the experiments and the numerical simulations both for a single-turbine setup as well as a two-turbine setup where the turbines are offset in the spanwise direction. A shift in the wake center caused by the tower wake is detected similar to experiments. The additional velocity deficit caused by the tower agrees well with the experimental data. The wake is captured well by Nek5000 in comparison with experiments both for the single wind turbine and in the two-turbine setup. The blade loading however shows large discrepancies for the high-turbulence, two-turbine case. While the experiments predicted higher thrust for the downstream turbine than for the upstream turbine, the opposite case was observed in Nek5000.

Place, publisher, year, edition, pages
Institute of Physics Publishing, 2017
Keywords
Incompressible flow, Navier Stokes equations, Numerical methods, Numerical models, Turbomachine blades, Turbulence, Vortex flow, Wakes, Wind turbines, Comparison with experiments, Homogeneous isotropic turbulence, Incompressible Navier Stokes equations, Inflow boundary conditions, Numerical dissipation, Reynolds-averaged navier-stokes simulations, Science and Technology, Spectral element method, Turbine components
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-216460 (URN)10.1088/1742-6596/854/1/012025 (DOI)000435276400025 ()2-s2.0-85023600314 (Scopus ID)
Conference
30 May 2017 through 1 June 2017
Note

QC 20171205

Available from: 2017-12-05 Created: 2017-12-05 Last updated: 2019-05-14Bibliographically approved
4. Blind test comparison on the wake behind a yawed wind turbine
Open this publication in new window or tab >>Blind test comparison on the wake behind a yawed wind turbine
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2018 (English)In: Wind Energy Science, ISSN 2213-3968, E-ISSN 2366-7443, Vol. 3, no 2, p. 883-903Article in journal (Refereed) Published
Abstract [en]

This article summarizes the results of the "Blind test 5" workshop, which was held in Visby, Sweden, in May 2017. This study compares the numerical predictions of the wake flow behind a model wind turbine operated in yaw to experimental wind tunnel results. Prior to the workshop, research groups were invited to predict the turbine performance and wake flow properties using computational fluid dynamics (CFD) methods. For this purpose, the power, thrust, and yaw moments for a 30 degrees yawed model turbine, as well as the wake's mean and turbulent streamwise and vertical flow components, were measured in the wind tunnel at the Norwegian University of Science and Technology (NTNU). In order to increase the complexity, a non-yawed downstream turbine was added in a second test case, while a third test case challenged the modelers with a new rotor and turbine geometry. Four participants submitted predictions using different flow solvers, three of which were based on large eddy simulations (LES) while another one used an improved delayed detached eddy simulation (IDDES) model. The performance of a single yawed turbine was fairly well predicted by all simulations, both in the first and third test cases. The scatter in the downstream turbine performance predictions in the second test case, however, was found to be significantly larger. The complex asymmetric shape of the mean streamwise and vertical velocities was generally well predicted by all the simulations for all test cases. The largest improvement with respect to previous blind tests is the good prediction of the levels of TKE in the wake, even for the complex case of yaw misalignment. These very promising results confirm the mature development stage of LES/DES simulations for wind turbine wake modeling, while competitive advantages might be obtained by faster computational methods.

Place, publisher, year, edition, pages
COPERNICUS GESELLSCHAFT MBH, 2018
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-239766 (URN)10.5194/wes-3-883-2018 (DOI)000450295200001 ()
Note

QC 20190109

Available from: 2019-01-09 Created: 2019-01-09 Last updated: 2019-05-14Bibliographically approved
5. Parametric dependencies of the yawed wind-turbine wake development
Open this publication in new window or tab >>Parametric dependencies of the yawed wind-turbine wake development
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-251414 (URN)
Note

QC 20190619

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-06-19Bibliographically approved
6. Near-wake structure of the yawed wind turbine
Open this publication in new window or tab >>Near-wake structure of the yawed wind turbine
2019 (English)Report (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-251413 (URN)
Note

QC 20190619

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-06-19Bibliographically approved
7. Yaw optimization potential of wind turbines in sheared flows.
Open this publication in new window or tab >>Yaw optimization potential of wind turbines in sheared flows.
2019 (English)Report (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-251412 (URN)
Note

QC 20190619

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-06-19Bibliographically approved
8. Tip-vortex breakdown of wind turbines subject to shear
Open this publication in new window or tab >>Tip-vortex breakdown of wind turbines subject to shear
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-251411 (URN)
Note

QC 20190619

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-06-19Bibliographically approved
9. Tip-vortex instabilities of two in-line wind turbines
Open this publication in new window or tab >>Tip-vortex instabilities of two in-line wind turbines
2019 (English)Conference proceedings (editor) (Refereed)
Place, publisher, year, edition, pages
Institute of Physics (IOP), 2019
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-251408 (URN)
Note

QC 20190619

Available from: 2019-05-14 Created: 2019-05-14 Last updated: 2019-06-19Bibliographically approved

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