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Actuator line simulations of a Joukowsky and Tjæreborg rotor using spectral element and finite volume methods
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.
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2016 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 753, no 8, article id 082011Article in journal (Refereed) Published
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. Vol. 753, no 8, article id 082011
Keywords [en]
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: urn:nbn:se:kth:diva-201777DOI: 10.1088/1742-6596/753/8/082011Scopus ID: 2-s2.0-84995394418OAI: oai:DiVA.org:kth-201777DiVA, id: diva2:1075305
Conference
5 October 2016 through 7 October 2016
Note

QC 20170217

Available from: 2017-02-17 Created: 2017-02-17 Last updated: 2017-05-23Bibliographically approved
In thesis
1. Wind turbine simulations using spectral elements
Open this publication in new window or tab >>Wind turbine simulations using spectral elements
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Understanding the flow around wind turbines is a highly relevant research question due to the increased interest in harvesting energy from renewable sources. This thesis approaches the topic by means of numerical simulations using the actuator line method and the incompressible Navier–Stokes equations in the spectral element code Nek5000. The aim is to gain enhanced understanding of the wind turbine wake structure and wind turbine wake interaction. A verification study of the method and implementation is performed against the finite volume solver EllipSys3D using two types of turbines, an idealized constant circulation turbine and the Tjæreborg turbine. It is shown that Nek5000 requires significantly lower resolution to accurately compute the wake development, however, at the cost of a smaller time step.The constant circulation turbine is investigated further with the goal of establishing guidelines for the use of the actuator line method in spectral element codes, where the mesh is inherently non-equidistant and currently used guidelines of force distribution based on Gaussian kernels are difficult to apply. It is shown that Nek5000 requires a larger kernel width in the fixed frame of reference to remove numerical instabilities. Further, the impact of different Gaussian widths on the wake development is investigated in the rotating frame of reference, showing that the convection velocity and the breakdown of the spiral tip and root vortices are dependent on the Gaussian width. In the second part, the flow around single and multiple wind-turbine setups at different operating conditions is investigated and compared with experimental results. The focus is placed on comparing the power and thrust coefficients and the wake development based on the time-averaged streamwise velocity and turbulent stresses. Further the influence of the tower model is investigated both upstream and downstream of the turbine. The results show that the wake is captured accurately in most cases. The loading exhibits a significant dependence on the Reynolds number at which the airfoil data is extracted. When the helical tip vortices are stable the turbulent stresses at the tip vortices are underestimated in the numerical simulations. This is due to the finite resolution and the projection of the actuator line forces in the numerical domain using a prescribed Gaussian width, which leads to lower induced velocities in the helical vortices.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. p. 32
Series
TRITA-MEK, ISSN 0348-467X ; 17/07
Keywords
wind turbine, wakes, wake interaction, computational fluid dynamics, actuator line method, spectral elements, free-stream turbulence
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-207630 (URN)978-91-7729-448-1 (ISBN)
Presentation
2017-06-07, E2, Lindstedtsvägen 3, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency
Note

QC 20170523

Available from: 2017-05-23 Created: 2017-05-22 Last updated: 2017-05-23Bibliographically approved

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