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High-order numerical simulations of wind turbine wakes
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.ORCID iD: 0000-0003-2687-8148
Danish Technical University .
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
Uppsala University.
Show others and affiliations
2017 (English)Report (Other academic)
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 much lower numerical dissipation than the more commonly used finite volume codes. The 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 the 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, investigate parametric influences and compare computational aspects with the existing finite volume codes. The results show good agreement between the experiments and the numerical simulations.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , p. 17
Keywords [en]
wind turbine wakes, wake interaction, spectral elements, Nek5000
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-207628OAI: oai:DiVA.org:kth-207628DiVA, id: diva2:1097804
Funder
Swedish Energy Agency
Note

QC 20170523

Available from: 2017-05-22 Created: 2017-05-22 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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