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Parametric study of the actuator line method in high-order codes
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.ORCID iD: 0000-0003-2687-8148
KTH, School of Engineering Sciences (SCI), Mechanics, Turbulence.
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
2017 (English)Report (Other academic)
Abstract [en]

The high accuracy of spectral element methods combined with low computationalcost and a high level of parallelization, makes them appealing for large-scale investigations of wind turbines and wind turbine interaction using the state-of-the-art actuator line method by Sørensen & Shen (2002). While the spectral element code Nek5000 has already been used for wind turbine simulations by e.g. Peet et al. (2013), Chatterjee & Peet (2015), Chatterjee & Peet (2016) and Kleusberg et al. (2016) for investigations of wind turbine wakes and wake interaction, the implications of the actuator line method in a high-order code and the effect of the involved parameters have not been studied in detail. This paper investigates the constant circulation turbine in the fixed and rotatingframe of reference. In the rotating frame of reference several wake parameters previously discussed e.g. by Ivanell et al. (2009) and Sarmast (2013) are revisitedand analyzed. Further, parametric studies are conducted in the fixed frame ofreference to investigate an observed instablility related to the spectral element width. The instablity is not a property of the spectral element discretization as it is also observed in other research using finite volume techniques. However, the decreased numerical dissipation and the non-equidistant grid used in spectral element methods leads to amplification of the instability. The parameters are investigated on a reduced two-dimensional test case and the conclusions transfered to the full actuator line setup. It is established that a Gaussian width of approximately five times the average grid spacing is necessary to reduce the effect of the instability related to the spectral element width when investigating sensitive flow cases. A force projection method proposed by Pinelli et al. (2010) is investigated as an alternative to the typically used Gaussian kernel. Finally, the influence of this instability is investigated when perturbations are applied tothe flow. Both small-scale perturbations that are introduced at the blade tips and low inflow turbulence which is imposed as an inlet condition are investigated.It is shown that when perturbations are introduced to the flow the large-scale wake behavior in the rotating and fixed frame of reference are similar and a Gaussian width which is 2.4 times the averaged grid spacing is sufficient.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , p. 35
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-207629OAI: oai:DiVA.org:kth-207629DiVA, id: diva2:1097805
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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