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Validation of the actuator line method using near wake measurements of the MEXICO rotor
KTH, School of Engineering Sciences (SCI), Mechanics. Uppsala University Campus Gotland, Sweden .
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2015 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 18, no 3, 499-514 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of the present work is to validate the capability of the actuator line method to compute vortex structures in the near wake behind the MEXICO experimental wind turbine rotor. In the MEXICO project/MexNext Annex, particle image velocimetry measurements have made it possible to determine the exact position of each tip vortex core in a plane parallel to the flow direction. Determining center positions of the vortex cores makes it possible to determine the trajectory of the tip vortices, and thus the wake expansion in space, for the analyzed tip speed ratios. The corresponding cases, in terms of tip speed ratios, have been simulated by large-eddy simulations using a Navier - Stokes code combined with the actuator line method. The flow field is analyzed in terms of wake expansion, vortex core radius, circulation and axial and radial velocity distributions. Generally, the actuator line method generates significantly larger vortex cores than in the experimental cases, but predicts the expansion, the circulation and the velocity distributions with satisfying results. Additionally, the simulation and experimental data are used to test three different techniques to compute the average axial induction in the wake flow. These techniques are based on the helical pitch of the tip vortex structure, 1D momentum theory and wake expansion combined with mass conservation. The results from the different methods vary quite much, especially at high values of λ.

Place, publisher, year, edition, pages
John Wiley & Sons, 2015. Vol. 18, no 3, 499-514 p.
Keyword [en]
ACL method, CFD, EllipSys3D, MEXICO rotor, MexNext
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-109520DOI: 10.1002/we.1714ISI: 000348899700008Scopus ID: 2-s2.0-84921881756OAI: oai:DiVA.org:kth-109520DiVA: diva2:583070
Note

QC 20150318. Updated from submitted to published.

Correction in:Wind Energ. 2015; 18:1683 DOI: 10.1002/we.1864, WOS:000358730800012

Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Numerical computations of wind turbine wakes and wake interaction: Optimization and control
Open this publication in new window or tab >>Numerical computations of wind turbine wakes and wake interaction: Optimization and control
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In the present thesis the wake flow behind wind turbines is analyzed numerically using large-eddy simulations. The wind turbine rotors are modeled by using either the actuator disc method or the actuator line method in which the blades are represented by body forces computed with airfoil data. Using these models, the boundary layers of the turbine blades are not resolved and most of the computational power is preserved to simulate the wake flow. The actuator disc method is used for the wake interaction studies of the Lillgrund wind farm. In this study the power production is simulated for two different wind directions and compared to measurements. A grid sensitivity study and a turbulence intensity study are also performed. As a last step the front row turbines are derated in an attempt to increase the total production of the farm. The results show that it is important to impose atmospheric conditions at the inlet in the simulations, otherwise production will be unrealistically low for some turbines in the farm. The agreement between the simulated and measured power is very good. The study based on derating the front row turbines does not show any positive increase on the farm production. The actuator line method is used for near wake analysis of the MEXICO rotor. In this study the near wake is simulated for five different flow cases and compared with particle image velocimetry (PIV) measurements. The analysis is performed by comparing size and circulation of the tip vortices, the radial and streamwise velocity distributions, the spatial expansion of the wake and the axial induction factor. The simulations and measurements generally are in agreement. In some cases, however, the measurements are affected by tunnel effects which are not captured in the simulations. In connection to the actuator disc method a power control strategy for operating conditions below rated power is implemented and tested. The strategy is first validated using an in-house developed blade element momentum code and then is implemented in the actuator disc method used in the EllipSys3D code. The initial tests show that the strategy responds as expected when changing the moment of inertia of the rotor and when varying the inlet conditions. Results from the implementation of the strategy in the actuator disc method in EllipSys3D show that the turbine adapts to the conditions it is operating in by changing its rotational velocity and power output when the inlet conditions are varied.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. vi, 56 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:18
Keyword
Actuator disc method, Actuator line method, Blade element momentum method, Wind turbine wake, Wake interaction, CFD, LES, EllipSys3D
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-109522 (URN)978-91-7501-624-5 (ISBN)
Presentation
2013-01-25, D33, Lindstedtsvägen 5, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20130111

Available from: 2013-01-11 Created: 2013-01-07 Last updated: 2013-01-11Bibliographically approved
2. Numerical computations of wind turbine wakes and wake interaction
Open this publication in new window or tab >>Numerical computations of wind turbine wakes and wake interaction
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

When wind turbines are placed in farms, wake effects reduce the overall power production. Also, turbine loads are significantly increased since turbulence levels are high within the wake flow. Therefore, when planning for a wind farm, it is imperative to have an understanding of the flow conditions in the farm in order to estimate the power losses and to optimize the durability of the turbines to be selected for the farm. The possibilities granted by numerical modeling and the development of computational capabilities give an opportunity to study these flow conditions in detail, which has been the key focus of this doctoral work.

The actuator disc method is used for predicting the power production of the Lillgrund wind farm. The results of the simulations are compared to measurements from the actual wind farm, which are found to be in very good agreement. However, some uncertainties are identified in the modeling of the turbine. One of the uncertainties is that a generic rotor is used in the Lillgrund case. In order to quantify the errors resulting from this generalization three different rotor configurations are simulated in various flow conditions. Generally, it can be stated that the choice of rotor configuration is not crucial if the intention of the simulations is to compute the mean wake characteristics subject to turbulent inflow. Another uncertainty is that the turbines in the Lillgrund case were simulated without a power controller. Therefore, a power controller is implemented and used in simulations. Generally, the controller reduces the thrust of the turbines, reduces turbulence intensity and increases velocity levels in the wake flow. However, the use of a controller was observed to have a small impact on the power production.

The effects of using the technique of imposing pregenerated turbulence and a prescribed boundary layer in the simulation are analyzed in order to verify its applicability in very long domains. It is observed that close to the plane of imposed turbulence, the conditions are mainly dependent on the imposed turbulence while far downstream the turbulence, regardless of its initial characteristics, is in near equilibrium with the prescribed wind shear.

The actuator line method is validated using measurements of the near wake behind the MEXICO rotor. The analysis is performed by comparing position, size and circulation of the tip vortices, as well as velocity distributions in the wake flow. The simulations and measurements are generally found to be in good agreement apart from the tip vortex size, which is greatly overestimated in the simulations.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. ix, 40 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2015:04
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-166658 (URN)978-91-7595-558-2 (ISBN)
Public defence
2015-06-04, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20150519

Available from: 2015-05-19 Created: 2015-05-13 Last updated: 2015-05-19Bibliographically approved

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