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  • 1. Breton, Simon-Philippe
    et al.
    Nilsson, Karl
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Olivares-Espinosa, Hugo
    Masson, Christian
    Dufresne, Louis
    Study of the effect of the presence of downstream turbines on upstream ones and use of a controller in CFD wind turbine simulation models2012Conference paper (Other academic)
  • 2. Breton, Simon-Philippe
    et al.
    Nilsson, Karl
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. Uppsala University Campus Gotland, Wind Energy, Sweden .
    Olivares-Espinosa, Hugo
    Masson, Christian
    Dufresne, Louis
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. Uppsala University Campus Gotland, Wind Energy, Sweden .
    Study of the influence of imposed turbulence on the asymptotic wake deficit in a very long line of wind turbines2014In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 70, 153-163 p.Article in journal (Refereed)
    Abstract [en]

    The influence of imposed turbulence on the development of the flow along a long row of wind turbines is studied, in search for an asymptotic wake deficit state. Calculations are performed using EllipSys3D, a CFD code that solves the Navier-Stokes equations in their incompressible form using a finite volume approach. In this code, the Large-Eddy Simulation technique is used for modeling turbulence, and the wind turbine rotors are represented as actuator discs whose loading is determined through the use of tabulated airfoil data by applying the blade-element method. Ten turbines are located along a row and separated from each other by seven rotor diameters, which is representative of the distance used in today's offshore wind farms. Turbulence is pre-generated with the Mann model, with imposed turbulence intensity levels of 4.5% and 8.9%. The aim with this study is to investigate features of the flow that depend solely on imposed turbulence and the presence of wind turbine rotors. For this reason, the turbines are isolated from their environment, and no effect from the presence of the atmospheric boundary layer is modeled, i.e., a non-sheared inflow is used. Analysis of the characteristics of the flow as a function of the position along the row of turbines is performed in terms of standard deviation of the velocity components, turbulence kinetic energy, mean velocity, and power spectra of the axial velocity fluctuations. The mean power production along the row of turbines is also used as an indicator. Calculations are performed below rated power, where a generator torque controller implemented in EllipSys3D renders it possible for the turbines to adapt to the flow conditions in which they operate. The results obtained for the standard deviation of the velocity components, turbulence kinetic energy, power and mean velocity as functions of downstream distance show that an asymptotic wake state seems close to be reached, in the conditions tested, near the end of the 10 turbine row. Significant changes towards this state are seen to happen faster when imposing turbulence in the domain. Power spectra of the axial velocity fluctuations are shown to provide interesting information about the turbulence in the flow, but are found not to be useful in determining if an asymptotic wake state is reached. (C) 2014 Elsevier Ltd. All rights reserved.

  • 3.
    Eriksson, O.
    et al.
    KTH. Uppsala University, Sweden .
    Mikkelsen, R.
    Hansen, K. S.
    Nilsson, K.
    KTH. Uppsala University, Sweden .
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. Uppsala University, Sweden .
    Analysis of long distance wakes of Horns Rev I using actuator disc approach2014In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 555, no 1, 012032Article in journal (Refereed)
    Abstract [en]

    The wake recovery behind the Horns Rev wind farm is analysed to investigate the applicability of Large Eddy Simulations (LES) in combination with an actuator disc method (ACD) for farm to farm interaction studies. Periodic boundary conditions on the lateral boundaries are used to model the wind farm (as infinitely wide), using only two columns of turbines. The meteorological conditions of the site are taken into account by introducing wind shear and pre-generated synthetic turbulence to the simulation domain using body forces. Simulations are carried out to study the power production and the velocity deficit in the farm wake. The results are compared to the actual power production as well as to wind measurements at 2 km and 6 km behind the wind farm. The simulated power production inside the farm shows an overall good correlation with the real production, but is slightly overpredicted in the most downstream rows. The simulations overpredict the wake recovery, namely the wind velocity, at long distances behind the farm. Further studies are needed before the presented method can be applied for the simulation of long distance wakes. Suggested parameters to be studied are the development of the turbulence downstream in the domain and the impact of the grid resolution.

  • 4. Eriksson, O.
    et al.
    Nilsson, K.
    KTH.
    Breton, S-P
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Large-eddy simulations of wind farm production and long distance wakes2015In: WAKE CONFERENCE 2015, 2015, Vol. 625, 012022Conference paper (Refereed)
    Abstract [en]

    The future development of offshore wind power will include many wind farms built in the same areas. It is known that wind farms produce long distance wakes, which means that we will see more occasions of farm to farm interaction, namely one wind farm operating in the wake of another wind farm. This study investigates how to perform accurate power predictions on large wind farms and how to assess the long distance wakes generated by these farms. The focus of this paper is the production's and wake's sensitivity to the extension of the grid as well as the turbulence when using Large-eddy simulations (LES) with pregenerated Mann turbulence. The aim is to determine an optimal grid which minimizes blockage effects and ensures constant resolution in the entire wake region at the lowest computational cost. The simulations are first performed in the absence of wind turbines in order to assess how the atmospheric turbulence and wind profile are evolving downstream (up to 12,000 m behind the position where the turbulence is imposed). In the second step, 10 turbines are added in the domain (using an actuator disc method) and their production is analyzed alongside the mean velocities in the domain. The blockage effects are tested using grids with different vertical extents. An equidistant region is used in order to ensure high resolution in the wake region. The importance of covering the entire wake structure inside the equidistant region is analyzed by decreasing the size of this region. In this step, the importance of the lateral size of the Mann turbulence box is also analyzed. In the results it can be seen that the flow is acceptably preserved through the empty domain if a larger turbulence box is used. The relative production is increased (due to blockage effects) for the last turbines using a smaller vertical domain, increased for a lower or narrower equidistant region (due to the smearing of the wake in the stretched area) and decreased when using a smaller turbulence box (due to decreased inmixing) The long distance wake behind the row is most impacted by the use of a smaller turbulence box, while the other simulation setups have less influence on these results. In summary, the results show the importance of having relatively large extensions of the domain, large extensions of the equidistant region and especially large extensions of the turbulence box.

  • 5.
    Ivanell, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. Uppsala University, Sweden.
    Leweke, T.
    Sarmast, S.
    Quaranta, H. U.
    Mikkelsen, R. F.
    Sorensen, J. N.
    Comparison between experiments and Large-Eddy Simulations of tip spiral structure and geometry2015In: WAKE CONFERENCE 2015, 2015, Vol. 625, 012018Conference paper (Refereed)
    Abstract [en]

    Results from Large-Eddy Simulations using the actuator line technique have been validated against experimental results. The experimental rotor wake, which forms the basis for the comparison, was studied in a recirculating free-surface water channel, where a helical vortex was generated by a single-bladed rotor mounted on a shaft. An investigation of how the experimental blade geometry and aerofoil characteristics affect the results was performed. Based on this, an adjustment of the pitch setting was introduced, which is still well within the limits of the experimental uncertainty. Excellent agreement between the experimental and the numerical results was achieved concerning the circulation, wake expansion and pitch of the helical tip vortex. A disagreement was found regarding the root vortex position and the axial velocity along the centre line of the tip vortex. This work establishes a good base for further studies of more fundamental stability parameters of helical rotor wakes.

  • 6.
    Ivanell, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Gotland University, Sweden.
    Mikkelsen, R.
    Sørensen, J. N.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stability analysis of the tip vortices of a wind turbine2008In: 46th AIAA Aerospace Sciences Meeting and Exhibit, 2008Conference paper (Refereed)
    Abstract [en]

    The aim of the present project is to get a better understanding of the stability properties of wakes generated by wind turbine rotors. To determine the stability properties of wind turbine wakes a numerical study on the stability of the tip vortices behind the Tjaereborg wind turbine has been carried out. The numerical model is based on large eddy simulations of the Navier-Stokes equations using the actuator line method to generate the wake and the tip vortices. To determine critical frequencies the flow is perturbed by inserting a harmonic perturbation. The results show that the instability is dispersive and that growth arise only for some specific frequencies and type of modes.

  • 7.
    Ivanell, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mikkelsen, Robert
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Actuator disc modelling of wake interaction in Horns Rev wind farmIn: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824Article in journal (Other academic)
    Abstract [en]

    Large eddy simulations of the Navier-Stokes equations are performed tosimulate the Horns Rev off shore wind farm 15 km outside the Danish westcoast. The aim is to achieve a better understanding of the wake interactioninside the farm. The simulations are performed by combining the in-housedeveloped computer code EllipSys3D with the actuator-disc methodology. Inthe actuator-disc method the blades are represented by a disc at which bodyforces representing the aerodynamic loading are introduced. The body forcesare determined by computing local angles of attack and tabulated aerofoil coefficients.The advantage of using the actuator-disc technique is that it is notnecessary to resolve blade boundary layers since the computational resourcesare devoted to simulating the dynamics of the flow structures.In the present study approximately 13.6 million mesh points are used toresolve the wake structure in the park. The park contains 80 wind turbinesdistributed over an area of about 20km2. Since it is not possible to simulate allturbines, the 2 central columns of turbines have been simulated with periodicboundary conditions. This corresponds to an infinitely wide farm with 10turbines in downstream direction. Simulations were performed within plusminus 15 degrees of the turbine alignment, making the wide farm approximationreasonable.The results from the CFD simulations are evaluated and the downstreamevolution of the velocity field is depicted. Special interest is given to whatextent the production is dependent on the inflow angle and turbulence level.The study shows that the applied method captures the main productionvariation within the wind farm. The result further demonstrates that levelsof production correlate well with measurements. However, in some cases thevariation of the measurement data is caused by variation of measurement conditionswith inflow angles.

  • 8.
    Ivanell, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mikkelsen, Robert
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Analysis of numerically generated wake structures2009In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 1, 63-80 p.Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations of the Navier-Stokes equations are performed to achieve a better understanding of the behaviour of wakes generated by wind turbines. The simulations are performed by combining the in-house developed computer code EllipSys3D with the actuator-line methodology. In the actuator-line method, the blades are represented by lines along which body forces representing the loading are introduced. The body forces are determined by computing local angles of attack and using tabulated aerofoil coefficients. The advantage of using the actuator-line technique is that it is not needed to resolve blade boundary layers and instead the computational resources are devoted to simulating the dynamics of the flow structures. In the present study, approximately 5 million mesh points are used to resolve the wake structure in a 120-degree domain behind the turbine. The results from the computational fluid dynamics (CFD) simulations are evaluated and the downstream evolution of the velocity field is depicted. Special interest is given to the structure and position of the tip vortices. Further, the circulation from the wake flow field is computed and compared to the distribution of circulation on the blades.

  • 9.
    Ivanell, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mikkelsen, Robert
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Stability analysis of the tip vortices of a wind turbine2010In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 13, no 8, 705-715 p.Article in journal (Refereed)
    Abstract [en]

     

    The aim of the present project is to obtain a better understanding of thestability properties of wakes generated by wind turbine rotors. To accomplishthis a numerical study on the stability of the tip vortices of the Tjaereborgwind turbine has been carried out. The numerical model is based on large eddysimulations of the Navier-Stokes equations using the actuator line method togenerate the wake and the tip vortices. To determine critical frequencies theflow is disturbed by inserting harmonic perturbations.The results show that the instability is dispersive and that growth arisesonly for some specific frequencies and type of modes. The study also providesevidence of a relationship between the turbulence intensity and the length ofthe near wake. The relationship however needs to be calibrated against measurements.

  • 10.
    Ivanell, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Mikkelsen, Robert
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Validation of methods using EllipSys3D2008Report (Other academic)
    Abstract [en]

    Numerical simulations of the Navier-Stokes equations are performed toachieve a better understanding of the behaviour of wakes generated by windturbines. The simulations are performed by combining the in-house developedcomputer code EllipSys3D with the actuator line and actuator disc methodologies.In the actuator methods the blades are represented by lines or a discat which body forces representing the loading are introduced. The body forcesare determined by computing local angles of attack and using tabulated aerofoilcoefficients. The advantage of using the actuator techniques is that theyare not needed to resolve blade boundary layers. Instead the computationalresources are devoted to simulating the dynamics of the flow structures. Inthe present study we run simulations using both the actuator line and theactuator disc methodologies. The influence from changing a number of parametersis evaluated. The study serves as a validation and sensitivity study toboth the actuator line and the actuator disc methods. The result shows thatboth methods produce realistic results. There is however a sensitivity to someparameters.

  • 11.
    Ivanell, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Mikkelsen, Robert
    Sørensen, J. N.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Three dimensional actuator disc modelling ofwind farm wake interaction2008In: European Wind Energy Conference and Exhibition, 2008, 1-10 p.Conference paper (Refereed)
    Abstract [en]

    Large eddy simulations of the Navier-Stokes equations are performed to achieve a better under-standing of the behaviour of wakes generated by wind turbines. The simulations are performed bycombining the in-house developed computer code EllipSys3D with the actuator-disc methodology.In the actuator-disc method the blades are represented by a disc at which body forces representingthe loading are introduced. The body forces are determined by computing local angles of attack andusing tabulated aerofoil coefficients. The advantage of using the actuator-disc technique is that itis not needed to resolve blade boundary layers and instead the computational resources are devotedto simulating the dynamics of the flow structures. In the present study approximately six millionsmesh points are used to resolve the wake structure in a park containing up to 9 turbines. Theresults from the CFD simulations are evaluated and the downstream evolution of the velocity fieldis depicted. Special interest is given to what extent the production is dependent of inflow angle.The result clearly indicates that turbulent inflow has a strong impact on the result and leads to amore complex flow structure.

  • 12.
    Ivanell, Stefan S. A.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Numerical computations of wind turbine wakes2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Numerical simulations of the Navier-Stokes equations are performed to achieve a better understanding of the behaviour of wakes generated by wind turbines. The simulations are performed by combining the in-house developed computer code EllipSys3D with the actuator line and disc methodologies. In the actuator line and disc methods the blades are represented by a line or a disc on which body forces representing the loading are introduced. The body forces are determined by computing local angles of attack and using tabulated aerofoil coefficients. The advantage of using the actuator disc technique is that it is not necessary to resolve blade boundary layers. Instead the computational resources are devoted to simulating the dynamics of the flow structures. In the present study both the actuator line and disc methods are used. Between approximately six to fourteen million mesh points are used to resolve the wake structure in a range from a single turbine wake to wake interaction in a farm containing 80 turbines. These 80 turbines are however represented by 20 actuator discs due to periodicity because of numerical limitations. In step one of this project the objective was to find a numerical method suitable to study both the flow structures in the wake behind a single wind turbine and to simulate complicated interaction between a number of turbines. The study resulted in an increased comprehension of basic flow features in the wake, but more importantly in the use of a numerical method very suitable for the upcoming purpose. The second objective of the project was to study the basic mechanisms controlling the length of the wake to obtain better understanding of the stability properties of wakes generated by wind turbine rotors. The numerical model was based on large eddy simulations of the Navier-Stokes equations using the actuator line method to generate the wake and the tip vortices. To determine critical frequencies the flow is disturbed by inserting a harmonic perturbation. The results showed that instability is dispersive and that growth occurs only for specific frequencies and mode types. The study also provides evidence of a relationship between the turbulence intensity and the length of the wake. The relationship however needs to be calibrated with measurements. In the last project objective, full wake interaction in large wind turbine farms was studied and verified to measurements. Large eddy simulations of the Navier-Stokes equations are performed to simulate the Horns Rev off-shore wind farm 15 km outside the Danish west coast. The aim is to achieve a better understanding of the wake interaction inside the farm. The simulations are performed by using the actuator disc methodology. Approximately 13.6 million mesh points are used to resolve the wake structure in the park containing 80 turbines. Since it is not possible to simulate all turbines, the 2 central columns of turbines have been simulated with periodic boundary conditions. This corresponds to an infinitely wide farm with 10 turbines in downstream direction. Simulations were performed within plus/minus 15 degrees of the turbine alignment. The infinitely wide farm approximation is thus reasonable. The results from the CFD simulations are evaluated and the downstream evolution of the velocity field is depicted. Special interest is given to what extent production is dependent on the inflow angle and turbulence level. The study shows that the applied method captures the main production variation within the wind farm. The result further demonstrates that levels of production correlate well with measurements. However, in some cases the variation of the measurement data is caused by the different measurement conditions during different inflow angles.

  • 13.
    Ivanell, Stefan S. A.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical computations of wind turbine wakes2005Licentiate thesis, monograph (Other scientific)
    Abstract [en]

    Numerical simulations using CFD methods are performed for wind turbine applications. The aim of the project is to get a better understanding of the wake behaviour, which is needed since today’s industrial design codes for wind power applications are based on the BEM (Blade Element Momentum) method. This method has been extended with a number of empirical corrections not based on physical flow features. The importance of accurate design models does also increase as the turbines become larger. Therefore, the research is today shifting toward a more fundamental approach, aiming at understanding basic aerodynamic mechanisms. The result from the CFD simulation is evaluated and special interest is given to the circulation and the position of vortices. From these evaluations, it will hopefully be possible to improve the engineering methods and base them, to a greater extent, on physical features instead of empirical corrections.

    The simulations are performed using the program ”EllipSys3D” developed at DTU (The Technical University of Denmark). The Actuator Line Method is used, where the blade is represented by a line instead of a large number of panels. The forces on that line are introduced by using tabulated aerodynamic coefficients. In this way, the computer resource is used more efficiently since the number of node points locally around the blade is decreased, and they can instead be concentrated in the wake behind the blades.

    An evaluation method to extract values of the circulation from the wake flow field is developed.

    The result shows agreement with classical theorems from Helmholtz, from which it follows that the wake tip vortex has the same circulation as the maximum value of the bound circulation on the blade.

  • 14.
    Ivanell, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Sørensen, J. N.
    Mikkelsen, Robert F.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical analysis of the tip and root vortex position in the wake of a wind turbine2007In: Science of Making Torque from Wind / [ed] Hansen, MOL; Hansen, KS, 2007, Vol. 75, 12035-12035 p.Conference paper (Refereed)
    Abstract [en]

    The stability of tip and root vortices are studied numerically in order to analyse the basic mechanism behind the break down of tip and root vortices. The simulations axe performed using the CFD program "EllipSys3D". In the computations the so-called Actuator Line Method is used, where the blades are represented by lines of body forces representing the loading. The forces on the lines are implemented using tabulated aerodynamic aerofoil data. In this way, computer resources are used more efficiently since the number of mesh points locally around the blade is decreased, and they are instead concentrated in the wake behind the blades. We here present results of computed flow fields and evaluate the flow behaviour in the wake. In particular we compare the position of the root vortices as to the azimuthal position of the tip votices.

  • 15.
    Ivanell, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Sørensen, Jens N.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Numerical computations of wind turbine wakes2007In: Wind Energy / [ed] Peinke, J; Schaumann, P; Barth, S, 2007, 259-263 p.Conference paper (Refereed)
  • 16.
    Kleusberg, Elektra
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Mikkelsen, R. F.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    High-Order Numerical Simulations of Wind Turbine Wakes2017In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 854, no 1, 012025Article in journal (Refereed)
    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.

  • 17.
    Kleusberg, Elektra
    et al.
    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.
    Sarmast, S.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Henningson, Dan Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Actuator line simulations of a Joukowsky and Tjæreborg rotor using spectral element and finite volume methods2016In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 753, no 8, 082011Article in journal (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.

  • 18.
    Nilsson, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Breton, Simon-Philippe
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Evaluation of the effects of using a power controller in LES/ACD simulations2015Report (Other academic)
  • 19.
    Nilsson, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Breton, Simon-Philippe
    Sørensen, Jens
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Airfoil data sensitivity analysis for actuator disc simulations used in wind turbine applications2014In: SCIENCE OF MAKING TORQUE FROM WIND 2014 (TORQUE 2014), 2014, Vol. 524, 012135- p.Conference paper (Refereed)
    Abstract [en]

    To analyse the sensitivity of blade geometry and airfoil characteristics on the prediction of performance characteristics of wind farms, large-eddy simulations using an actuator disc (ACD) method are performed for three different blade/airfoil configurations. The aim of the study is to determine how the mean characteristics of wake flow, mean power production and thrust depend on the choice of airfoil data and blade geometry. In order to simulate realistic conditions, pre-generated turbulence and wind shear are imposed in the computational domain. Using three different turbulence intensities and varying the spacing between the turbines, the flow around 4-8 aligned turbines is simulated. The analysis is based on normalized mean streamwise velocity, turbulence intensity, relative mean power production and thrust. From the computations it can be concluded that the actual airfoil characteristics and blade geometry only are of importance at very low inflow turbulence. At realistic turbulence conditions for an atmospheric boundary layer the specific blade characteristics play an minor role on power performance and the resulting wake characteristics. The results therefore give a hint that the choice of airfoil data in ACD simulations is not crucial if the intention of the simulations is to compute mean wake characteristics using a turbulent inflow.

  • 20.
    Nilsson, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Eriksson, Ola
    Svensson, Nina
    Breton, Simon-Philippe
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Large-eddy simulations of the evolution of imposed turbulence in prescribed boundary layers in a very long domainManuscript (preprint) (Other academic)
  • 21.
    Nilsson, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Uppsala University Campus Gotland, Sweden.
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Uppsala University Campus Gotland, Sweden.
    Hansen, Kurt S.
    Mikkelsen, Robert
    Sørensen, Jens N.
    Breton, Simon-Philippe
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Large-eddy simulations of the Lillgrund wind farm2015In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 18, no 3, 449-467 p.Article in journal (Refereed)
    Abstract [en]

    The power production of the Lillgrund wind farm is determined numerically using large-eddy simulations and compared with measurements. In order to simulate realistic atmospheric conditions, pre-generated turbulence and wind shear are imposed in the computational domain. The atmospheric conditions are determined from data extracted from a met mast, which was erected prior to the establishment of the farm. In order to allocate most of the computational power to the simulations of the wake flow, the turbines are modeled using an actuator disc method where the discs are imposed in the computational domain as body forces which for every time step are calculated from tabulated airfoil data. A study of the influence of imposed upstream ambient turbulence is performed and shows that higher levels of turbulence results in slightly increased total power production and that it is of great importance to include ambient turbulence in the simulations. By introducing ambient atmospheric turbulence, the simulations compare very well with measurements at the studied inflow angles. A final study aiming at increasing the farm production by curtailing the power output of the front row turbines and thus letting more kinetic energy pass downstream is performed. The results, however, show that manipulating only the front row turbines has no positive effect on the farm production, and therefore, more complex curtailment strategies are needed to be tested.

  • 22. Nilsson, Karl
    et al.
    Shen, W. Z.
    Sørensen, J. N.
    Breton, S. -P
    Ivanell, Stefan
    Erratum: Validation of the actuator line method using near wake measurements of the Mexico rotor (Wind Energy (2015) 18 (499-514))2015In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 18, no 9Article in journal (Refereed)
  • 23.
    Nilsson, Karl
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. Uppsala University Campus Gotland, Sweden .
    Shen, Wen Z.
    Sørensen, Jens N.
    Breton, Simon-Philippe
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. Uppsala University Campus Gotland, Sweden.
    Validation of the actuator line method using near wake measurements of the MEXICO rotor2015In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 18, no 3, 499-514 p.Article in journal (Refereed)
    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.

  • 24. Sarmast, S.
    et al.
    Shen, W. Z.
    Zhu, W. J.
    Mikkelsen, R. F.
    Breton, S. P.
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. Uppsala University, Wind Energy Section.
    Validation of the actuator line and disc techniques using the New Mexico measurements2016In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 753, no 3, 032026Article in journal (Refereed)
    Abstract [en]

    Actuator line and disc techniques are employed to analyse the wake obtained in the New Mexico wind turbine experiment. The New Mexico measurement campaign done in 2014 is a follow-up to the MEXICO campaign, which was completed in 2006. Three flow configurations in axial flow condition are simulated and both computed loads and velocity fields around the rotor are compared with detailed PIV measurements. The comparisons show that the computed loadings are generally in agreement with the measurements under the rotor's design condition. Both actuator approaches under-predicted the loading in the inboard part of blade in stall condition as only 2D airfoil data were used in the simulations. The predicted wake velocities generally agree well with the PIV measurements. In the experiment, PIV measurements are also provided close to the hub and nacelle. To study the effect of hub and nacelle, numerical simulations are performed both in the presence and absence of the hub geometry. This study shows that the large hub used in the experiment has only small effects on overall wake behaviour.

  • 25.
    Sarmast, Sasan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. DTU Wind Energy, Denmark Technical University, Denmark .
    Chivaee, Hamid Sarlak
    Ivanell, Stefan
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Wind Energy Campus Gotland, Department of Earth Sciences, Uppsala University, Sweden.
    Mikkelsen, Robert F.
    Comparison of the near-wake between a simplied vortex model and actuator line simulations of a horizontal-axis wind turbineManuscript (preprint) (Other academic)
  • 26.
    Sarmast, Sasan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. DTU Wind Energy, Denmark Technical University, Denmark .
    Chivaee, Hamid Sarlak
    Ivanell, Stefan
    KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Wind Energy Campus Gotland, Department of Earth Sciences, Uppsala University, Sweden.
    Mikkelsen, Robert F.
    Numerical investigation of the wake interaction between two model wind turbines with span-wise offset2014In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 524, no 1Article in journal (Refereed)
    Abstract [en]

    Wake interaction between two model scale wind turbines with span-wise offset is investigated numerically using Large Eddy Simulation (LES) and the results are validated against the experimental data. An actuator line technique is used for modeling the rotor. The investigated setup refers to a series of experimental measurements of two model scale turbines conducted by NTNU in low speed wind tunnel in which the two wind turbines are aligned with a span-wise offset resulting in half wake interaction. Two levels of free-stream turbulence are tested, the minimum undisturbed level of about Ti 0.23% and a high level of about Ti = 10% using a passive upstream grid. The results show that the rotor characteristics for both rotors are well captured numerically even if the downstream rotor operates into stall regimes. There are however some difficulties in correct prediction of the thrust level. The interacting wake development is captured in great details in terms of wake deficit and streamwise turbulence kinetic energy. The present work is done in connection with Blind test 3 workshops organized jointly by NOWITECH and NORCOWE.

  • 27.
    Sarmast, Sasan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Tech Univ Denmark.
    Dadfar, Reza
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Mikkelsen, R. F.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala Univ, Sweden.
    Sorensen, Jens N.
    Henningson, Dans S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Mutual inductance instability of the tip vortices behind a wind turbine2014In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 755, 705-731 p.Article in journal (Refereed)
    Abstract [en]

    Two modal decomposition techniques are employed to analyse the stability of wind turbine wakes. A numerical study on a single wind turbine wake is carried out focusing on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations using the actuator line (ACL) method to simulate the wake behind the Tj ae reborg wind turbine. The wake is perturbed by low-amplitude excitation sources located in the neighbourhood of the tip spirals. The amplification of the waves travelling along the spiral triggers instabilities, leading to breakdown of the wake. Based on the grid configurations and the type of excitations, two basic flow cases, symmetric and asymmetric, are identified. In the symmetric setup, we impose a 120 degrees symmetry condition in the dynamics of the flow and in the asymmetric setup we calculate the full 360 degrees wake. Different cases are subsequently analysed using dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD). The results reveal that the main instability mechanism is dispersive and that the modal growth in the symmetric setup arises only for some specific frequencies and spatial structures, e.g. two dominant groups of modes with positive growth (spatial structures) are identified, while breaking the symmetry reveals that almost all the modes have positive growth rate. In both setups, the most unstable modes have a non-dimensional spatial growth rate close to pi/2 and they are characterized by an out-of-phase displacement of successive helix turns leading to local vortex pairing. The present results indicate that the asymmetric case is crucial to study, as the stability characteristics of the flow change significantly compared to the symmetric configurations. Based on the constant non-dimensional growth rate of disturbances, we derive a new analytical relationship between the length of the wake up to the turbulent breakdown and the operating conditions of a wind turbine.

  • 28.
    Sarmast, Sasan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Dadfar, Reza
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mikkelsen, Robert F.
    DTU wind energy.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Ivanell, Stehan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Henningson, Dans S.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Instability of the tip vortices behind a wind turbine: a wind turbineManuscript (preprint) (Other academic)
  • 29.
    Sarmast, Sasan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Ivanell, Stehan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mikkelsen, Robert F.
    DTU wind energy.
    Henningson, Dans S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Instability of the helical tip vortices behind: a wind turbineArticle in journal (Other academic)
    Abstract [en]

    A numerical study on a single wind turbine wake has been carried out focusing on the instability onset of the trailed tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations of the Navier– Stokes equations using the actuator line method to simulate the wake and the tip vortices, using the EllipSys3D general purpose 3D Navier–Stokes solver. Data from the Tjæreborg wind turbine is used in the analysis. Dynamic mode decomposition (DMD) is utilized for analysis of the wind turbine near wake. This method allows for extraction of dominant coherent structures from the flow, leading to an improved understanding of the flow physics and underlying instability mechanisms.

  • 30.
    Sarmast, Sasan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. Technical University of Denmark, Denmark.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Mikkelsen, Robert F.
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala University, Sweden.
    Comparison of the near-wake between actuator-line simulations and a simplified vortex model of a horizontal-axis wind turbine2016In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 19, no 3, 471-481 p.Article in journal (Refereed)
    Abstract [en]

    The flow around an isolated horizontal-axis wind turbine is estimated by means of a new vortex code based on the Biot-Savart law with constant circulation along the blades. The results have been compared with numerical simulations where the wind turbine blades are replaced with actuator lines. Two different wind turbines have been simulated: one with constant circulation along the blades, to replicate the vortex method approximations, and the other with a realistic circulation distribution, to compare the outcomes of the vortex model with real operative wind-turbine conditions (Tjaereborg wind turbine). The vortex model matched the numerical simulation of the turbine with constant blade circulation in terms of the near-wake structure and local forces along the blade. The results from the Tjaereborg turbine case showed some discrepancies between the two approaches, but overall, the agreement is qualitatively good, validating the analytical method for more general conditions. The present results show that a simple vortex code is able to provide an estimation of the flow around the wind turbine similar to the actuator-line approach but with a negligible computational effort.

  • 31.
    Sarmast, Sasan
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mikkelsen, Robert F.
    DTU wind energy.
    Ivanell, Stehan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    A comparison between a simplified vortex model and actuator line simulations of a horizontal axis wind turbineManuscript (preprint) (Other academic)
  • 32.
    Sorensen, Jens N.
    et al.
    Uppsala University.
    Mikkelsen, Robert F.
    DTU Wind Energy.
    Henningson, Dan S.
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Ivanell, Stehan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre. Uppsala University.
    Sarmast, Sasan
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Andersen, Soren J.
    DTU Wind Energy.
    Simulation of wind turbine wakes using the actuator line technique2015In: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, ISSN 1364-503X, Vol. 373, 2035- p.Article in journal (Refereed)
    Abstract [en]

    The actuator line technique was introduced as a numerical tool to be employed in combination with large eddy simulations to enable the study of wakes and wake interaction in wind farms. The technique is today largely used for studying basic features of wakes as well as for making performance predictions of wind farms. In this paper, we give a short introduction to the wake problem and the actuator line methodology and present a study in which the technique is employed to determine the near-wake properties of wind turbines. The presented results include a comparison of experimental results of the wake characteristics of the flow around a three-bladed model wind turbine, the development of a simple analytical formula for determining the near-wake length behind a wind turbine and a detailed investigation of wake structures based on proper orthogonal decomposition analysis of numerically generated snapshots of the wake.

  • 33. Sørensen, J. N.
    et al.
    Mikkelsen, R.
    Sarmast, Sasan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Ivanell, Stefan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. Uppsala University.
    Henningson, Dan
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Determination of wind turbine near-wake length based on stability analysis2014In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 524, no 1, 012155Article in journal (Refereed)
    Abstract [en]

    A numerical study on the wake behind a wind turbine is carried out focusing on determining the length of the near-wake based on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations using the actuator line (ACL) method. The wake is perturbed by applying stochastic or harmonic excitations in the neighborhood of the tips of the blades. The flow field is then analyzed to obtain the stability properties of the tip vortices in the wake of the wind turbine. As a main outcome of the study it is found that the amplification of specific waves (traveling structures) along the tip vortex spirals is responsible for triggering the instability leading to wake breakdown. The presence of unstable modes in the wake is related to the mutual inductance (vortex pairing) instability where there is an out-of-phase displacement of successive helix turns. Furthermore, using the non-dimensional growth rate, it is found that the pairing instability has a universal growth rate equal to π/2. Using this relationship, and the assumption that breakdown to turbulence occurs once a vortex has experienced sufficient growth, we provide an analytical relationship between the turbulence intensity and the stable wake length. The analysis leads to a simple expression for determining the length of the near wake. This expression shows that the near wake length is inversely proportional to thrust, tip speed ratio and the logarithmic of the turbulence intensity.

1 - 33 of 33
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