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Stefan Ivanell – One of the best experts on this subject based on the ideXlab platform.

  • Analytical body forces in numerical Actuator Disc model of wind turbines
    Renewable Energy, 2020
    Co-Authors: Jens Nørkær Sørensen, Karl Nilsson, Stefan Ivanell, Henrik Asmuth, Robert Flemming Mikkelsen
    Abstract:

    Abstract An analytical model for representing body forces in numerical Actuator Disc models of wind turbturbines is developed and validated. The model is based on the assumption that the rotor Disc is subject to a constant circulation modified for tip and root effects. The model comprises expressions for both the axial and the azimuthal force distributions, and is generalized to be utilized for all kinds of inflow, including wind shear, turbulence, and shadow effects in wind farms. The advantage of the model is that it does not depend on any detailed knowledge concerning the wind turbturbine being analysed, but only requires knowledge regarding the rated wind speed and nameplate capacity. To validate the analytical model, results are compared to numerically generated results using detailed information regarding geometry and airfoil data for the 2 MW Tjaereborg wind turbturbine and the 10 MW DTU reference turbine. The comparisons show very good agreement between the loadings using the new analytical model and the airfoil data based method for the two tested wind turbturbines, demonstrating that the analytical model is a simple and reliable way of introducing body forces in Actuator Disc simulations without any prior knowledge of the wind turbturbine being analysed.

  • Wind Farm Power Production Assessment : Introduction of a New Actuator Disc Method and Comparison with Existing Models in the Context of a Case Study
    Applied Sciences, 2019
    Co-Authors: Nikolaos Simisiroglou, Heracles Polatidis, Stefan Ivanell
    Abstract:

    The aim of the present study is to perform a comparative analysis of two Actuator Disc methods (ACD) and two analytical wake models for wind farm power production assessment. To do so, wind turbturbine

  • Wind farm power production assessment : a comparative analysis of two Actuator Disc methods and two analytical wake models
    , 2018
    Co-Authors: Nikolaos Simisiroglou, Heracles Polatidis, Stefan Ivanell
    Abstract:

    Abstract. The aim of the present study is to perform a comparative analysis of two Actuator Disc methods (ACD) and two analytical wake models for wind farm power production assessment. To do so wind turbturbine power production data from the Lillgrund offshore wind farm in Sweden is used. The measured power production for individual wind turbturbines is compared with results from simulations, done in the WindSim software, using two ACD methods (old and new) and two analytical wake models widely used within the wind industry (Jensen and Larsen wake models). It was found that the new ACD method and the Larsen model outperform the other method and model in most cases. Furthermore, results from the new ACD method show a clear improvement in the estimated power production in comparison to the old ACD method. The Jensen method seems to overestimate the power deficit for all cases. The new ACD method, despite it’s simplicity, is capable of capturing the power production within the given error margin although it tends to underestimate the power deficit.

Jens Nørkær Sørensen – One of the best experts on this subject based on the ideXlab platform.

  • Analytical body forces in numerical Actuator Disc model of wind turbines
    Renewable Energy, 2020
    Co-Authors: Jens Nørkær Sørensen, Karl Nilsson, Stefan Ivanell, Henrik Asmuth, Robert Flemming Mikkelsen
    Abstract:

    Abstract An analytical model for representing body forces in numerical Actuator Disc models of wind turbines is developed and validated. The model is based on the assumption that the rotor Disc is subject to a constant circulation modified for tip and root effects. The model comprises expressions for both the axial and the azimuthal force distributions, and is generalized to be utilized for all kinds of inflow, including wind shear, turbulence, and shadow effects in wind farms. The advantage of the model is that it does not depend on any detailed knowledge concerning the wind turbine being analysed, but only requires knowledge regarding the rated wind speed and nameplate capacity. To validate the analytical model, results are compared to numerically generated results using detailed information regarding geometry and airfoil data for the 2 MW Tjaereborg wind turbine and the 10 MW DTU reference turbine. The comparisons show very good agreement between the loadings using the new analytical model and the airfoil data based method for the two tested wind turbines, demonstrating that the analytical model is a simple and reliable way of introducing body forces in Actuator Disc simulations without any prior knowledge of the wind turbine being analysed.

  • The General Momentum Theory
    Research Topics in Wind Energy, 2015
    Co-Authors: Jens Nørkær Sørensen
    Abstract:

    In the axial momentum theory presented in the previous chapter, the rotational flow was ignored and the rotor was replaced by a pressure jump represented by an Actuator Disc. To develop further the momentum theory, the Actuator Disc equations are modified by introducing rotational velocities to the flow.

  • Comparison between PIV measurements and computations of the near-wake of an Actuator Disc
    Journal of Physics: Conference Series, 2014
    Co-Authors: Søren Juhl Andersen, Robert Flemming Mikkelsen, Jens Nørkær Sørensen, L. E. M. Lignarolo, Daniele Ragni, C.j. Simao Ferreira, G.j.w. Van Bussel
    Abstract:

    Experimental stereoscopic PIV measurements in the wake of a two-bladed rotor and a porous Actuator Disc are compared to numerical simulation of an Actuator Disc. Compared to previous literature, the focus of the present analysis is on the near wake, where the Actuator Discs fail to represent the complex flow structures correctly, which affects the downstream representation of the full wake behind a real rotor. The near wake region is characterised by the instability and breakdown of the tip-vortex helical system, which constitutes the onset of a stronger mixing process. The comparison focuses on the turbulent structures in the shear layer at the borders of the wake through the analysis of the Reynolds stresses and by employing POD on two separate regions. The analysis shows that the Actuator Discs fail to capture the details of the complex flow behind a rotor, but that the experimental and numerical Actuator Discs are generally comparable at a certain distance behind the Actuator Disc. This project is intended to provide the basis for understanding the origin of the limitations of the current wake models based on the Actuator Disc assumption.

G.a.m. Van Kuik – One of the best experts on this subject based on the ideXlab platform.

  • New dynamic‐inflow engineering models based on linear and nonlinear Actuator Disc vortex models
    Wind Energy, 2019
    Co-Authors: Delphine De Tavernier, Carlos Simao Ferreira, G.a.m. Van Kuik, G. Schepers
    Abstract:

    Two new engineering models are presented for the aerodynamic induction of a wind turbturbine underdynamicthrust.ThemodelsaredevelopedusingthedifferentialformofDuhamelintegrals of indicial responses of Actuator Disc type vortex models. The time constants of the indicial functions are obtained by the indicial responses of a linearand a nonlinearActuatorDisc model. The new dynamic-inflow engineering models are verified against the results of a Computational Fluid Dynamics (CFD) model and compared against the dynamic-inflow engineering models of Pitt-Peters, Oye, and Energy Research Center of the Netherlands (ECN), for several load cases. Comparisons of all models show that two time constants are necessary to predict the dynamic induction. The amplitude and phase delay of the velocity distribution shows a strong radial dependency. Verifying the models against results from the CFD model shows that the model based on the linearActuatorDisc vortex modelpredicts a similarperformance as the Oye model. The model based on the nonlinear Actuator Disc vortex model predicts the dynamic induction betterthantheothermodelsconcerningbothphasedelayandamplitude,especiallyathighload

  • new dynamic inflow engineering models based on linear and nonlinear Actuator Disc vortex models
    Wind Energy, 2019
    Co-Authors: Delphine De Tavernier, Carlos Simao Ferreira, G.a.m. Van Kuik, G. Schepers
    Abstract:

    Two new engineering models are presented for the aerodynamic induction of a wind turbturbine underdynamicthrust.ThemodelsaredevelopedusingthedifferentialformofDuhamelintegrals of indicial responses of Actuator Disc type vortex models. The time constants of the indicial functions are obtained by the indicial responses of a linearand a nonlinearActuatorDisc model. The new dynamic-inflow engineering models are verified against the results of a Computational Fluid Dynamics (CFD) model and compared against the dynamic-inflow engineering models of Pitt-Peters, Oye, and Energy Research Center of the Netherlands (ECN), for several load cases. Comparisons of all models show that two time constants are necessary to predict the dynamic induction. The amplitude and phase delay of the velocity distribution shows a strong radial dependency. Verifying the models against results from the CFD model shows that the model based on the linearActuatorDisc vortex modelpredicts a similarperformance as the Oye model. The model based on the nonlinear Actuator Disc vortex model predicts the dynamic induction betterthantheothermodelsconcerningbothphasedelayandamplitude,especiallyathighload

  • The dynamic wake of an Actuator Disc undergoing transient load: A numerical and experimental study
    Renewable Energy, 2019
    Co-Authors: Carlos Simao Ferreira, G.a.m. Van Kuik
    Abstract:

    Abstract The currently most used theory for rotor aerodynamics — Blade Element Momentum is based on the assumption of stationary wake conditions. However, an unsteady rotor loading results in an unsteady wake flow field. This work aims to study the impact of an unsteady Actuator Disc on the wake flow field using a free wake vortex ring model. The numerical results are compared to a wind tunnel measurement, where the wake flow of an Actuator Disc model undergoing transient load was obtained. The numerical results complement the experimental work while providing information such as the vorticity field and contributions from different vortex elements. The velocity at different locations is compared between the experimental and numerical results. The observed velocity peaks in the experimental results are also observed in the numerical results. A steeper ramp time results in a steeper velocity transient slope, and in turn in a larger amplitude of peak values. It is revealed that the rolling-up processes is the main cause for the velocity difference at various locations and in the three cases by decomposing velocity induced by different vortex element.

Robert Flemming Mikkelsen – One of the best experts on this subject based on the ideXlab platform.

  • Analytical body forces in numerical Actuator Disc model of wind turbines
    Renewable Energy, 2020
    Co-Authors: Jens Nørkær Sørensen, Karl Nilsson, Stefan Ivanell, Henrik Asmuth, Robert Flemming Mikkelsen
    Abstract:

    Abstract An analytical model for representing body forces in numerical Actuator Disc models of wind turbines is developed and validated. The model is based on the assumption that the rotor Disc is subject to a constant circulation modified for tip and root effects. The model comprises expressions for both the axial and the azimuthal force distributions, and is generalized to be utilized for all kinds of inflow, including wind shear, turbulence, and shadow effects in wind farms. The advantage of the model is that it does not depend on any detailed knowledge concerning the wind turbine being analysed, but only requires knowledge regarding the rated wind speed and nameplate capacity. To validate the analytical model, results are compared to numerically generated results using detailed information regarding geometry and airfoil data for the 2 MW Tjaereborg wind turbine and the 10 MW DTU reference turbine. The comparisons show very good agreement between the loadings using the new analytical model and the airfoil data based method for the two tested wind turbines, demonstrating that the analytical model is a simple and reliable way of introducing body forces in Actuator Disc simulations without any prior knowledge of the wind turbine being analysed.

  • Analysis of long distance wakes of Horns Rev I using Actuator Disc approach
    Journal of Physics: Conference Series, 2014
    Co-Authors: Ola Eriksson, Robert Flemming Mikkelsen, Karl Nilsson, Kurt Schaldemose Hansen, Stehan Ivanell
    Abstract:

    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 bounboundary 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.

  • Comparison between PIV measurements and computations of the near-wake of an Actuator Disc
    Journal of Physics: Conference Series, 2014
    Co-Authors: Søren Juhl Andersen, Robert Flemming Mikkelsen, Jens Nørkær Sørensen, L. E. M. Lignarolo, Daniele Ragni, C.j. Simao Ferreira, G.j.w. Van Bussel
    Abstract:

    Experimental stereoscopic PIV measurements in the wake of a two-bladed rotor and a porous Actuator Disc are compared to numerical simulation of an Actuator Disc. Compared to previous literature, the focus of the present analysis is on the near wake, where the Actuator Discs fail to represent the complex flow structures correctly, which affects the downstream representation of the full wake behind a real rotor. The near wake region is characterised by the instability and breakdown of the tip-vortex helical system, which constitutes the onset of a stronger mixing process. The comparison focuses on the turbulent structures in the shear layer at the borders of the wake through the analysis of the Reynolds stresses and by employing POD on two separate regions. The analysis shows that the Actuator Discs fail to capture the details of the complex flow behind a rotor, but that the experimental and numerical Actuator Discs are generally comparable at a certain distance behind the Actuator Disc. This project is intended to provide the basis for understanding the origin of the limitations of the current wake models based on the Actuator Disc assumption.

L. E. M. Lignarolo – One of the best experts on this subject based on the ideXlab platform.

  • Validation of four LES and a vortex model against stereo-PIV measurements in the near wake of an Actuator Disc and a wind turbine
    Renewable Energy, 2016
    Co-Authors: L. E. M. Lignarolo, Daniele Ragni, Gijs Van Kuik, Søren Juhl Andersen, Dhruv Mehta, Richard J. A. M. Stevens, Ali Emre Yilmaz, Charles Meneveau, Carlos J. Ferreira, Johan Meyers
    Abstract:

    In this paper we report the results of a workshop organised by the Delft University of Technology in 2014, aiming at the comparison between different state-of-the-art numerical models for the simulation of wind turbturbine wakes. The chosen benchmark case is a wind tunnel measurement, where stereoscopic Particle Image Velocimetry was employed to obtain the velocity field and turbulence statistics in the near wake of a two-bladed wind turbturbine model and of a porous Disc, which mimics the numerical Actuator used in the simulations. Researchers have been invited to simulate the experimental case based on the Disc drag coefficient and the inflow characteristics. Four large eddy simulation (LES) codes from different institutions and a vortex model are part of the comparison. The purpose of this benchmark is to validate the numerical predictions of the flow field statistics in the near wake of an Actuator Disc, a case that is highly relevant for full wind farm applications. The comparison has shown that, despite its extreme simplicity, the vortex model is capable of reproducing the wake expansion and the centreline velocity with very high accuracy. Also all tested LES models are able to predict the velocity deficit in the very near wake well, contrary to what was expected from previous literature. However, the resolved velocity fluctuations in the LES are below the experimentally measured values.

  • Wind turbine and Actuator Disc wake: Two experimental campaigns
    , 2015
    Co-Authors: L. E. M. Lignarolo, Daniele Ragni, C.j. Simao Ferreira, G.j.w. Van Bussel
    Abstract:

    The present paper is the summary of 3 years of research on the wake aerodynamics of horizontal axis wind turbturbine at Delft University of Technology, the Netherlands. In particular, the main results and the conclusions of two experimental campaigns are collected. The underlying research question is: how do the near-wake turbulent flow structures affect the re-energising of the far wake and to what extend is the Actuator Disc assumption valid for the representation of the near wake dynamics? In the first experiments, stereo particle image velocimetry is used for analysing the turbulent velovelocity field in the near and transition wake of a small two-bladed wind turbturbine model. The results showed the important role of the tip-vortex helix instability (leapfrogging) in the mixing process and in the re-energising of the wake. The tip-vortex instability and breakdown, in fact, give rise to a more efficient turbulent mixing. In the second campaign, the same measurement technique is used for acquiring data in the near wake of the wind turbturbine model and in the near wake of a porous Disc, emulating the numerical Actuator Disc. The results show a good match velocity fields between wind turbturbine and Actuator Disc, but show a different turbulence intensity and turbulent mixing. The analysis suggest the possibility to extend the use of the Actuator Disc model in numerical simulation until the very near wake, provided that the turbulent mixing is correctly represented.

  • Potential flow solutions for energy extracting Actuator Disc flows
    Wind Energy, 2015
    Co-Authors: G.a.m. Van Kuik, L. E. M. Lignarolo
    Abstract:

    The Actuator Disc is the oldest representation of a rotor, screw or propeller. Performance prediction is possible by applying momentum theory, giving integrated values for power and velocity. Computational fluid dynamics has provided much more flow details, but a full potential flow solution zooming in on these flow details was still absent. With the wake boundary Discretized by vortex rings, flow states for energy extracting Discs have been obtained for thrust coefficients up to 0.998. Boundary conditions are met with an accuracy of a few ‰. Results from momentum theory are confirmed. Most rotor design codes use momentum theory in annulus or differential form, assuming that the axial velocity vx at the Disc is uniform. However, the absolute velocity |v| is found to be uniform, and arguments for this are presented. The non-uniformity of vx is an inherent part of the flow solution caused by, in terms of momentum theory, the pressure acting at the annuli. This makes the annuli not independent from each other as assumed in current design codes. Although this was already known, it is now confirmed up to the highest thrust coefficients. Optimizing a rotor design should be carried out for the non-uniform distribution of vx. To enable this, an equation for the non-uniformity as function of thrust and radial position is presented, being a surface-fit to the calculated data. Qualitatively, the non-uniform distribution does the same as the Prandtl–Glauert–Shen tip correction applied to a uniform distribution. Copyright © 2015 John Wiley & Sons, Ltd.