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Stefan Ivanell - One of the best experts on this subject based on the ideXlab platform.
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Analytical body forces in numerical Actuator Disc model of wind turbines
Renewable Energy, 2020Co-Authors: Jens Nørkær Sørensen, Stefan Ivanell, Karl Nilsson, Henrik Asmuth, Robert Flemming MikkelsenAbstract: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.
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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, 2019Co-Authors: Nikolaos Simisiroglou, Heracles Polatidis, Stefan IvanellAbstract: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 turbine ...
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Wind farm power production assessment : a comparative analysis of two Actuator Disc methods and two analytical wake models
2018Co-Authors: Nikolaos Simisiroglou, Heracles Polatidis, Stefan IvanellAbstract: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 turbine power production data from the Lillgrund offshore wind farm in Sweden is used. The measured power production for individual wind turbines 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.
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Validation of the Actuator Disc approach using small-scale model wind turbines
Wind Energy Science, 2017Co-Authors: Nikolaos Simisiroglou, Simon-philippe Breton, Stefan IvanellAbstract:Abstract. The aim of the present study is the validation of the implementation of an Actuator Disc (ACD) model in the computational fluid dynamics (CFD) code PHOENICS. The flow behaviour for three wind turbine cases is investigated numerically and compared to wind tunnel measurements: (A) the flow around a single model wind turbine, (B) the wake interaction between two in-line model wind turbines for a uniform inflow of low turbulence intensity and (C) the wake interaction between two in-line model wind turbines at different separation distances in a uniform or sheared inflow of high turbulence intensity. This is carried out using Reynolds-averaged Navier–Stokes (RANS) simulations and an ACD technique in the CFD code PHOENICS. The computations are conducted for the design condition of the rotors using four different turbulence closure models and five different thrust distributions. The computed axial velocity field as well as the turbulence kinetic energy are compared with hot-wire anemometry (HWA) measurements. For the cases with two in-line wind turbines, the thrust coefficient is also computed and compared with measurements. The results show that for different inflow conditions and wind turbine spacings the proposed method is able to predict the overall behaviour of the flow with low computational effort. When using the k-e and Kato–Launder k-e turbulence models the results are generally in closer agreement with the measurements.
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Validation of the Actuator Disc and Actuator line techniques for yawed rotor flows using the New Mexico experimental data
Journal of Physics: Conference Series, 2017Co-Authors: Simon-philippe Breton, Wen Zhong Shen, Stefan IvanellAbstract:Experimental data acquired in the New MEXICO experiment on a yawed 4.5m diameter rotor model turbine are used here to validate the Actuator line (AL) and Actuator Disc (AD) models implemented in the Large Eddy Simulation code EllipSys3D in terms of loading and velocity field. Even without modelling the geometry of the hub and nacelle, the AL and AD models produce similar results that are generally in good agreement with the experimental data under the various configurations considered. As expected, the AL model does better at capturing the induction effects from the individual blade tip vortices, while the AD model can reproduce the averaged features of the flow. The importance of using high quality airfoil data (including 3D corrections) as well as a fine grid resolution is highlighted by the results obtained. Overall, it is found that both models can satisfactorily predict the 3D velocity field and blade loading of the New MEXICO rotor under yawed inflow.
Jens Nørkær Sørensen - One of the best experts on this subject based on the ideXlab platform.
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Analytical body forces in numerical Actuator Disc model of wind turbines
Renewable Energy, 2020Co-Authors: Jens Nørkær Sørensen, Stefan Ivanell, Karl Nilsson, Henrik Asmuth, Robert Flemming MikkelsenAbstract: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.
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The General Momentum Theory
Research Topics in Wind Energy, 2015Co-Authors: Jens Nørkær SørensenAbstract: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.
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Comparison between PIV measurements and computations of the near-wake of an Actuator Disc
Journal of Physics: Conference Series, 2014Co-Authors: Søren Juhl Andersen, Robert Flemming Mikkelsen, Jens Nørkær Sørensen, L. E. M. Lignarolo, Daniele Ragni, C.j. Simao Ferreira, G.j.w. Van BusselAbstract: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.
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Airfoil data sensitivity analysis for Actuator Disc simulations used in wind turbine applications
Journal of Physics: Conference Series, 2014Co-Authors: Karl Nilsson, Simon-philippe Breton, Jens Nørkær Sørensen, Stefan IvanellAbstract: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.
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analysis of wake states by a full field Actuator Disc model
Wind Energy, 1998Co-Authors: Jens Nørkær Sørensen, Wen Zhong Shen, X. MunduateAbstract:Various wake status have been analysed by a numerical method that combines the Actuator Disc principle with the Navier–Stokes equations. Results are compared with one-dimensional momentum theory and experiments. The computations are in excellent agreement with one-dimensional momentum theory for rotors working in the windmill brake state as well as in the propeller and hover states. The computations demonstrate that the turbulent wake and vortex ring states are unstable regimes for a rotor with constant loading and that these states, after a complicated transient phase, settle to a steady state. Copyright © 1998 John Wiley & Sons, Ltd.
G.a.m. Van Kuik - One of the best experts on this subject based on the ideXlab platform.
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New dynamic‐inflow engineering models based on linear and nonlinear Actuator Disc vortex models
Wind Energy, 2019Co-Authors: Delphine De Tavernier, Carlos Simao Ferreira, G.a.m. Van Kuik, G. SchepersAbstract:Two new engineering models are presented for the aerodynamic induction of a wind turbine 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
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new dynamic inflow engineering models based on linear and nonlinear Actuator Disc vortex models
Wind Energy, 2019Co-Authors: Delphine De Tavernier, Carlos Simao Ferreira, G.a.m. Van Kuik, G. SchepersAbstract:Two new engineering models are presented for the aerodynamic induction of a wind turbine 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
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The dynamic wake of an Actuator Disc undergoing transient load: A numerical and experimental study
Renewable Energy, 2019Co-Authors: Carlos Simao Ferreira, G.a.m. Van KuikAbstract: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.
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Potential flow solutions for energy extracting Actuator Disc flows
Wind Energy, 2015Co-Authors: G.a.m. Van Kuik, L. E. M. LignaroloAbstract: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.
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On Actuator Disc force fields generating wake vorticity
2009Co-Authors: G.a.m. Van Kuik, A.h. Van ZuijlenAbstract:Actuator Disc calculations can be divided in two categories: force models where, for a prescribed force field, the flow is calculated using a CFD method, and kinematic models, where the wake is calculated based on wake boundary conditions and the force field is known when the velocities are known. In both categories, but specifically for the kinematical models, results are reported that differ some 10% from momentum models. Furthermore, most calculations which give details about the flow through the Disc do not satisfy the condition derived by Xyros & Xyros (2007) that the axial velocity through the Disc is uniform for Discs with a uniform surface load. Apart from this, the inconsistency in the momentum models Discussed by van Kuik (2003) is still unresolved. These observations raise the questions: what is the relation between force- and flow field, what are the requirements for a steady axisymmetric force field to generate vorticity in an Euler flow?
Robert Flemming Mikkelsen - One of the best experts on this subject based on the ideXlab platform.
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Analytical body forces in numerical Actuator Disc model of wind turbines
Renewable Energy, 2020Co-Authors: Jens Nørkær Sørensen, Stefan Ivanell, Karl Nilsson, Henrik Asmuth, Robert Flemming MikkelsenAbstract: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.
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Analysis of long distance wakes of Horns Rev I using Actuator Disc approach
Journal of Physics: Conference Series, 2014Co-Authors: Ola Eriksson, Robert Flemming Mikkelsen, Karl Nilsson, Kurt Schaldemose Hansen, Stehan IvanellAbstract: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.
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Comparison between PIV measurements and computations of the near-wake of an Actuator Disc
Journal of Physics: Conference Series, 2014Co-Authors: Søren Juhl Andersen, Robert Flemming Mikkelsen, Jens Nørkær Sørensen, L. E. M. Lignarolo, Daniele Ragni, C.j. Simao Ferreira, G.j.w. Van BusselAbstract: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.
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validation and modification of the blade element momentum theory based on comparisons with Actuator Disc simulations
Wind Energy, 2010Co-Authors: Helge Aagaard Madsen, Mads Dossing, Robert Flemming MikkelsenAbstract:A comprehensive investigation of the Blade Element Momentum (BEM) model using detailed numerical simulations with an axis symmetric Actuator Disc (AD) model has been carried out. The present implementation of the BEM model is in a version where exactly the same input in the form of non-dimensional axial and tangential load coefficients can be used for the BEM model as for the numerical AD model. At a rotor Disc loading corresponding to maximum power coefficient, we found close correlation between the AD and BEM model as concerns the integral value of the power coefficient. However, locally along the blade radius, we found considerable deviations with the general tendency, that the BEM model underestimates the power coefficient on the inboard part of the rotor and overestimates the coefficient on the outboard part. A closer investigation of the deviations showed that underestimation of the power coefficient on the inboard part could be ascribed to the pressure variation in the rotating wake not taken into account in the BEM model. We further found that the overestimation of the power coefficient on the outboard part of the rotor is due to the expansion of the flow causing a non-uniform induction although the loading is uniform. Based on the findings we derived two small engineering sub-models to be included in the BEM model to account for the physical mechanisms causing the deviations. Finally, the influence of using the corrected BEM model, BEMcor on two rotor designs is presented. Copyright © 2009 John Wiley & Sons, Ltd.
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Actuator Disc Simulations of Influence of Wind Shear on Power Production of Wind Turbines
2010Co-Authors: Niels Troldborg, Mac Gaunaa, Robert Flemming MikkelsenAbstract:The influence of ground proximity and/or wind shear on the power production of a wind turbine is studied using full Navier-Stokes simulations combined with an Actuator Disc approach. The work reveals that the local power coefficient of a Disc operating in a sheared inflow at a given local thrust coefficient is higher than in a corresponding uniform inflow. When a ground surface is present, it is shown that the increased power production, caused by the wind shear, decreases with decreasing hub height. If the incoming wind is uniform, then the ground surface does not affect the power production significantly.
L. E. M. Lignarolo - One of the best experts on this subject based on the ideXlab platform.
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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, 2016Co-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 MeyersAbstract: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 turbine 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 turbine 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.
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Wind turbine and Actuator Disc wake: Two experimental campaigns
2015Co-Authors: L. E. M. Lignarolo, Daniele Ragni, C.j. Simao Ferreira, G.j.w. Van BusselAbstract:The present paper is the summary of 3 years of research on the wake aerodynamics of horizontal axis wind turbine 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 velocity field in the near and transition wake of a small two-bladed wind turbine 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 turbine 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 turbine 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.
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Potential flow solutions for energy extracting Actuator Disc flows
Wind Energy, 2015Co-Authors: G.a.m. Van Kuik, L. E. M. LignaroloAbstract: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.
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Turbulent mixing in wind turbine and Actuator Disc wakes: experiments and POD analysis
33rd Wind Energy Symposium, 2015Co-Authors: L. E. M. Lignarolo, Daniele Ragni, Carlos Simao Ferreira, G.j.w. Van BusselAbstract:The Actuator Disc is a common approach to simplify the numerical and experimental aerodynamic simulation of a wind turbine. Previous research proved that this approach underestimates the wake losses in wind farm simulations. The velocity field of a wind turbine and a porous Disc (emulating the Actuator Disc) has been measured in a wind tunnel using stereo particle image velocimetry. The data analysis provided the time-average velocity and turbulence intensity fields and mean-flow kinetic-energy fluxes at the boundary of the wake. These properties have been compared for the wind turbine and the Actuator Disc wake. The results show a different turbulence intensity and turbulent mixing. The proper orthogonal decomposition technique is employed as a filter for separating the coherent periodic fluctuations due to the tip vortex passage from the random turbulent fluctuations. This will allow to understand what is the role of the two flow structures in the wake mixing process.
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Comparison between PIV measurements and computations of the near-wake of an Actuator Disc
Journal of Physics: Conference Series, 2014Co-Authors: Søren Juhl Andersen, Robert Flemming Mikkelsen, Jens Nørkær Sørensen, L. E. M. Lignarolo, Daniele Ragni, C.j. Simao Ferreira, G.j.w. Van BusselAbstract: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.
