| 1. |
- Ivanell, Stefan, et al.
(författare)
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Stability analysis of the tip vortices of a wind turbine
- 2008
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Ingår i: 46th AIAA Aerospace Sciences Meeting and Exhibit. - 9781563479373
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 2. |
- Kleusberg, Elektra, et al.
(författare)
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Actuator line simulations of a Joukowsky and Tjæreborg rotor using spectral element and finite volume methods
- 2016
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Ingår i: SCIENCE OF MAKING TORQUE FROM WIND (TORQUE 2016). - : Institute of Physics (IOP).
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 3. |
- Andersen, S.J., et al.
(författare)
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Comparison of Engineering Wake Models with CFD Simulations
- 2014
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 524, s. 012161-
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Tidskriftsartikel (refereegranskat)abstract
- The engineering wake models by Jensen [1] and Frandsen et al. [2] are assessed for different scenarios simulated using Large Eddy Simulation and the Actuator Line method implemented in the Navier-Stokes equations. The scenarios include the far wake behind a single wind turbine, a long row of turbines in an atmospheric boundary layer, idealised cases of an infinitely long row of wind turbines and infinite wind farms with three different spacings. Both models include a wake expansion factor, which is calibrated to fit the simulated wake velocities. The analysis highlights physical deficiencies in the ability of the models to universally predict the wake velocities, as the expansion factor can be fitted for a given case, but with not apparent transition between the cases. 1.
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| 4. |
- Andersen, S. J., et al.
(författare)
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Quantifying variability of Large Eddy Simulations of very large wind farms
- 2015
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Ingår i: Wake Conference 2015. - : IOP Publishing. ; , s. 012027-
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Konferensbidrag (refereegranskat)abstract
- Large Eddy Simulations are inherently dynamic as the largest scales are resolved and the smallest scales are modeled temporally. This raises challenges for simulations including very large scales such as atmospheric flows, which require very long simulation times. Simple averages fail at capturing these dynamics and potentially yield misleading interpretations concerning the capabilities of different models when tested in blind tests or in benchmarking exercises such as Wakebench, where results from different flow models are compared. This article will present results from very large wind farm simulations using Actuator Disc (AD) and Line (AL) models for two different turbine spacings with turbulent inflow. The results of each numerical flow model include a certain variability, and it will be examined if different models result in comparable probability distributions.
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| 5. |
- Andersen, Soren Juhl, et al.
(författare)
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Statistics of LES Simulations of Large Wind Farms
- 2016
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Ingår i: SCIENCE OF MAKING TORQUE FROM WIND (TORQUE 2016). - : IOP PUBLISHING LTD.
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Konferensbidrag (refereegranskat)abstract
- Numerous large eddy simulations are performed of large wind farms using the actuator line method, which has been fully coupled to the aero-elastic code, Flex5. The higher order moments of the flow field inside large wind farms is examined in order to determine a representative reference velocity. The statistical moments appear to collapse and hence the turbulence inside large wind farms can potentially be scaled accordingly. The thrust coefficient is estimated by two different reference velocities and the generic C-T expression by Frandsen. A reference velocity derived from the power production is shown to give very good agreement and furthermore enables the very good estimation of the thrust force using only the steady C-T-curve, even for very short time samples. Finally, the effective turbulence inside large wind farms and the equivalent loads are examined.
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| 6. |
- Arnqvist, Johan, 1985-, et al.
(författare)
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Investigation of Turbulence Accuracy When Modeling Wind in Realistic Forests Using LES
- 2019
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Ingår i: Progress In Turbulence Viii. - Cham : SPRINGER INTERNATIONAL PUBLISHING AG. - 9783030221966 - 9783030221959 ; , s. 291-296
-
Konferensbidrag (refereegranskat)abstract
- This study presents an evaluation of wind field simulations, in neutral atmospheric conditions, above a heterogeneous forest. The calculations were performed with Large-Eddy Simulation (LES) code OpenFOAM, with explicit modelling of the forest through drag coefficient and forest density. The findings indicate that a large modelling domain is needed in order to reproduce the measurements in different wind directions, since the effect of far upwind forest characteristics influence the wind and turbulence profiles. It is further shown that even though the low resolution of the LES simulations lead to slightly misrepresented single point turbulence characteristics, two point turbulence characteristics are well predicted due to spatial filtering of the small scales.
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| 7. |
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| 8. |
- Asmuth, Henrik, et al.
(författare)
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Actuator line simulations of wind turbine wakes using the lattice Boltzmann method
- 2020
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Ingår i: Wind Energy Science. - : Copernicus GmbH. - 2366-7443 .- 2366-7451. ; 5:2, s. 623-645
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Tidskriftsartikel (refereegranskat)abstract
- The high computational demand of large-eddy simulations (LESs) remains the biggest obstacle for a wider applicability of the method in the field of wind energy. Recent progress of GPU-based (graphics processing unit) lattice Boltzmann frameworks provides significant performance gains alleviating such constraints. The presented work investigates the potential of LES of wind turbine wakes using the cumulant lattice Boltzmann method (CLBM). The wind turbine is represented by the actuator line model (ALM). The implementation is validated and discussed by means of a code-to-code comparison to an established finite-volume Navier–Stokes solver. To this end, the ALM is subjected to both laminar and turbulent inflow while a standard Smagorinsky sub-grid-scale model is employed in the two numerical approaches. The resulting wake characteristics are discussed in terms of the first- and second-order statistics as well the spectra of the turbulence kinetic energy. The near-wake characteristics in laminar inflow are shown to match closely with differences of less than 3 % in the wake deficit. Larger discrepancies are found in the far wake and relate to differences in the point of the laminar-turbulent transition of the wake. In line with other studies, these differences can be attributed to the different orders of accuracy of the two methods. Consistently better agreement is found in turbulent inflow due to the lower impact of the numerical scheme on the wake transition. In summary, the study outlines the feasibility of wind turbine simulations using the CLBM and further validates the presented set-up. Furthermore, it highlights the computational potential of GPU-based LBM implementations for wind energy applications. For the presented cases, near-real-time performance was achieved using a single, off-the-shelf GPU on a local workstation.
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| 9. |
- Asmuth, Henrik, et al.
(författare)
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Assessment of Weak Compressibility in Actuator Line Simulations of Wind Turbine Wakes
- 2020
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 1618
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Tidskriftsartikel (refereegranskat)abstract
- The trend of increasing rotor diameters and tip-speeds has brought about concerns of non-negligible compressibility effects in wind turbine aerodynamics. The investigation of such effects on wakes is particularly difficult when using actuator line models (ALM). This is because crucial regions of the flow, i.e. the direct vicinity of the blade, are not simulated but represented by body forces. To separately assess the impact of compressibility on the wake and the ALM itself, we conduct large-eddy simulations (LES) where the forces of the ALM are prescribed and based on the local sampled velocity (standard procedure), respectively. The LES are based on the weakly-compressible Lattice Boltzmann Method (LBM). Further to the comparison of (near-)incompressible to compressible simulations we investigate cases with artificially increased compressibility. This is commonly done in weakly-compressible approaches to reduce the computational demand. The investigation with prescribed forces shows that compressibility effects in the wake flow are negligible. Small differences in the wake velocity (of max. 1%) are found to be related to local compressibility effects in the direct vicinity of the ALM. Most significantly, compressibility is found to affect the sampled velocity and thereby accuracy of the ALM.
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| 10. |
- Asmuth, Henrik
(författare)
-
Efficient Large-eddy Simulation for Wind Energy Applications
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modelling the interaction of wind turbines with the ambient flow is essential for almost all technical aspects of wind energy exploitation. Large-eddy simulation (LES) is the most detailed approach feasible to model this complex interaction of wind turbines with the atmospheric boundary layer and the wakes of upstream turbines. Despite more than twenty years of fundamental research on wind turbine modelling with LES, applications of the method remain limited to academic use cases to date. The main bottleneck hindering a broader adoption of LES in the industrial practice is the large computational demand of the method. Nevertheless, it holds enormous potential for addressing various modelling challenges arising from current trends in wind energy.A promising alternative to classical numerical approaches for LES is the lattice Boltzmann method (LBM). In particular, GPU-based (graphics processing unit) implementations of the method provide significant performance gains and have enabled unprecedented computational efficiencies for LES in different fields of fluid dynamics. Still, the LBM´s potential for wind energy applications remains untapped due to open questions, some of which are specific to the field. This thesis addresses two specific problems in applications of LES to wind turbine and farm simulations. First, is the representation of wind turbines with the actuator line technique. And, second, is the modelling of the surface shear stress in simulations of atmospheric boundary layers. Both aspects are crucial to enable LES for wind energy applications with the LBM, as is usually done with conventional approaches.As for the former, an LBM implementation of the actuator line model is applied in multiple studies on wind turbine wakes. Code-to-code comparisons and experimental validations show that the model can accurately capture the aerodynamic forces acting on the turbine blades as well as the wake characteristics. For the simulation of boundary layer flows a novel LBM-specific wall model is developed. The model, referred to as inverse momentum exchange method, imposes the surface shear stress at the first offwall grid points by adjusting the slip velocity in bounce-back boundary schemes. Simulations are compared to theoretical, numerical, and experimental reference data of isothermal boundary layer flows. It is consistently found that both mean quantities and higherorder turbulence statistics can be well-captured by wall-modelled lattice Boltzmann LES using the presented wall model and the employed cumulant collision scheme.The results presented illustrate that the LBM is a suitable approach for state-of-the-art LES of wind turbine wakes and boundary layer flows. Moreover, the applied method is shown to be robust, and, above all, extremely computationally efficient. Based on the observed computational efficiencies, it is concluded that industry LES for wind energy applications is possible with GPU-based LBM solvers. Furthermore, additional studies presented in this thesis illustrate further potentials of the method. Such are applications of reinforcement learning to wind farm control or large-scale data generation for the training of deep learning models for wake predictions.
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| 11. |
- Asmuth, Henrik, et al.
(författare)
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How Fast is Fast Enough? : Industry Perspectives on the Use of Large-eddy Simulation in Wind Energy
- 2023
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Ingår i: WAKE CONFERENCE 2023. - : Institute of Physics Publishing (IOPP).
-
Konferensbidrag (refereegranskat)abstract
- The use of graphics processing units (GPUs) has facilitated unprecedented performance gains for computational fluids dynamics in recent years. In many industries this has enabled the integration of large-eddy simulation (LES) in the engineering practice. Flow modelling in the wind industry though still primarily relies on models with significantly lower fidelity. This paper seeks to investigate the reasons why wind energy applications of LES are still an exception in the industrial practice. On that account, we present a survey among industry experts on the matter. The survey shows that the large runtimes and computational costs of LES are still seen as a main obstacle. However, other reasons such as a lack of expertise and user experience, the need for more validation, and lacking trust in the potential benefits of LES reveal that computational efficiency is not the only concern. Lastly, we present an exemplary simulation of a generic offshore wind farm using a GPU-resident Lattice Boltzmann LES framework. The example shows that the runtime requirements stated by a large part of the respondents can already now be fulfilled with reasonable hardware effort.
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| 12. |
- Asmuth, Henrik, et al.
(författare)
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The Actuator Line Model in Lattice Boltzmann Frameworks : Numerical Sensitivity and Computational Performance
- 2019
-
Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 1256
-
Tidskriftsartikel (refereegranskat)abstract
- The growing use of large-eddy simulations for the modelling of wind farms makes the need for efficient numerical frameworks more essential than ever. GPU-accelerated implementations of the Lattice Boltzmann Method (LBM) have shown to provide significant performance gains over classical Navier-Stokes-based computational fluid dynamics. Yet, their use in the field of wind energy remains limited to date. In this fundamental study the cumulant LBM is scrutinised for actuator line simulations of wind turbines. The numerical sensitivity of the method in a simple uniform inflow is investigated with respect to spatial and temporal resolution as well as the width of the actuator line’s regularisation kernel. Comparable accuracy and slightly better stability properties are shown in relation to a standard Navier-Stokes implementation. The results indicate the overall suitability of the cumulant LBM for wind turbine wake simulations. The potential of the LBM for future wind energy applications is clarified by means of a brief comparison of computational performance.
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| 13. |
- Asmuth, Henrik, et al.
(författare)
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Wall-modeled lattice Boltzmann large-eddy simulation of neutral atmospheric boundary layers
- 2021
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Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 33:10, s. 105111-105111
-
Tidskriftsartikel (refereegranskat)abstract
- The lattice Boltzmann method (LBM) sees a growing popularity in the field of atmospheric sciences and wind energy, largely due to itsexcellent computational performance. Still, LBM large-eddy simulation (LES) studies of canonical atmospheric boundary layer flows remainlimited. One reason for this is the early stage of development of LBM-specific wall models. In this work, we discuss LBM–LES of isothermalpressure-driven rough-wall boundary layers using a cumulant collision model. To that end, we also present a novel wall modeling approach,referred to as inverse momentum exchange method (iMEM). The iMEM enforces a wall shear stress at the off-wall grid points by adjustingthe slip velocity in bounce-back boundary schemes. In contrast to other methods, the approach does not rely on the eddy viscosity, nor doesit require the reconstruction of distribution functions. Initially, we investigate different aspects of the modeling of the wall shear stress, i.e.,an averaging of the input velocity as well as the wall-normal distance of its sampling location. Particularly, sampling locations above the firstoff-wall node are found to be an effective measure to reduce the occurring log-layer mismatch. Furthermore, we analyze the turbulence statis-tics at different grid resolutions. The results are compared to phenomenological scaling laws, experimental, and numerical references. Theanalysis demonstrates a satisfactory performance of the numerical model, specifically when compared to a well-established mixed pseudo-spectral finite difference (PSFD) solver. Generally, the study underlines the suitability of the LBM and particularly the cumulant LBM forcomputationally efficient LES of wall-modeled boundary layer flows.
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| 14. |
- Asmuth, Henrik, et al.
(författare)
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Wind Turbine Response in Waked Inflow: A Modelling Benchmark Against Full-Scale Measurements
- 2022
-
Ingår i: Renewable energy. - : Elsevier. - 0960-1481 .- 1879-0682. ; 191, s. 868-887
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Tidskriftsartikel (refereegranskat)abstract
- Predicting the power and loads of wind turbines in waked inflow conditions still presents a major modelling challenge. It requires the accurate modelling of the atmospheric flow conditions, wakes of upstream turbines and the response of the turbine of interest. Rigorous validations of model frameworks against measurements of utility-scale wind turbines in such scenarios remain limited to date. In this study, six models of different fidelity are compared against measurements from the DanAero experiment. The two benchmark cases feature a full-wake and partial-wake scenario, respectively. The simulations are compared against local pressure forces and inflow velocities measured on several blade sections of the downstream turbine, as well as met mast measurements and standard SCADA data. Regardless of the model fidelity, reasonable agreements are found in terms of the wake characteristics and turbine response. For instance, the azimuth variation of the mean aerodynamic forces acting on the blade was captured with a mean relative error of 15–20%. While various model-specific deficiencies could be identified, the study highlights the need for further full-scale measurement campaigns with even more extensive instrumentation. Furthermore, it is concluded that validations should not be limited to integrated and/or time-averaged quantities that conceal characteristic spatial or temporal variations.
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| 15. |
- Barney, Andrew
(författare)
-
Energy planning for islands : Guiding island energy transition and decision-making
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The importance and benefits of transitioning away from fossil fuel based energy systems is becoming ever clearer. The transition is especially crucial for geographic islands that typically have limited, or no, access to mainland energy networks. This limited access means that they must rely on costly, economically and environmentally, imported fossil fuels for energy production. This isolation and reliance on imported fuels makes islands particularly energy insecure, which is only expected to worsen because of climate change. At the same time, much of the current guidance and policy developed to help plan for islands’ energy transition is based on the conditions and circumstances present on the mainland or on energy planning that focuses primarily on technical or economic parameters. To plan for a smoother, more successful transition away from fossil fuel based energy production on islands, more data is needed about islanders themselves and their local circumstances as well as planning guidance flexible enough to fit the variable needs of island planners that goes beyond a techno-economic focus.In this PhD Thesis and in response to these transition issues, an energy planning platform, REACT-DECARB, has been developed to specifically address the needs of island energy planners. This platform seeks to be holistic in its approach to facilitate the island energy transition planning procedure by considering typical island characteristics and energy transition processes while also including key technical, social, environmental and economic dimensions and granting planners flexibility in how they incorporate them. Additionally, surveys and interviews with island residents are conducted to gain an understanding of islanders’ motivations, priorities and awareness in relation to energy and energy transition as well as to determine if these can be of guidance to island energy transition planners.The applications of the REACT-DECARB platform on geographic islands representing eight different countries from around the EU with varying populations, land areas and climates identified energy transition opportunities and obstacles specific to the islands where the platform was applied as well as to other islands seeking to transition their energy systems. Moreover, these applications of the planning platform demonstrated its ability to help island planners in the development of holistic energy transition paths. The platform takes island energy transition planning beyond a focus on techno-economics by including a wider range of planning dimensions allowing them to decide the methods best suited to their island’s needs. Further, the interactions with island residents demonstrated that their relationships to energy could be informative in the development of energy transition plans as well as likely being critical to their success. Islanders’ understandings, motivations, priorities and awareness were found to be not only directly useful in guiding planners during the designing of island energy transition plans but can also serve to inform planners in how best to educate local residents about and engage them in local energy transition projects. Ultimately, the work in this Thesis contributes to the efforts to make sure islands’, and islanders’, needs and perspectives are considered and included as a part of the wider energy transition.
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| 16. |
- Barney, Andrew
(författare)
-
Integrated regional energy planning for islands : Development of a decision-aiding methodological framework and application
- 2022
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Clear and inclusive planning guidance for island planners seeking transitions away from traditional fossil fuel based energy systems is currently needed. At present, the guidance from the scientific community has largely been provided piecemeal and with significant focus on the specifics of single islands and, more often than not, on proposed energy systems’ techno-economics. To address these gaps in applicability and encourage a more balanced energy project analysis the REACT-DECARB integrated energy planning and decision-aiding platform has been developed. REACT-DECARB seeks to work as a comprehensive planning platform for island energy transition and this thesis investigates the ability of the platform to provide the support to planners needed to facilitate their transition planning. To do so the core concepts, descriptions and applications of decision making as well as a number of energy planning tools are presented. The platform is then described and its individual steps are applied to eight geographic islands within the EU. These applications of the platform identified key opportunities for the islands, such as significant RES potential, while also finding potential obstacles, such as land use conflicts. Further, it highlighted the high economic cost of seeking full electricity autonomy and of using substantial battery capacities. The platform additionally identified the importance local preferences can potentially have on the ranking of future energy scenarios as well as the impact lifetime project environmental costs can have on overall system environmental impacts. In summation, the REACT-DECARB platform is found to be an effective tool in directing island energy decarbonisation efforts by guiding local energy planners in what should be included in their planning and how these key elements can be incorporated in the decisions made. Further, the platform is found to be flexible enough to be applied to more than the specific circumstances of a single island and allows planners to adjust their approaches to its different steps as needed and to better match their capabilities.
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| 17. |
- Bastankhah, Majid, et al.
(författare)
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A fast-running physics-based wake model for a semi-infinite wind farm
- 2024
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 985
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a new generation of fast-running physics-based models to predict the wake of a semi-infinite wind farm, extending infinitely in the lateral direction but with finite size in the streamwise direction. The assumption of a semi-infinite wind farm enables concurrent solving of the laterally averaged momentum equations in both streamwise and spanwise directions. The developed model captures important physical phenomena such as vertical top-down transport of energy into the farm, variable wake recovery rate due to the farm-generated turbulence and also wake deflection due to turbine yaw misalignment and Coriolis force. Of special note is the model's capability to predict and shed light on the counteracting effect of Coriolis force causing wake deflections in both positive and negative directions. Moreover, the impact of wind farm layout configuration on the flow distribution is modelled through a parameter called the local deficit coefficient. Model predictions were validated against large-eddy simulations extending up to 45 km downstream of wind farms. Detailed analyses were performed to study the impacts of various factors such as incoming turbulence, wind farm size, inter-turbine spacing and wind farm layout on the farm wake.
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| 18. |
- Breton, Simon-Philippe, et al.
(författare)
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A survey of modelling methods for high-fidelity wind farm simulations using large eddy simulation
- 2017
-
Ingår i: Philosophical Transactions. Series A. - : ROYAL SOC. - 1364-503X .- 1471-2962. ; 375:2091
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Forskningsöversikt (refereegranskat)abstract
- Large eddy simulations (LES) of wind farms have the capability to provide valuable and detailed information about the dynamics of wind turbine wakes. For this reason, their use within the wind energy research community is on the rise, spurring the development of new models and methods. This review surveys the most common schemes available to model the rotor, atmospheric conditions and terrain effects within current state-of-the-art LES codes, of which an overview is provided. A summary of the experimental research data available for validation of LES codes within the context of single and multiple wake situations is also supplied. Some typical results for wind turbine and wind farm flows are presented to illustrate best practices for carrying out high-fidelity LES of wind farms under various atmospheric and terrain conditions. This article is part of the themed issue 'Wind energy in complex terrains'.
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| 19. |
- Breton, Simon-Philippe, et al.
(författare)
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Comparative CFD study of the effect of the presence of downstream turbines on upstream ones using a rotational speed control system
- 2014
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Ingår i: Science of Making Torque from Wind 2012. - : IOP Publishing.
-
Konferensbidrag (refereegranskat)abstract
- The effect of a downstream turbine on the production of a turbine located upstream of the latter is studied in this work. This is done through the use of two CFD simulation codes, namely OpenFOAM and EllipSys3D, which solve the Navier-Stokes equations in their incompressible form using a finite volume approach. In both EllipSys3D and Open Foam, the LES (Large Eddy Simulation) technique is used for modelling turbulence. The wind turbine rotors are modelled as actuator disks whose loading is determined through the use of tabulated airfoil data by applying the blade-element method. A generator torque controller is used in both simulation methods to ensure that the simulated turbines adapt, in terms of rotational velocity, to the inflow conditions they are submited to. Results from both simulation codes, although they differ slightly, show that the downstream turbine affects the upstream one when the spacing between the turbines is small. This is also suggested to be the case looking at measurements performed at the Lillgrund offshore wind farm, whose turbines are located unusually close to each other. However, for distances used in today's typical wind farms, this effect is shown by our calculations not to be significant.
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| 20. |
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| 21. |
- Breton, Simon-Philippe, et al.
(författare)
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Study Of The Influence Of Atmospheric turbulence On The Asymptotic wake Deficit In A very Long Line Of Wind Turbines
- 2013
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Ingår i: Proceedings of the 2013 International Conference on Aerodynamics of Offshore Wind Energy Systems and Wakes (ICOWES 2013). - Denmark. ; , s. 420-434
-
Konferensbidrag (refereegranskat)abstract
- The influence of atmospheric 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 modelling turbulence, and the wind turbine rotors are represented as actuator disks 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 pregenerated with the Mann model, with imposed turbulent levels of 4.5 and 8.9%. The turbines are in this study isolated from their environment, as no effect from the ground is modeled. This makes the proposed study of the asymptotic wake state behavior easier. Analysis of the characteristics of the flow as a function of the position along the row of turbines is performed in terms of turbulence intensity, mean velocity, and power spectra of the velocity fluctuations. 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 inlet conditions in which they operate.The results obtained for the turbulence intensity, power and mean velocity as a function of downstream distance show that an asymptotic wake state seems close to be reached near the end of the 10 turbine row. They also show a certain dependency on the imposed level of turbulence. Uncertainties obtained in the power spectra of the velocity fluctuations suggest that further investigation is necessary.
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| 22. |
- Breton, Simon-Philippe, et al.
(författare)
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Study of the influence of imposed turbulence on the asymptotic wake deficit in a very long line of wind turbines
- 2014
-
Ingår i: Renewable energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 70, s. 153-163
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 23. |
- Breton, Simon-Philippe, et al.
(författare)
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Validation of the actuator disc and actuator line techniques for yawed rotor flows using the New MEXICO experimental data
- 2017
-
Konferensbidrag (refereegranskat)abstract
- 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 loadingand 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.
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| 24. |
- Diaz, Gonzalo Pablo Navarro, et al.
(författare)
-
Actuator line model using simplified force calculation methods
- 2023
-
Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 8:3, s. 363-382
-
Tidskriftsartikel (refereegranskat)abstract
- To simulate transient wind turbine wake interaction problems using limited wind turbine data, two new variants of the actuator line technique are proposed in which the rotor blade forces are computed locally using generic load data. The proposed models, which are extensions of the actuator disk force models proposed by Navarro Diaz et al. (2019a) and Sorensen et al. (2020), only demand thrust and power coefficients and the tip speed ratio as input parameters. In the paper the analogy between the actuator disk model (ADM) and the actuator line model (ALM) is shown, and from this a simple methodology to implement local forces in the ALM without the need for knowledge of blade geometry and local airfoil data is derived. Two simplified variants of ALMs are proposed, an analytical one based on Sorensen et al. (2020) and a numerical one based on Navarro Diaz et al. (2019a). The proposed models are compared to the ADM using analogous data, as well as to the classical ALM based on blade element theory, which provides more detailed force distributions by using airfoil data. To evaluate the local force calculation, the analysis of a partial-wake interaction case between two wind turbines is carried out for a uniform laminar inflow and for a turbulent neutral atmospheric boundary layer inflow. The computations are performed using the large eddy simulation facility in Open Source Field Operation and Manipulation (OpenFOAM), including Simulator for Wind Farm Applications (SOWFA) libraries and the reference National Renewable Energy Laboratory (NREL) 5 MW wind turbine as the test case. In the single-turbine case, computed normal and tangential force distributions along the blade showed a very good agreement between the employed models. The two new ALMs exhibited the same distribution as the ALM based on geometry and airfoil data, with minor differences due to the particular tip correction needed in the ALM. For the challenging partially impacted wake case, both the analytical and the numerical approaches manage to correctly capture the force distribution at the different regions of the rotor area, with, however, a consistent overestimation of the normal force outside the wake and an underestimation inside the wake. The analytical approach shows a slightly better performance in wake impact cases compared to the numerical one. As expected, the ALMs gave a much more detailed prediction of the higher-frequency power output fluctuations than the ADM. These promising findings open the possibility to simulate commercial wind farms in transient inflows using the ALM without having to get access to actual wind turbine and airfoil data, which in most cases are restricted due to confidentiality.
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| 25. |
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| 26. |
- Eriksson, Ola, et al.
(författare)
-
Analysis of long distance wakes behind a row of turbines – a parameter study
- 2014
-
Ingår i: Science of Making Torque from Wind. - : IOP Publishing. ; , s. 012152-
-
Konferensbidrag (refereegranskat)abstract
- Large Eddy Simulations (LES) of the long distance wake behind a row of 10 turbines are conducted to predict wake recovery. The Navier-Stokes solver EllipSys3D is used in combination with the actuator disc concept. Neutral atmospheric conditions are assumed in combination with synthetic turbulence using the Mann method. Both the wind shear profile and turbulence are introduced into the flow field using body forces. Previous simulations using the same simulation method to model the Horns Rev wind farm showed a higher wake recovery at long distances compared to measurements. The current study investigates further the sensitivity to parameters such as the grid resolution, Reynolds number, the turbulence characteristics as well as the impact of using different internal turbine spacings. The clearest impact on the recovery behind the farm could be seen from the background turbulence. The impact of the wind shear on the turbulence level in the domain needs further studies. A lower turbulence level gives lower wake recovery as expected. A lower wake recovery can also be seen for a higher grid resolution. The Reynolds number, apart from when using a very low value, has a small impact on the result. The variation of the internal spacing is seen to have a relatively minor impact on the farm wake recovery.
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| 27. |
- Eriksson, Ola, et al.
(författare)
-
Analysis of long distance wakes of Horns Rev I using actuator disc approach
- 2014
-
Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 555:1
-
Tidskriftsartikel (refereegranskat)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 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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| 28. |
- Eriksson, Ola, 1979-, et al.
(författare)
-
Impact of Wind Veer and the Coriolis Force for an Idealized Farm to Farm Interaction Case
- 2019
-
Ingår i: Applied Sciences. - : MDPI AG. - 2076-3417. ; 9:5
-
Tidskriftsartikel (refereegranskat)abstract
- The impact of the Coriolis force on the long distance wake behind wind farms is investigated using Large Eddy Simulations (LES) combined with a Forced Boundary Layer (FBL) technique. When using the FBL technique any mean wind shear and turbulent fluctuations can be added with body forces. The wind shear can also include the mean wind veer due to the Coriolis force. The variation of the Coriolis force due to local deviations from the mean profile, e.g., from wakes, is not taken into account in the FBL. This can be corrected for with an extra source term in the equations, hereon defined as the Coriolis correction. For a row of 4 turbines it is shown that the inclusion of the wind veer turns the wake to the right, while including the Coriolis correction turns it to the left. When including both wind veer and Coriolis correction the impact of wind veer dominates. For an idealized farm to farm interaction case, two farms of 4 * 4 turbines with 6 km in between, it can be seen that when including wind veer and the Coriolis correction a approximately 3% increase in the relative production for a full wake direction can be seen and only a slightly smaller increase can be seen when including only wind veer. The results indicate that FBL can be used for studies of long distance wakes without including a Coriolis correction but efforts need to be taken to use a wind shear with a correct mean wind veer.
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| 29. |
- Eriksson, Ola, 1979-, et al.
(författare)
-
Large-eddy simulations of the evolution of imposed turbulence in forced boundary layers in a very long domain
- 2020
-
Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 23:6, s. 1482-1493
-
Tidskriftsartikel (refereegranskat)abstract
- The technique of using imposed turbulence in combination with a forced boundary layer in order to model the atmospheric boundary layer is analyzed for a very long domain using large-eddy simulations with different combinations of prescribed velocity profiles and pregenerated turbulence fields based on the Mann model. The ambient flow is first studied in the absence of wind turbines. The velocity profiles undergo a transition throughout the domain with a velocity increase of 10% to 15% close to the ground far downstream in the domain. The turbulence characteristics close to the turbulence plane are, as expected, similar to those of the added Mann turbulence. The turbulence will then undergo a transition throughout the domain to finally reach a balance with the shear profile at a certain downstream distance. This distance is found to depend on the turbulence level of the added Mann turbulence planes. A lower Mann turbulence level generally results in a shorter "balancing" distance. Secondly, a row of 10 turbines is imposed in the simulations at different distances from the plane of turbulence in order to determine how the distance affects wake conditions and power production levels. Our results show that a "balancing" distance is needed between the turbulence plane and the first turbine in the row in order to ensure nonchanging ambient conditions throughout the turbine row. This introduces an increase in the computational costs. The computational cost for the forced boundary technique is normally lower compared with using precursor simulations, for longer domains; however, this needs to be verified further.
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| 30. |
- Eriksson, Ola, et al.
(författare)
-
Large-eddy simulations of wind farm production and long distance wakes
- 2015
-
Ingår i: WAKE CONFERENCE 2015. - : IOP Publishing.
-
Konferensbidrag (refereegranskat)abstract
- 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.
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| 31. |
- Eriksson, Ola, 1979-
(författare)
-
Numerical Computations of Wakes Behind Wind Farms : A tool to study Farm to Farm interaction
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- More and larger wind farms are planned offshore. As the most suitable sites to build are limited these new wind farms will be constructed near to each other in so called wind farm clusters. Behind the wind turbines in these farms there is a disrupted flow of air called a wake that is characterized by reduced wind speed and increased turbulence. These individual turbine wakes combine to form a farm wake that can travel long distances. In wind farm clusters farm to farm interaction will occur, i.e. the long distance wake from one wind farm will impact the wind conditions for other farms in the surrounding area.This thesis contains numerical studies of these long distance wakes. In these studies Large Eddy Simulations (LES) using an Actuator Disc method (ACD) are used. A prescribed boundary layer is used where the wind shear is introduced using body forces. The turbulence, based on the Mann model, is introduced as fluctuating body forces upstream of the farm. A neutral atmosphere is assumed. The applied method has earlier been used for studies of wake effects inside farms but not for the longer distances needed for the study of farm to farm interaction. Parameter studies are setup to analyze how to best use the model for the study of long distance wakes with regards to 1) numerical and physical parameters in the model, 2) the extension of the domain and turbulence as well as the characteristics of the flow far downstream and 3) the downstream development of turbulence with different combinations of wind shear and turbulence level.Using an initial simulation setup a wind farm was studied and preliminary results were obtained. These results were subsequently improved upon by applying the simulation setup adjustments indicated in the parameter studies. A comparison with a mesoscale model was also done. The mesoscale model was shown to be relevant for studies of long distance wakes in another study comparing LES and WRF. Finally an idealized farm to farm interaction case was studied with focus on the impact of including the Coriolis force in the simulations.Combining LES with a mesoscale model is of interest to study further.
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| 32. |
- Eriksson, Ola, 1979-
(författare)
-
Numerical Computations of Wakes Behind Wind Farms
- 2015
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- More and larger wind farms are planned offshore. As the most suitable build sites are limited wind farms will be constructed near to each other in so called wind farm clusters. Behind the wind turbines in these farms there is a disrupted flow of air called a wake that is characterized by reduced wind speed and increased turbulence. These individual turbine wakes combine to form a farm wake that can travel a long distance. In wind farm clusters farm to farm interaction will occur, i.e. the long distance wake from one wind farm will impact the wind conditions for other farms in the surrounding area.The thesis contains numerical studies of these long distance wakes. In this study Large Eddy Simulations (LES) using an Actuator Disc method (ACD) are used. A prescribed boundary layer is used where the wind shear is introduced using body forces. The turbulence, based on the Mann model, is introduced as fluctuating body forces upstream of the farm. A neutral atmosphere is assumed. The applied method has earlier been used for studies of wake effects inside farms but not for the longer distances needed for farm to farm interaction.Numerical studies are performed to get better knowledge about the use of this model for long distance wakes. The first study compares the simulation results with measurements behind an existing farm. Three parameter studies are thereafter setup to analyze how to best use the model. The first parameter study examines numerical and physical parameters in the model. The second one looks at the extension of the domain and turbulence as well as the characteristics of the flow far downstream. The third one gathers information on the downstream development of turbulence with different combinations of wind shear and turbulence level. The impact of placing the turbines at different distances from the turbulence plane is also studied. Finally a second study of an existing wind farm is performed and compared with a mesoscale model. The model is shown to be relevant also for studies of long distance wakes. Combining LES with a mesoscale model can be of interest.
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| 33. |
|
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| 34. |
- Eriksson, Ola, 1979-, et al.
(författare)
-
The Long distance wake behind Horns Rev I studied using large eddy simulations and a wind turbine parameterization in WRF
- 2017
-
Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 854
-
Tidskriftsartikel (refereegranskat)abstract
- The aim of the present paper is to obtain a better understanding of long distance wakes generated by wind farms as a first step towards a better understanding of farm to farm interaction. The Horns Rev I (HR) wind farm is considered for this purpose, where comparisons are performed between microscale Large Eddy Simulations (LES) using an Actuator Disc model (ACD), mesoscale simulations in the Weather Research and Forecasting Model (WRF) using a wind turbine parameterization, production data as well as wind measurements in the wind farm wake. The LES is manually set up according to the wind conditions obtained from the mesoscale simulation as a first step towards a meso/microscale coupling.The LES using an ACD are performed in the EllipSys3D code. A forced boundary layer (FBL) approach is used to introduce the desired wind shear and the atmospheric turbulence field from the Mann model. The WRF uses a wind turbine parameterization based on momentum sink. To make comparisons with the LESs and the site data possible an idealized setup of WRF is used in this study.The case studied here considers a westerly wind direction sector (at hub height) of 270 ± 2.5 degrees and a wind speed of 8 ± 0.5 m/s. For both the simulations and the site data a neutral atmosphere is considered. The simulation results for the relative production as well as the wind speed 2 km and 6 km downstream from the wind farm are compared to site data. Further comparisons between LES and WRF are also performed regarding the wake recovery and expansion.The results are also compared to an earlier study of HR using LES as well as an earlier comparison of LES and WRF. Overall the results in this study show a better agreement between LES and WRF as well as better agreement between simulations and site data.The procedure of using the profile from WRF as inlet to LES can be seen as a simplified coupling of the models that could be developed further to combine the methods for cases of farm to farm interaction.
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| 35. |
- Eriksson, Ola, et al.
(författare)
-
Wake downstream of the Lillgrund wind farm - A Comparison between LES using the actuator disc method and a Wind farm Parametrization in WRF
- 2015
-
Ingår i: WAKE CONFERENCE 2015. - : IOP Publishing. ; , s. 012028-
-
Konferensbidrag (refereegranskat)abstract
- Simulations of the Lillgrund wind farm (located between Malmo and Copenhagen) are performed using both Large Eddy Simulation (LES) and mesoscale simulations in WRF. The aim is to obtain a better understanding of wakes generated by entire wind farms in order to improve the understanding of farm to farm interactions. The study compares the results from the two used models for the energy production and the wake characteristics downstream of the wind farm. A comparison is also performed with regards to the production data from the Lillgrund wind farm which has been filtered to be comparable to the case used in the simulations. The studied case, based on a prerun in WRF without any wind farm, has an inflow angle of 222 +/- 2.5 deg, a wind speed at hub height of 9.8 m/s and a near neutral atmosphere. A logarithmic wind shear is used in LES and the turbulence intensity is 5.9%. The WRF simulations use a parameterization for wind farms. The wind farm is treated by the model as a sink of the resolved atmospheric momentum. The total energy extraction and the electrical power are respectively proportional to specified thrust and power coefficients. The generated turbulent kinetic energy are the difference between the total and the electrical power. The LES are performed using the EllipSys3D code applying the actuator disc methodology for representing the presence of the rotors. Synthetic atmospheric turbulence is generated with the Mann model. Both the atmospheric turbulence and the wind shear are introduced using body forces. The production was found to be better estimated in LES. WRF show a slightly higher recovery behind the farm. The internal boundary layer is for the compared simulation setups higher in LES while the wake expansion is about the same in both models. The results from the WRF parameterization could potentially be improved by increasing the grid resolution. For farm to farm interaction a combination of the two methods is found to be of interest.
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| 36. |
- Forsting, Alexander R. Meyer, et al.
(författare)
-
On the accuracy of predicting wind-farm blockage
- 2023
-
Ingår i: Renewable energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 214, s. 114-129
-
Tidskriftsartikel (refereegranskat)abstract
- To assess the uncertainty in blockage quantification, this study proposes a comparison of farm blockage predictions from wind-tunnel experiments, Reynolds Averaged Navier-Stokes based simulations using multiple numerical setups, and analytical models. The influence of the numerical setup is demonstrated to be small if a consistent definition of blockage (able to sort out systematic errors) is used. The effect of domain confinement and turbulence intensity is investigated assessing their range of variability. Different analytical models performed similarly in comparison to the numerical data, demonstrating the best accuracy for realistic spacing between the turbines and supporting their use as reliable engineering tools.
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| 37. |
- Hallgren, Christoffer, et al.
(författare)
-
Brief communication : On the definition of the low-level jet
- 2023
-
Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 8:11, s. 1651-1658
-
Tidskriftsartikel (refereegranskat)abstract
- Low-level jets (LLJs) are examples of non-logarithmic wind speed profiles affecting wind turbine power production, wake recovery, and structural/aerodynamic loading. However, there is no consensus regarding which definition should be applied for jet identification. In this study we argue that a shear definition is more relevant to wind energy than a falloff definition. The shear definition is demonstrated and validated through the development of a European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation reanalysis (ERA5) LLJ climatology for six sites. Identification of LLJs and their morphology, frequency, and intensity is critically dependent on the (i) vertical window of data from which LLJs are extracted and (ii) the definition employed.
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| 38. |
- Hallgren, Christoffer, et al.
(författare)
-
Classification and properties of non-idealized coastal wind profiles - an observational study
- 2022
-
Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 7:3, s. 1183-1207
-
Tidskriftsartikel (refereegranskat)abstract
- Non-idealized wind profiles frequently occur over the Baltic Sea and are important to take into consideration for offshore wind power, as they affect not only the power production but also the loads on the structure and the behavior of the wake behind the turbine. In this observational study, we classified non-idealized profiles as the following wind profiles having negative shear in at least one part of the lidar wind profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum and negative profiles. Using observations spanning over 3 years, we show that these non-idealized profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28-300 m layer) mostly occurred during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the strong shear zone of low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favorable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase in the wind speed from below.
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| 39. |
- Hallgren, Christoffer, et al.
(författare)
-
Looking for an Offshore Low-Level Jet Champion among Recent Reanalyses : A Tight Race over the Baltic Sea
- 2020
-
Ingår i: Energies. - : MDPI. - 1996-1073. ; 13:14
-
Tidskriftsartikel (refereegranskat)abstract
- With an increasing interest in offshore wind energy, focus has been directed towards large semi-enclosed basins such as the Baltic Sea as potential sites to set up wind turbines. The meteorology of this inland sea in particular is strongly affected by the surrounding land, creating mesoscale conditions that are important to take into consideration when planning for new wind farms. This paper presents a comparison between data from four state-of-the-art reanalyses (MERRA2, ERA5, UERRA, NEWA) and observations from LiDAR. The comparison is made for four sites in the Baltic Sea with wind profiles up to 300 m. The findings provide insight into the accuracy of reanalyses for wind resource assessment. In general, the reanalyses underestimate the average wind speed. The average shear is too low in NEWA, while ERA5 and UERRA predominantly overestimate the shear. MERRA2 suffers from insufficient vertical resolution, which limits its usefulness in evaluating the wind profile. It is also shown that low-level jets, a very frequent mesoscale phenomenon in the Baltic Sea during late spring, can appear in a wide range of wind speeds. The observed frequency of low-level jets is best captured by UERRA. In terms of general wind characteristics, ERA5, UERRA, and NEWA are similar, and the best choice depends on the application.
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| 40. |
- Hallgren, Christoffer, et al.
(författare)
-
Machine learning methods to improve spatial predictions of coastal wind speed profiles and low-level jets using single-level ERA5 data
- 2024
-
Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 9:4, s. 821-840
-
Tidskriftsartikel (refereegranskat)abstract
- Observations of the wind speed at heights relevant for wind power are sparse, especially offshore, but with emerging aid from advanced statistical methods, it may be possible to derive information regarding wind profiles using surface observations. In this study, two machine learning (ML) methods are developed for predictions of (1) coastal wind speed profiles and (2) low-level jets (LLJs) at three locations of high relevance to offshore wind energy deployment: the US Northeastern Atlantic Coastal Zone, the North Sea, and the Baltic Sea. The ML models are trained on multiple years of lidar profiles and utilize single-level ERA5 variables as input. The models output spatial predictions of coastal wind speed profiles and LLJ occurrence. A suite of nine ERA5 variables are considered for use in the study due to their physics-based relevance in coastal wind speed profile genesis and the possibility to observe these variables in real-time via measurements. The wind speed at 10 ma.s.l. and the surface sensible heat flux are shown to have the highest importance for both wind speed profile and LLJ predictions. Wind speed profile predictions output by the ML models exhibit similar root mean squared error (RMSE) with respect to observations as is found for ERA5 output. At typical hub heights, the ML models show lower RMSE than ERA5 indicating approximately 5 % RMSE reduction. LLJ identification scores are evaluated using the symmetric extremal dependence index (SEDI). LLJ predictions from the ML models outperform predictions from ERA5, demonstrating markedly higher SEDIs. However, optimization utilizing the SEDI results in a higher number of false alarms when compared to ERA5.
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| 41. |
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| 42. |
- Hallgren, Christoffer, et al.
(författare)
-
The smoother the better? : A comparison of six post-processing methods to improve short-term offshore wind power forecasts in the Baltic Sea
- 2021
-
Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 6:5, s. 1205-1226
-
Tidskriftsartikel (refereegranskat)abstract
- With a rapidly increasing capacity of electricity generation from wind power, the demand for accurate power production forecasts is growing. To date, most wind power installations have been onshore and thus most studies on production forecasts have focused on onshore conditions. However, as offshore wind power is becoming increasingly popular it is also important to assess forecast quality in offshore locations. In this study, forecasts from the high-resolution numerical weather prediction model AROME was used to analyze power production forecast performance for an offshore site in the Baltic Sea. To improve the AROME forecasts, six post-processing methods were investigated and their individual performance analyzed in general as well as for different wind speed ranges, boundary layer stratifications, synoptic situations and in low-level jet conditions. In general, AROME performed well in forecasting the power production, but applying smoothing or using a random forest algorithm increased forecast skill. Smoothing the forecast improved the performance at all wind speeds, all stratifications and for all synoptic weather classes, and the random forest method increased the forecast skill during low-level jets. To achieve the best performance, we recommend selecting which method to use based on the forecasted weather conditions. Combining forecasts from neighboring grid points, combining the recent forecast with the forecast from yesterday or applying linear regression to correct the forecast based on earlier performance were not fruitful methods to increase the overall forecast quality.
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| 43. |
- Hallgren, Christoffer, et al.
(författare)
-
The winds are twisting: analysis of strong directional shear across the rotor plane using coastal lidar measurements and ERA5
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The change of wind direction with height (the directional shear) affects both the power production from a wind turbine, wake effects and aerodynamic loading. In this study, a climatology of the relative occurrence of strong directional shear over Scandinavia is created using 43 years of hourly ERA5 data covering the height range of a modern wind turbine and at wind speeds of operation. It is shown that strong directional shear (≥15° over the rotor) is occurring 20-30% of the time over land and 10-25% of the time over the extended Baltic Sea. The height of the atmospheric boundary-layer and the wind speed at hub height are identified as the most important predictors for strong directional shear, with low boundary-layer heights and weak winds being the main causes. Associated with this, a strong land-sea seasonality is observed. Further, ERA5 is validated against lidar soundings from two coastal sites, both indicating a major underestimation in the distribution of the directional shear in ERA5. Especially in strongly stratified boundary-layers ERA5 struggles, with 25% of the data having errors exceeding 24° and 28° for Östergarnsholm and Utö respectively.
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| 44. |
- Hanssen-Bauer, O. W., et al.
(författare)
-
Comparison of three DWM-based wake models at above-rated wind speeds
- 2023
-
Ingår i: WAKE CONFERENCE 2023. - : Institute of Physics Publishing (IOPP).
-
Konferensbidrag (refereegranskat)abstract
- In this study we investigate three mid-fidelity wind turbine wake models based on the dynamic wake meandering (DWM) model principle, and compare their performance with a reference dataset, produced with large-eddy simulations using the actuator line model. The models are compared with respect to flow field, power, and loads on a row of four 5MW reference turbines experiencing above-rated wind conditions. In general, the DWM models show fairly good agreement with large-eddy simulation for the time-averaged flow fields, blade forces and power, with increasing differences along the turbine row. Also when comparing fatigue loads of blade root moments, the differences between the models increase further into the row, with deviations up to 25 % of the reference case. However, while the development in blade root moment fatigue along the turbine row is predominantly driven by the energy content at the frequency corresponding to the turbine's rotational period (1P) for the DWM models, the large-eddy simulation results suggest that the key drivers for the blade root and tower loads are the increase in meandering and energy at higher frequencies (> 1P) deeper into the turbine row. For the tower loads, the DWM models highly underestimate the fatigue for the waked turbines. From these results, we suggest priorities for future model developments so that robust model implementations can be used in wind farm design and operation.
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| 45. |
- Hanssen-Bauer, O. W., et al.
(författare)
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Dependence of wind turbine loads on inlet flow field
- 2020
-
Ingår i: Science of making torque from wind (TORQUE 2020), pts 1-5. - : IOP Publishing.
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Konferensbidrag (refereegranskat)abstract
- In wind farm simulations, the inflow wind field plays a crucial role in the accuracy of both power production, structural load predictions and the turbulent wake development behind wind turbines. Three different inflow wind field generation techniques, namely the Mann model, a reduced order based model described herein and LES data, are used in this study to characterise the relation between the inflow and the structural response of the wind turbine. In addition, the wake development under different inflow conditions are studied. The turbulence statistics of the reduced-order model and the LES data are similar to each other while the Mann turbulence has different turbulence profiles and spectral characteristics. An in-house developed aeroelastic code, 3Dfloat, is used for structural response analysis. The differences between the inflow fields are mainly attributed to the turbulence intensity profiles, and differences in their spectral characteristics.
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| 46. |
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| 47. |
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| 48. |
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| 49. |
- Ivanell, Stefan, et al.
(författare)
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Actuator disc modelling of wake interaction in Horns Rev wind farm
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Ingår i: Wind Energy. - 1095-4244 .- 1099-1824.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 50. |
- Ivanell, Stefan, et al.
(författare)
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Analysis of numerically generated wake structures
- 2009
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Ingår i: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 1, s. 63-80
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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