| 1. |
- Abedi, Hamidreza, 1979, et al.
(author)
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Assessment of Wind Field Generation Methods on Predicted Wind Turbine Power Production Using a Free Vortex Filament Wake Approach
- 2022
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In: Journal of Solar Energy Engineering, Transactions of the ASME. - : ASME International. - 1528-8986 .- 0199-6231. ; 144:2
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Journal article (peer-reviewed)abstract
- The generated power and thrust of a wind turbine strongly depend on the flow field around the turbine. In the present study, three different inflow methods, i.e., a time series (TS) from large eddy simulation (LES) of atmospheric boundary layer flow field, a synthetic turbulent flow field using the Mann model (MM), and a steady-state mean wind profile with shear, are integrated with the free vortex filament wake method to investigate the effect of wind field generation methods on the wind turbine performance where the impact of the turbine and the trailing wake vortices on the turbulent flow fields is ignored. For this purpose, an in-house vortex lattice free wake (VLFW) code is developed and used to predict the aerodynamic loads on rotor blades. The NREL 5-MW reference wind turbine is used for the VLFW simulations. For a fair assessment of different inflow generation methods on power production of a wind turbine, it is not sufficient that the generated wind fields employed in the TS and MM methods have the same streamwise mean velocity and turbulence intensity at hub height. Instead, the generated inflows must have equivalent power-spectral densities especially at low frequencies since the rotor blades essentially respond to the large-scale fluctuations (macroscopic scales) rather than small-scale fluctuations (microscopic scales). A faster energy decay rate of LES inflow leads to a higher energy content in the TS method at low frequencies (associated with the macroscopic dynamics of the rotor blades). This extra kinetic energy results in a slightly higher mean power production while using the TS method although the inflow conditions at hub height/rotor plane are the same for both the TS and MM methods. Moreover, the impact of simulation time (the length of time integration) on the power production of a wind turbine (exposed to an unsteady inflow) must be taken into account. A short simulation time remarkably affects the mean wind speed over the rotor area for identical turbulent inflows. For Taylor's hypothesis application using a single LES flow field, the results show a significant difference in the mean powers corresponding to the different realizations due to large turbulent fluctuations.
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| 2. |
- Nebenführ, Bastian, 1984
(author)
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Aerodynamic and Aeroacoustic Analysis of a Multi-Element Airfoil using Hybrid RANS/LES Modeling Approaches
- 2012
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Licentiate thesis (other academic/artistic)abstract
- Global hybrid RANS/LES simulations were used for computing the turbulent flow around a three-element airfoil. Based on the results of the flow computations, a comprehensive analysis of the flow properties was performed. The mean pressure distribution obtained from the simulations, was found to be in excellent agreement with experiments, when using an adequate spanwise domain extent and spanwise grid resolution. Initially, strong delays of the slat shear layer instability were observed, which could be overcome partly by adapting the grid resolution.Moreover, unsteady flow data from the hybrid RANS/LES simulations was extracted at a permeable surface in the near-field of the airfoil and used as input for wave propagation calculations. Those calculations were done via three acoustic analogies, namely the Kirchhoff analogy, the Ffowcs-Williams and Hawkings analogy and the Curle analogy. In the present acoustical analysis, the performance of the three acoustic analogies is investigated. The noise contribution of all three elements of the airfoil was studied as well as their emission behavior.Along with the aerodynamic and aeroacoustic analysis of the high-lift flow, hybrid RANS/LES modeling has been considered. As the so-called gray area problem of hybrid RANS\slash LES methods was encountered for the shear layer emanating from the slat cusp, several alleviation techniques were implemented and tested for a fundamental mixing layer flow.
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| 3. |
- Nebenführ, Bastian, 1984, et al.
(author)
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Development of a reduced-order model for wind turbine response to atmospheric turbulence in forest regions
- 2014
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In: Proceedings of 6th International Symposium on Computational Wind Engineering, Hamburg, Germany, 8-12 June, 2014.
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Conference paper (peer-reviewed)abstract
- A reduced-order structural model of a wind turbine has been developed and coupled with wind shear and turbulence fields generated computationally by Large-Eddy Simulation (LES). The wind turbine is composed of rigid but rotating blades on top of a flexible tower structure. Turbulence data from LES over a forest and over low-roughness flat terrain were tested in the proposed model. It was found that, while maintaining the same mean wind speed at the hub height, turbulence appears strongly increased in the atmospheric boundary layer above a forest. Subsequently, the dynamic loads on the wind turbine structure are more than doubled for the case with forest.
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| 4. |
- Nebenführ, Bastian, 1984, et al.
(author)
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Hybrid RANS-LES Simulation of Turbulent High-Lift Flow in Relation to Noise Generation
- 2012
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In: Notes on Numerical Fluid Mechanics and Multidisciplinary Design. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1612-2909 .- 1860-0824. ; 117
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Conference paper (peer-reviewed)abstract
- Turbulence-resolving simulations have been performed using hybrid RANS-LES approaches for the turbulent flow around a three-element high-lift configuration. The main purpose is to explore the effect of some modeling-related numerical aspects on the simulation of resolved velocity and pressure fluctuations as potent noise-generating sources. Along with a presentation of resolved instantaneous and mean flow features, the impact of the time step and the spanwise extent of the computational domain is investigated. It is shown that the temporal resolution and the spanwise extension of the computational domain impose effects not only on the prediction of mean flow, but more significantly on the correlation of resolved turbulent structures, which may consequently affect the accuracy of flow-generated noise properties.
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| 5. |
- Nebenführ, Bastian, 1984, et al.
(author)
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Hybrid RANS/LES Simulations for Aerodynamic and Aeroacoustic Analysis of a Multi-Element Airfoil
- 2013
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In: 19th AIAA/CEAS Aeroacoustics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624102134 ; , s. 64-
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Conference paper (peer-reviewed)abstract
- A hybrid RANS/LES modeling approach is used for simulating the turbulent flow around a three-element airfoil in high-lift configuration. A detailed analysis of the flow is made, based on the simulation outcome. A comprehensive aeroacoustic analysis involving all three elements of the airfoil is also presented. To provide input data for acoustic analogies, the results of the simulation are sampled at a permeable stationary surface near the airfoil and at the airfoil itself. The far-field noise signature of the high-lift airfoil is computed with the help of the Kirchhoff integral surface method, the Ffowcs-Williams and Hawkings method for a stationary, permeable surface, and the Curle method. The sound pressure level spectrum exhibits a broad-banded shape with several narrow-banded tonal peaks at low Strouhal numbers. The broad-banded peak at high Strouhal numbers, which is typically associated with vortex shedding behind the blunt slat trailing edge, was also captured. Using Curle's acoustic analogy, the noise emission pattern of the three elements is explored, isolated from each other, revealing that both slat and flap act as dipoles. By refining the used grid, the flow results are significantly improved in terms of slat shear layer instability and resolved turbulent content as compared to our previous work.
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| 6. |
- Nebenführ, Bastian, 1984, et al.
(author)
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Influence of a forest canopy on the neutral atmospheric boudary layer - A LES study
- 2014
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In: ETMM10: 10th International ERCOFTAC Symposium on Turbulence Modelling and Measurements.
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Conference paper (peer-reviewed)abstract
- Large-eddy simulation was used to investigate the influence of a forest on the neutral atmospheric boundary layer. This can yield important information for the design and placement of wind turbines in forest regions. Two simulations were conducted: one with and one without the forest. The simulation with forest showed good agreement with field measurements. It was found that the forest significantly increases both the wind shear and the turbulence intensity at the hub-height of an imaginary wind turbine. Both quantities clearly exceed the recent wind turbine design criteria.
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| 7. |
- Nebenführ, Bastian, 1984, et al.
(author)
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Large-Eddy Simulation for Wind Turbine Fatigue Load Calculation in Forest Regions
- 2013
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In: Proceedings of 9th PhD Seminar on Wind Energy in Europe, September 18-20, 2013, Uppsala University Campus Gotland, Sweden.
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Conference paper (other academic/artistic)abstract
- Large-Eddy Simulations have been used for predicting theatmospheric boundary layer in neutral stability under the influence of a forest. The objective is to generate input fields for load calculations that accurately represent the flow conditions above forest regions. In the simulations, the forest is modeled in terms of an additional drag force in the lowest part of the domain. This forest model has proved to be working reliably and being versatile, as it is easily adapted to many different sorts of trees. Two different LES cases are considered in the present work: one regular LES that can be considered the baseline case and a LES with additional forcing from synthetic turbulence in the upper part of the canopy. The mean velocity profile is shown to agree well with measurements from a test site in southern Sweden for the forcing case. However, the forcing case is not able to predict the shear stress profile well.
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| 8. |
- Nebenführ, Bastian, 1984, et al.
(author)
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Large-Eddy Simulation Study of Thermally Stratified Canopy Flow
- 2015
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In: Boundary-Layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 156:2, s. 253-276
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Journal article (peer-reviewed)abstract
- A number of large-eddy simulations (LES) are performed for the calculation of the airflow over a horizontally homogeneous forest canopy for a wide range of thermal stability classes. For the first time, results from LES of a stably stratified canopy are also presented. Simulation results compare favourably to recent field measurements over a pine forest in south-eastern Sweden. The simple heat source model was found to perform adequately and to yield within-canopy heat-flux profiles typically observed for stable conditions in the field. Evidence was found for a layer of unstably stratified air in the canopy trunk space under stable stratification. The importance of a secondary wind-speed maximum is emphasized in stable conditions. Examination of the budget equation of turbulent kinetic energy (TKE) revealed that, during stable stratification, pressure transport plays an increasingly important role in supplying the canopy region with TKE.
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| 9. |
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| 10. |
- Nebenführ, Bastian, 1984, et al.
(author)
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Prediction of wind-turbine fatigue loads in forest regions based on turbulent LES inflow fields
- 2017
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In: Wind Energy. - : Wiley. - 1099-1824 .- 1095-4244. ; 20:6, s. 1003-1015
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Journal article (peer-reviewed)abstract
- Large-eddy simulations (LES) were used to predict the neutral atmospheric boundary layer over a sparse and a dense forest, as well as over grass-covered flat terrain. The forest is explicitly represented in the simulations through momentum sink terms. Turbulence data extracted from the LES served then as inflow turbulence for the simulation of the dynamic structural response of a generic wind turbine. In this way, the impact of forest density, wind speed and wind-turbine hub height on the wind-turbine fatigue loads was studied. Results show for example significantly increased equivalent fatigue loads above the two forests. Moreover, a comparison between LES turbulence and synthetically generated turbulence in terms of load predictions was made and revealed that synthetic turbulence was able to excite the same spectral peaks as LES turbulence but lead to consistently lower equivalent fatigue loads. Copyright (c) 2016 John Wiley & Sons, Ltd.
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| 11. |
- Nebenführ, Bastian, 1984
(author)
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Turbulence-resolving simulations for engineering applications
- 2015
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Doctoral thesis (other academic/artistic)abstract
- Most fluid flows of industrial interest are turbulent and their accurate representation may be of vital importance for the design process of new products. To date, steady RANS methods are usually employed for the simulation of turbulent flows of everyday engineering problems. These methods base the description of turbulence characteristics on mean-flow gradients and only provide a solution for the mean flow. However, there are applications that require instantaneous flow information, for which the use of unsteady, turbulence-resolving simulation techniques is indispensable. In this thesis, the latter have been applied to predict two flow problems of industrial importance. Additionally to providing the flow solution, the simulation method's capability of producing input data for subsequent multi-disciplinary analysis was evaluated.In the first case, hybrid RANS/LES methods were used for simulating the complex flow around a three-element airfoil with deployed high-lift devices. Instantaneous flow properties were extracted from the simulation via a sampling surface and served as input for a subsequent aeroacoustic analysis of the airfoil using acoustic analogies. It was found that the chosen hybrid RANS/LES simulation technique was well-suited for computing the flow. Moreover, decoupling the flow simulation and the noise propagation enables aeroacoustic analysis and farfield-noise prediction for complex geometries at relevant Reynolds numbers. The slat was confirmed to be a major contributor to high-lift noise. Careful placement of the sampling surface, so as to enclose all turbulent noise sources, seems to be of paramount importance, in particular for using the Kirchhoff analogy.The second case dealt with LES simulations of the atmospheric boundary layer above and inside forest regions. Also from these simulations, instantaneous turbulence data were extracted and used in subsequent fatigue-load calculations for a wind turbine. It was expected that the presence of a forest leads to stronger atmospheric turbulence and increased wind shear, compared to flow over low-roughness flat terrain. By simulating the atmospheric boundary layer with and without a forest, this expectation could be verified and it was possible to quantify the effect of the forest on the wind-turbine fatigue loads. It could be shown that typical loads are increased by a factor of almost three in terms of root-mean-square values and equivalent fatigue loads.
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| 12. |
- Peng, S. H., et al.
(author)
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Lessons learned from hybrid RANS-LES computations of a three-element airfoil flow
- 2013
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In: 21st AIAA Computational Fluid Dynamics Conference. - Reston, Virginia : American Institute of Aeronautics and Astronautics.
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Conference paper (peer-reviewed)abstract
- Some important aspects of hybrid RANS-LES modelling for a high-lift flow have been discussed on the basis of a number of simulations with different computational settings tabulated in nine cases. By means of comparative studies of these cases grouped in different sets, the effect of the simulation strategies invoked in the computations has been investigated, including local transition (specified around the leading edges of the high-lift elements), hybrid RANS-LES methods, temporal resolution, as well as the extension of computational domain in the spanwise (homogeneous) direction and the grid resolution in this direction. In exploring the effects of these issues, the lessons learned from these computations are also speculated.
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