| 1. |
- Abedi, Hamidreza, 1979, et al.
(författare)
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Numerical modelling of neutral atmospheric boundary layer flow through heterogeneous forest canopies in complex terrain (a case study of a Swedish wind farm)
- 2021
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Ingår i: Renewable Energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 180, s. 806-828
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Tidskriftsartikel (refereegranskat)abstract
- This paper exposes the risk of generalization of wind conditions from a single met-mast measurement to be representative of the actual flow field in a wind farm situated in complex terrain. As a case study, Large-Eddy Simulation (LES) of the neutral Atmospheric Boundary Layer (ABL) flow for a mid-western Sweden wind farm is performed. The site-specific complex topography and the forest properties like the Plant Area Density and the tree heights are extracted from the Airborne Laser Scanning (ALS) 3D data, thus the forest is heterogeneous. To emphasize the impact of the local topography and surface roughness on the wind field, the wind turbines are not included in the numerical simulations. The predicted wind speeds using LES are compared to wind speed from the nacelle-mounted anemometers taken from the wind farm's turbine SCADA data, focusing on the wake-free turbines. A sufficient degree of match is observed, supporting the accuracy of the numerical simulations. The results show that inflow variables i.e., mean wind speed, shear exponent and turbulence intensity vary at each wind turbine location justifying the need for turbine-specific assessment of the wind resource in a wind farm located in forested complex terrain. The inter-turbine (between turbines in the wind farm) differences in wind resource is quantified in terms of the difference in turbine-specific structural and mechanical loads by running wind turbine mechanical simulations using the extracting the wind fields predicted by the LES. The results show that not only inter-turbine loads varying significantly in the wind farm, but the turbine loads also differ significantly if a homogeneous assumption is made for the forest. Most importantly, it was found that the homogeneous forest assumption predicted a higher turbulence intensity compared to a heterogeneous forest resulting.
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| 2. |
- Sarkar, Saptarshi, 1992, et al.
(författare)
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Site-specific analysis of on wind turbines in complex terrain: A case study
- 2021
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Ingår i: WESC2021, Wind Energy Science Conference , The Conference Book (Intro) with the Book of Abstracts (Theme 01 - 10). ; Theme 02, s. 2-111-2-112
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This study aims to analyse and understand how the structural and drivetrain loads on the turbines can vary within a wind farm situated in a complex terrain. To this end, the flow field is studied using computational fluid and aero-elastic simulations and a modified version of the FAST software is used to determine the turbine response Verification of the numerical results is performed by using data from turbine SCADA system for a case study of Röbergsfjället wind farm with 8 turbines located in the mid-western part of Sweden.
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| 3. |
- Wickström, Anders, et al.
(författare)
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Analysis of wind turbines under harsh operation conditions
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This project focuses on the harsh operational conditions that sometimes lead to gearbox and bearing failures before their expected lifespan have been reached. Individual wind turbines that are subjected to extreme wind conditions and other severe operational conditions have been identified in close cooperation with wind turbine operators. Methods for determining the operational severity from data acquired during operation have been developed. Raw measured data have been processed and compared to the assumed data that were used in the wind turbine design process. Feedbacks from maintenance reports have been used to strengthen the relevance of these indices. A generic V90 turbine model has been created for the simulations using both FAST and VIDYN aero elastic simulation codes. The wind turbine dynamical behaviour has been analysed in arbitrary wind conditions and grid dynamics. The assessment has been made using 17 wind turbines of same design but with different operational conditions. The data from these turbines have been compared and comparison has also been made using simulated data for complex and flat terrains. The results show that the complex terrain increases the fatigue on the gearbox shaft and the fluctuation of wind direction in the complex terrain is much higher than for the flat terrain, resulting in more destructive fatigue loads. Simulation also shows that de-rating of the turbine has a significant impact on the loads in the drive train but no significant impact on the fatigue loads on the nacelle. There are different systems running for capturing operational data during operation. Normally the system stores only one measurement data point at each 20 seconds, i.e. 0.05 Hz. Then it is difficult to observe any details related to loads based on turbulence or other environment or technical conditions. For that reason, the project has managed to get higher resolution measurement data, 1 Hz sample rate, for specific turbines in the available fleets. The complex topography of a specific wind farm was extracted from LASer data and it was imported into the Computational Fluid Dynamics (CFD) software, STAR-CCM+, to generate the computational wind grid for the numerical simulations. The on-site meteorology mast data, provided by project partners, were used to determine the dominant mean wind speed/direction and turbulence intensity. A system simulation model in FAST or VIDYN can reasonably well predict the hub forces and mechanical torque of a Vestas V90 turbine under different kind of wind load in operation. A turbine designer is allowed to use any specific wind conditions as input for the load calculations. It might be outside the wind classes specified in IEC-61400-1 and therefore denoted class S, where the specific input conditions are stated. Using the calculated data for the complex terrain, a class 3S turbine has been defined with turbulence intensity, TI=0.17, shear α = 0.34 in comparison with standard wind turbine class 3A with TI=0.16 and α = 0.20. When using these input conditions, a fatigue load comparison and evaluation has been done. It shows that both drive torque and tower top bending moment increase with about 6 percent when class3S is used.
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| 4. |
- Abedi, Hamidreza, 1979
(författare)
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Assessment of flow characteristics over complex terrain covered by the heterogeneous forest at slightly varying mean flow directions: (A case study of a Swedish wind farm)
- 2023
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Ingår i: Renewable Energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 202, s. 537-553
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Tidskriftsartikel (refereegranskat)abstract
- The impact of heterogeneous forest canopies and complex terrain on the horizontal distortion of the inflow is studied. Large-Eddy Simulation (LES) of the neutral Atmospheric Boundary Layer (ABL) flow is performed for a wind farm in Sweden for three cases associated with three different wind directions at the range of the static yaw misalignment (≃±6∘) where the yaw control system is not activated. The ground topography and forest properties for the numerical modeling are extracted from the Airborne Laser Scanning (ALS) 3D data. The wind turbines within the wind farm are introduced using the actuator disk model. To focus on the airflow deflection only by the complex terrain and vegetation, the study is limited to upstream wind turbines without any wake interaction. The predicted mean wind speed and turbulence intensity for the upstream wind turbines are compared against the nacelle-mounted anemometers taken from the wind farm's turbine SCADA data. To quantify the additional load and moments induced at the rotor blades by the horizontal misalignment of the incoming flow, aero-structural simulation of the upstream wind turbines in the wind farm for all three cases is performed. The results show that the horizontal distortion of the inflow over the rotor swept area is usually kept below the range of static yaw misalignment (≃6∘) for the majority of the upstream wind turbines for all three cases. However, the impact of a large vertical shear exponent leading to misinterpretation of the results must be taken into consideration. Furthermore, the load imbalance of the rotor due to the vertical wind shear has the least direct contribution to the yaw moment. However, for a mean vertical shear exponent larger than α=0.25, contrary to expectation, a positive mean yaw moment under the positive-yawed inflow may be observed.
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| 5. |
- Abedi, Hamidreza, 1979, et al.
(författare)
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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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Ingår i: Journal of Solar Energy Engineering, Transactions of the ASME. - : ASME International. - 1528-8986 .- 0199-6231. ; 144:2
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Tidskriftsartikel (refereegranskat)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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| 6. |
- Abedi, Hamidreza, 1979, et al.
(författare)
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Development of blade element momentum (BEM) method for hydropower
- 2022
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Ingår i: IOP Conference Series: Earth and Environmental Science. - : IOP Publishing. - 1755-1307 .- 1755-1315. ; 1079:1
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Konferensbidrag (refereegranskat)abstract
- The BEM method is extensively used for analyzing the aerodynamic performance of wind turbines and marine propellers. It is computationally fast and is easily implemented while it can give fairly accurate results. Application of the BEM method to predict the forces acting on rotor blades for a model scale axial shaft-driven Counter-Rotating Pump-Turbine (CRPT) is investigated. Some modifications have been proposed to adopt the classical BEM method for CRPT machine and the results are validated against results from Computational Fluid Dynamics (CFD). The results display that the proposed modifications can improve the loading predicted by BEM. However, the improvements are more pronounced in pump mode rather than turbine mode.
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| 7. |
- Abedi, Hamidreza, 1979, et al.
(författare)
-
Development of Free Vortex Wake Method for Aerodynamic Loads on Rotor Blades
- 2014
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Ingår i: EWEA 2014: Europe’s Premier Wind Energy Event, Barcelona, Spain.
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Konferensbidrag (refereegranskat)abstract
- The aerodynamics of a wind turbine is governed by the flowaround the rotor, where the prediction of air loads on rotorblades in different operational conditions and its relation to rotor structural dynamics is crucial for design purposes. One of the most important challenges in wind turbine aerodynamics is therefore to accurately predict the forces on the blade, where the blade and wake are modeled by different approaches such as the Blade Element Momentum (BEM) theory, the vortex method and Computational Fluid Dynamics (CFD).A free vortex wake method, based on the potential, inviscidand irrotational flow, is developed to study the aerodynamicloads. The results are compared with the BEM method,the GENUVP code and CFD.
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| 8. |
- Abedi, Hamidreza, 1979, et al.
(författare)
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Development of Free Vortex Wake Method for Yaw Misalignment Effect on the Thrust Vector and Generated Power
- 2014
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Ingår i: 32nd AIAA Applied Aerodynamics Conference 2014; Atlanta, GA; United States; 16 June 2014 through 20 June 2014. - Reston, Virginia : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- Wind power is currently one of the most reliable new energy sources serving as an alternative to fossil fuel generated electricity and is known as a widely distributed clean and renewable source of energy. It is now the world's fastest growing energy source and has also become one of the most rapidly expanding industries. The aerodynamics of a wind turbine is governed by the flow around the rotor, where the prediction of air loads on rotor blades in different operational conditions and their relation to rotor structural dynamics is crucial for design purposes. One of the challenges in wind turbine aerodynamics is the yaw condition where the undisturbed upstream flow is not perpendicular to the rotor plane, giving a non-uniform blade load which is contrary to the axisymmetric flow assumption in the BEM (Blade Element Momentum) method. However, there are some engineering methods modifying the BEMmethod for yaw misalignment situations,1 where they often calculate the skewed axial induction factor as an average value over the rotor disk which is insensitive to the blade rotation direction. On the other hand, experiments show that the thrust vector for a positive yaw misalignment differs from that for a negative yaw misalignment. A free vortex wake method, based on the potential, inviscid and irrotational flow, is developed to study the deviation of thrust vector relative to rotor shaft. The results are compared with the BEM method2 and experimental data. A two-bladed variable speed wind turbine, the Hönö wind turbine,3 is used for this study.
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| 9. |
- Abedi, Hamidreza, 1979, et al.
(författare)
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Development of Free Vortex Wake Model for Wind Turbine Aerodynamics under Yaw Condition
- 2016
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Ingår i: 34th Wind Energy Symposium, 2016; San Diego; United States; 4 January 2016 through 8 January 2016. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103957
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Konferensbidrag (refereegranskat)abstract
- The aerodynamics of a wind turbine is governed by the flow around the rotor. One of the most severe operating conditions for a wind turbine is the yaw misalignment, when the mean upstream flow is not perpendicular to the rotor plane. This asymmetrical flow changes significantly the velocity field around the rotor blades which in turn reduces power production of the wind turbine. It also makes a periodic load variation along the rotor blade which accordingly increases the fatigue load from the design point of view. In this paper, the effect of the skewed wake, due to the yaw misalignment, on the wake aerodynamics of wind turbine is studied. For this purpose, an in-house Vortex Lattice Free Wake (VLFW) code, based on the potential, inviscid and irrotational flow, is developed. The results are compared with the MEXICO wind tunnel measurements. For the axial traverses, there is a good agreement between the measured axial (w) and tangential (u) velocity components and the simulations. Although the magnitude of the mean radial velocity component (v) is fairly well predicted, its fluctuation is not captured by the simulation. Moreover, for the radial traverses, the simulations are remarkably verified by the measurements.
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| 10. |
- Abedi, Hamidreza, 1979
(författare)
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Development of Vortex Filament Method for Aerodynamic Loads on Rotor Blades
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wind power is currently one of the most reliable new energy sources serving as an alternative tofossil fuel generated electricity and is known as a widely distributed clean and renewable sourceof energy. It is now the world’s fastest growing energy source and has also become one of themost rapidly expanding industries.The aerodynamics of a wind turbine are governed by the flow around the rotor, where theprediction of air loads on rotor blades in different operational conditions and its relation to rotorstructural dynamics is crucial for design purposes. One of the most important challenges in windturbine aerodynamics is therefore to accurately predict the forces on the blade, where the bladeand wake are modeled by different approaches such as the Blade Element Momentum (BEM)theory, the vortex method and Computational Fluid Dynamics (CFD). Here, the application ofthe vortex filament method for wind turbine aerodynamic performance is used. Different blademodels such as the lifting line and the lifting surface with prescribed and free wake models arestudied. The main purpose is to find the proper combination of blade and wake models for theaerodynamic loads as well as the computational time in order to develop an accurate and efficientaerodynamic tool. The results of the different approaches are compared with the BEM methodand GENUVP code (see the acknowledgments).
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