| 1. |
- 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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| 2. |
- Abedi, Hamidreza, 1979, et al.
(författare)
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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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| 3. |
- 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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| 4. |
- 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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| 5. |
- Abedi, Hamidreza, 1979, et al.
(författare)
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Enhancement of Free Vortex Filament Method for Aerodynamic Loads on Rotor Blades
- 2017
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Ingår i: Journal of Solar Energy Engineering, Transactions of the ASME. - : ASME International. - 1528-8986 .- 0199-6231. ; 6B
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Tidskriftsartikel (refereegranskat)abstract
- The aerodynamics of a wind turbine is governed by the flow around the rotor, where theprediction of air loads on rotor blades in different operational conditions and its relationto rotor structural dynamics is one of the most important challenges in wind turbine rotorblade design. Because of the unsteady flow field around wind turbine blades, predictionof aerodynamic loads with high level of accuracy is difficult and increases the uncertaintyof load calculations. An in-house vortex lattice free wake (VLFW) code, based on theinviscid, incompressible, and irrotational flow (potential flow), was developed to studythe aerodynamic loads. Since it is based on the potential flow, it cannot be used to predictviscous phenomena such as drag and boundary layer separation. Therefore, it must becoupled to tabulated airfoil data to take the viscosity effects into account. Additionally, adynamic approach must be introduced to modify the aerodynamic coefficients forunsteady operating conditions. This approach, which is called dynamic stall, adjusts thelift, the drag, and the moment coefficients for each blade element on the basis of the two dimensional(2D) static airfoil data together with the correction for separated flow. Twodifferent turbines, NREL and MEXICO, are used in the simulations. Predicted normaland tangential forces using the VLFW method are compared with the blade elementmomentum (BEM) method, the GENUVP code, and the MEXICO wind tunnel measurements. The results show that coupling to the 2D static airfoil data improves the load andpower predictions while employing the dynamic stall model to take the time-varying operating conditions into consideration is crucial.
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| 6. |
- Abedi, Hamidreza, 1979, et al.
(författare)
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Numerical Studies of the Upstream Flow Field around a Horizontal Axis Wind Turbine
- 2015
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Ingår i: 33rd Wind Energy Symposium, Kissimmee, Florida. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103445 ; , s. 12-
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Konferensbidrag (refereegranskat)abstract
- The aerodynamics of a wind turbine is governed by the flow around the rotor. Prediction of the velocityfield, both upstream and downstream, is one of the challenges for wind turbine performance in terms of theaerodynamic loads and the generated power at different operational conditions. For simplicity, the wind velocityat the rotor plane is assumed to be equal to far upstream flow where the interaction of the rotor bladeswith upstream flow, close to the rotor plane, is not taken into account. This paper aims to study the effect ofthe rotor blade azimuthal position and the trailing wake, on upstream and downstream flow near to the rotorplane. For this purpose, an in-house Vortex Lattice Free Wake (VLFW) code, based on the potential, inviscidand irrotational flow, is developed. The results are compared with the MEXICO wind tunnel measurements.They show that the wind speed decreases in the axial direction upstream the rotor plane because of the inducedvelocity field by the rotor blades and the trailing wake vortices. This leads to a power reduction of thewind turbine. Furthermore, contrary to the traditional actuator disk model, the VLFW simulations predictsa tangential velocity component upstream the rotor due to the blade rotation which is in agreement with themeasurement data. Finally, it is found that the flow field downstream and upstream the rotor blades dependson the blade azimuthal direction.
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| 7. |
- Abedi, Hamidreza, 1979, et al.
(författare)
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Vortex Method Application for Aerodynamic Loads on Rotor Blades
- 2013
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Ingår i: EWEA 2013: Europe’s Premier Wind Energy Event, Vienna, 4-7 February 2013. - 9781632663146 ; 2, s. 912-921
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Konferensbidrag (refereegranskat)abstract
- Today, wind power is one of the most reliable new energysource serving as an alternative to fossil-fuel generated electricityand is known as widely-distributed clean and renewable energysource. It is now the world’s fastest growing energy sourceand has also become one of the most rapidly expanding industries.The aerodynamics of a wind turbine are governed by theflow around the rotor where the prediction of air loads on rotorblades in different operational conditions and its relation torotor structural dynamics is crucial for design purposes. Therefore,one of the most important challenges in wind turbine aerodynamicsis to predict the forces on the blade accurately wherethe blade and wake are modeled by different approaches suchas Blade Element Momentum (BEM) theory, vortex method andComputational Fluid Dynamics (CFD). In this paper, the applicationof vortex method for wind turbine aerodynamic performanceis used. Different blade models such as lifting line andlifting surface with prescribed wake model are studied. The mainpurpose is to find the proper combination of blade and wakemodel influencing the aerodynamic loads as well as the computationaltime efficiency. The results of different approaches arecompared with the GENUVP code. (See acknowledgements)
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| 8. |
- Altintas, Atilla, 1979, et al.
(författare)
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A new approximation to modulation-effect analysis based on empirical mode decomposition
- 2019
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 31:2
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Tidskriftsartikel (refereegranskat)abstract
- The modulation effect, namely, the amplification or attenuation of near-wall small-scale (SS) structures by outer large-scale (LS) structures, is one of two commonly accepted ways that outer LS turbulent fluctuations can influence near-wall ones. Mode decomposition based on filtering is widely used to analyze the modulation effect. In the present study, a new approximation is proposed based on empirical mode decomposition (EMD) to investigate the aforementioned amplitude modulation effect. Both methods are used, and their results are compared for two-point and single-point analyses. It has been shown that the LS and SS signals that are decomposed by filtering and EMD follow identical paths. Despite the similarities of the signals, the suggested method exhibits a slightly higher correlation coefficient R compared to the method based on filtering for the two-point analysis. For the one-point analysis, however, the suggested method gives a rational correlation coefficient for the one-point analysis compared to the two-point analysis, while the existing method seems far from giving a rational correlation coefficient value, which is too low compared to that of the two-point analysis. The suggested method is relevant to many recent studies that questioned the reliability of calculating the correlation coefficient with the existing method. The variation of R for identical signals extends the discussion of the correlation-coefficient calculations to the very first process, namely, obtaining LS and SS data from the original signal.
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| 9. |
- Altıntaş, Atilla, et al.
(författare)
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A new approximation to modulation effect analysis based on empirical mode decomposition method
- 2018
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Ingår i: 10th International Conference on Computational Fluid Dynamics, ICCFD 2018 - Proceedings.
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Konferensbidrag (refereegranskat)abstract
- 'Modulation effect' which represents the amplification or attenuation of the SS (small-scale) structures near the wall by the outer LS (large-scale) structures is one of the two commonly accepted influence of the outer large-scale energy containing motions on the near wall turbulent fluctuations. Mode decomposition based on filtering suggested by Mathis et al. [1] is a widely used method for the analysis of the modulation effect. In the present study a new approximation is proposed based on the empirical mode decomposition (EMD) to investigate the aforementioned amplitude modulation effect. Both Mathis et al. [1] and suggested methods are being used and results are compared for two-point and single-point analysis. It has been shown that the LS and SS signals that are decomposed by filtering and EMD follow identical path. Although the similarities of the signals, the correlation coefficient, R, differ for the two methods. We observed that the suggested method in the present work exhibits a slightly higher correlation coefficient compared to the method based on filtering for the two-point analysis. However for the one-point analysis existing method seems far away to give a rational correlation coefficient value, which is too low compared to the suggested method. Therefore we believe that the suggested method which decomposes the signal by EMD instead of filtering adresses many recent studies that questions the reliability of the calculation of the correlation coefficient, R, by existing method. We believe that the variation of the correlation coefficient, R, for the identical signals extends the discussion of the correlation coefficient calculations to the very first process, that is obtaining LS and SS from the original signal.
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| 10. |
- Altintas, Atilla, 1979, et al.
(författare)
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Direct numerical simulation analysis of spanwise oscillating Lorentz force in turbulent channel flow at low Reynolds number
- 2017
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Ingår i: Acta Mechanica. - : Springer Science and Business Media LLC. - 1619-6937 .- 0001-5970. ; 228:4, s. 1269-1286
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Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations of a turbulent channel flow at low Reynolds number ($Re_{\tau} = 180$, based on the driving pressure gradient and channel half width) are performed. Some results are also presented for $Re_{\tau} = 400$. In this work we apply an idealized spanwise Lorentz force near the lower wall of the channel and compared the results for the applied force and no-force cases in both the upper half and the lower half of the channel. We have studied two-point correlations to explain the effect of the Lorentz force on streamwise vortices and streaky structures. Despite the observation of the clear stabilization of the streaky structures in the vicinity of the wall, the existence of the streamwise vortices is explained by the well-known turbulence regeneration cycle, which improves the understanding of streaky and streamwise vortex structure formation on turbulence generation. Spanwise oscillating Lorentz force effects on the Rankine vortex structures are investigated. Our results lead us to establish an explanation on the effect of sweep and ejection events on the mean vortex structures in the flow field. A mean vortex structure is defined by the time-averaged location of the local minimum and maximum of the streamwise r.m.s. vorticity. We also depict turbulence production rates for both cases and compared the lower and upper half of the channel.
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