| 1. |
- 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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| 2. |
- 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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| 3. |
- Asmuth, Henrik
(författare)
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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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| 4. |
- 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).
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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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| 5. |
- Asmuth, Henrik, et al.
(författare)
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The Actuator Line Model in Lattice Boltzmann Frameworks : Numerical Sensitivity and Computational Performance
- 2019
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 1256
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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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| 6. |
- Asmuth, Henrik, et al.
(författare)
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WakeNet 0.1 : A Simple Three-dimensional Wake Model Based on Convolutional Neural Networks
- 2022
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Ingår i: Journal of Physics, Conference Series. - : Institute of Physics Publishing (IOPP). - 1742-6588 .- 1742-6596. ; 2265:2
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Tidskriftsartikel (refereegranskat)abstract
- Deep convolutional neural networks are a promising machine learning approach for computationally efficient predictions of flow fields. In this work we present a simple modelling framework for the prediction of the time-averaged three-dimensional flow field of wind turbine wakes. The proposed model requires the mean inflow upstream of the turbine, aerodynamic data of the turbine and the tip-speed ratio as input data. The output comprises all three mean velocity components as well as the turbulence intensity. The model is trained with the flow statistics of 900 actuator line large-eddy simulations of a single turbine in various inflow and operating conditions. The model is found to accurately predict the characteristic features of the wake flow. The overall accuracy and efficiency of the model render it as a promising approach for future wind turbine wake predictions.
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| 7. |
- 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
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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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| 8. |
- Asmuth, Henrik, et al.
(författare)
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Wind Turbine Response in Waked Inflow: A Modelling Benchmark Against Full-Scale Measurements
- 2022
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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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| 9. |
- Diaz, Gonzalo Pablo Navarro, et al.
(författare)
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Actuator line model using simplified force calculation methods
- 2023
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Ingår i: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 8:3, s. 363-382
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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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| 10. |
- Hanssen-Bauer, O. W., et al.
(författare)
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Comparison of three DWM-based wake models at above-rated wind speeds
- 2023
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Ingår i: WAKE CONFERENCE 2023. - : Institute of Physics Publishing (IOPP).
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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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