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Numerical Heat Transfer And Fluid Flow

Wind energy, together with other renewable energy sources, are expected to grow substantially in the coming decades and play a key role in mitigating climate change and achieving energy sustainability. One of the main challenges in optimizing the design, operation, control, and grid integration of wind farms is the prediction of their performance, owing to the complex multiscale two-way interactions between wind farms and the turbulent atmospheric boundary layer (ABL). From a fluid mechanical perspective, these interactions are complicated by the high Reynolds number of the ABL flow, its inherent unsteadiness due to the diurnal cycle and synoptic-forcing variability, the ubiquitous nature of thermal effects, and the heterogeneity of the terrain. Particularly important is the effect of ABL turbulence on wind-turbine wake flows and their superposition, as they are responsible for considerable turbine power losses and fatigue loads in wind farms. These flow interactions affect, in turn, the structure of the ABL and the turbulent fluxes of momentum and scalars. This review summarizes recent experimental, computational, and theoretical research efforts that have contributed to improving our understanding and ability to predict the interactions of ABL flow with wind turbines and wind farms.

https://link.springer.com/content/pdf/10.1007/s10546-019-00473-0.pdf

Wind-Turbine and Wind-Farm Flows: A Review

Similar to other renewable energy sources, wind energy is characterized by a low power density. Hence, for wind energy to make considerable contributions to the world's overall energy supply, large wind farms (on- and offshore) consisting of arrays of ever larger wind turbines are being envisioned and built. From a fluid mechanics perspective, wind farms encompass turbulent flow phenomena occurring at many spatial and temporal scales. Of particular interest to understanding mean power extraction and fluctuations in wind farms are the scales ranging from 1 to 10 m that comprise the wakes behind individual wind turbines, to motions reaching 100 m to kilometers in scale, inherently associated with the atmospheric boundary layer. In this review, we summarize current understanding of these flow phenomena (particularly mean and second-order statistics) through field studies, wind tunnel experiments, large-eddy simulations, and analytical modeling, emphasizing the most relevant features for wind farm design and operation.

Flow Structure and Turbulence in Wind Farms

Machine learning techniques have been successfully applied for the intelligent fault diagnosis of rolling bearings in recent years. This study has developed an Improved Multi-Scale Convolutional Neural Network integrated with a Feature Attention mechanism (IMS-FACNN) model to address the poor performance of traditional CNN-based models under unsteady and complex working environments. The proposed IMS-FACNN has a good extrapolation performance because of the novel IMS coarse grained procedure with training interference and the introduced the feature attention mechanism, which improves the model's generalization ability. The proposed IMS-FACNN model has a better performance than existing methods in all the examined scenarios including diagnosing the bearing fault of a real wind turbine. The results show that the reliability and superiority of the IMS-FACNN model in diagnosing faults of rolling bearings.

Fault diagnosis of rolling bearings using an Improved Multi-Scale Convolutional Neural Network with Feature Attention mechanism

A critical review on the simulations of wind turbine aerodynamics focusing on hybrid RANS-LES methods

This report describes the results from the third phase of IEA TCP Wind Task 29 Mexnext, denoted as Mexnext-III, in which 22 parties from 10 countries participated. The main impact of the Mexnext-III project lies in the insights offered in the accuracy of design models by which it is possible to assess design calculations in a more thorough way. Moreover improved design models and recommendations on model use are provided which will reduce uncertainties in design calculations. Based on the project results it can be stated that current state of the art design models provide accurate aerodynamic load predictions for design conditions in constant axial and uniform inflow, while more scatter is observed in modeling the turbulent wake state, separated flow, 3d effects and tip effects. A prerequisite for accurate results is a careful consideration of input parameters and output post-processing. This especially holds for CFD simulations, of which the results are highly dependent on the large number of input- and output related choices. Guidelines to reduce the associated uncertainties have been given. Improvements have been made to engineering models describing amongst others yawed and dynamic inflow effects. However, it is shown that still large uncertainties exist in oblique, dynamic and/or nonuniform inflow conditions. Following this result it is recommended to further improve unsteady aerodynamic modeling in the fourth phase of IEA TCP Wind Task 29. Dedicated aerodynamic measurements, mainly from the New MEXICO experiment have been used to validate, improve and understand aerodynamic models. In the New MEXICO experiment pressure and flow field measurements were taken on a 4.5 meter diameter rotor in the large German Dutch Wind Tunnel, DNW. The New MEXICO experiment is a follow-up of the MEXICO experiment which was the subject of analysis in the first phase Mexnext-I. In Mexnext-I significant and unexpected discrepancies were found between simulations and measurements in terms of blade loads and induced velocities. Resolving the reason for these discrepancies and reducing them has been accomplished in Mexnext-III. Thereto, the entire New MEXICO database has been documented and delivered to the Mexnext-III consortium. With this database a large comparison exercise has been performed, featuring six simulation cases in axial and yawed inflow conditions, containing results of over 20 codes ranging from BEM to CFD. More than 10 different variable types ranging from lifting line variables to pressures, loads and velocities have been compared for the different conditions, resulting in over 250 comparison plots. Apart from benchmarking much effort was spent on understanding the the flow physics of various phenomena. The result is a unique insight in the current status and accuracy of rotor aerodynamic modeling. More specifically it can be concluded that most questions from Mexnext-I have been answered and so a much better understanding of the aerodynamics of the MEXICO rotor in terms of blade aerodynamics as well as near wake aerodynamics has been gained. This is illustrated by the better agreement between calculations and measurements compared to the agreement in Mexnext-I. This is true for pragmatic engineering models but also for high fidelity models. Opposite to the situation after the first MEXICO campaign, the relation between loads and velocities now fulfills the momentum balance, not only in axial direction but even in the tangential direction.

Final Report of IEA Wind Task 29 Mexnext (Phase 3)

The compressible block-structured flow solver FLOWer of the German Aerospace Center (DLR) has been extended towards state of the art detached-eddy simulation (DES) methods in order to conduct hybrid RANS/LES simulations of flow around rotary wings. The large-eddy simulation (LES) capabilities of the code are demonstrated for decaying isotropic turbulence. Excessive numerical dissipation is avoided by using the fifth-order WENO scheme and an appropriate low-Mach number correction. The DES implementations are validated first for the well documented test cases backward facing step and NACA0021 airfoil at \(60^\circ \) incidence, before increasing the complexity by simulating the flow around the MEXICO model wind turbine operating in stalled conditions and comparing with experimental data. From the latter, recommendations on the numerical settings are derived to successfully set up eddy resolving simulations for wind turbine or helicopter applications.

Hybrid RANS/LES Capabilities of the Flow Solver FLOWer—Application to Flow Around Wind Turbines

This article presents various detached eddy simulation (DES) results of a commercial wind turbine under multiple inflow conditions in complex and flat terrain. Challenges regarding the meshing process of wind turbines in complex terrain are described and an approach to overcome those is presented. The main focus of the evaluation is blade load and power response to inflow turbulence and terrain effects, e.g. the change of the inclination angle or the speed-up due to a hill. To separate the different influences, the complexity of the simulation setup is increased stepwise. Starting with a baseline simulation in flat terrain and uniform inflow over adding atmospheric turbulence to a complex terrain simulation of a fully meshed rotating 3D wind turbine under atmospheric inflow.

https://iopscience.iop.org/article/10.1088/1742-6596/524/1/012134/pdf

CFD Studies on Wind Turbines in Complex Terrain under Atmospheric Inflow Conditions

This paper presents results of detailed 3D CFD simulations of two 5MW wind turbines sited in the German wind farm Alpha Ventus which are located behind each other at half-wake conditions. The focus of interest in this study is put on wake – turbine interaction, in order to derive the main shadow effects and their influence on blade loads and power response of the downstream turbine. For this purpose, Detached Eddy Simulations (DES) were performed using the flow solver FLOWer from DLR (German Aerospace Center). To consider all relevant aerodynamic effects, the main turbine components are represented as direct model with resolved boundary layers. Measurement-based turbulent inflow conditions are prescribed to realistically account for the atmospheric boundary layer. In order to analyze the flow conditions in front of the downstream turbine, wake propagation and velocity spectra are evaluated and compared with the undisturbed atmospheric boundary layer. Their impact on loads and power production and their corresponding fluctuations is discussed by comparing these with the upstream turbine. It was found, that fatigue loads occurring at half-wake conditions are significantly higher for the downstream turbine, since blade load fluctuations are highly amplified by the unsteady wake of the upstream turbine.

https://iopscience.iop.org/article/10.1088/1742-6596/524/1/012143/pdf

CFD Simulations on Interference Effects between Offshore Wind Turbines

https://escholarship.org/content/qt621820b3/qt621820b3.pdf?t=p2pcl5

Heat transfer enhancement in a ribbed channel: Development of turbulence closures

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