Report on the final panel discussion on computational aeroacoustics

Some important conclusions about future prospects for aeroacoustics in general, and for computational aeroacoustics in particular, that were reached in the course of the Final Panel Discussion of the Workshop on Computational Aeroacoustics held from 6 to 9 April 1992 by ICASE and NASA Langley Research Center are summarized by the panel chairman. Aeroacoustics must now be involved in interactions with computational fluid dynamics (as applied not only to deterministic flows but also to the statistical characteristics of turbulence), while additionally incorporating rigorous comparisons with experiment. The new Computational Aeroacoustics will press forward in two parallel ways. In one of them, CFD will be used to determine aeroacoustic source strengths, the associated radiation being derived by the Acoustic Analogy approach in one of its forms. In the other, a direct Computational Aeroacoustics will apply CFD techniques over a region extending beyond the flow field so as to include at least the beginnings of the acoustic far field. There are some particularly important areas of study, including rotor noise, boundary-layer noise, and the noise of supersonic jets, where it is strongly recommended that use of both methods is continued. On the other hand, important problems of the diffraction of radiation from aeroacoustic sources around complicated aircraft shapes will require the use of comprehensively Computational Aeroacoustics, while Acoustic Analogy methods seem better suited to estimating subsonic jet noise. The study of model problems to allow comparisons with experiment will be valuable in both lines of attack

Modélisation des sources d'émission acoustique : aide à l'identification de paramètres pertinents pour décrire l'endommagement des matériaux

L’Emission Acoustique (EA) est une technique de contrôle non-destructif capable de détecter les différents mécanismes d’endommagement se produisant lors de sollicitations mécaniques. Dans les matériaux composites, un des enjeux actuel est de relier chaque signal enregistré au mécanisme source (rupture de fibre, fissuration matricielle, décohésion fibre/matrice, etc.). Cependant, le signal source subit lors de sa propagation jusqu’au capteur de nombreuses transformations altérant sa forme et par conséquent les descripteurs extraits de ce signal (amplitude, énergie, etc.). Dans le contexte de la classification des phénomènes sources, la modélisation de ces transformations s’avère donc indispensable. Une étude par éléments finis a été réalisée à l’aide du logiciel ABAQUS© en simulant une éprouvette de traction de type haltère (dimensions : 200x24x3,7 mm3). Pour l’instant, l’étude a été réalisée sans prendre en compte une éventuelle évolution des propriétés élasto-mécaniques du milieu de propagation avec l’endommagement. L’objectif principal de cette étude est de quantifier l’influence de la propagation sur le signal enregistré. Des sources de nature différente en termes de temps de montée, de fréquence et d’énergie ont été générées. Le déplacement à la surface matériau a été calculé à différentes distances de la source. Les descripteurs les moins affectés sont les descripteurs fréquentiels (fréquence de pic et barycentre fréquentiel) qui rendent une bonne image de la source et sont peu affectés par la propagation. Par contre, nous avons pu montrer que l’énergie du signal, dont la valeur est très influencée par la géométrie de l’éprouvette, est très affectée par les réflexions. En outre, l’énergie ne décroissant pas de manière exponentielle avec la distance de propagation pour cette géométrie, il n’est pas possible d’établir une relation simple entre l’énergie du signal d’EA et l’énergie de la source. Enfin, l’étude de l’impact d’un endommagement progressif de l’éprouvette sur le signal d’EA est en cours d’analyse

A new methodology for automating acoustic emission detection of metallic fatigue fractures in highly demanding aerospace environments: An overview

The acoustic emission (AE) phenomenon has many attributes that make it desirable as a structural health monitoring or non-destructive testing technique, including the capability to continuously and globally monitor large structures using a sparse sensor array and with no dependency on defect size. However, AE monitoring is yet to fulfil its true potential, due mainly to limitations in location accuracy and signal characterisation that often arise in complex structures with high levels of background noise. Furthermore, the technique has been criticised for a lack of quantitative results and the large amount of operator interpretation required during data analysis. This paper begins by introducing the challenges faced in developing an AE based structural health monitoring system and then gives a review of previous progress made in addresing these challenges. Subsequently an overview of a novel methodology for automatic detection of fatigue fractures in complex geometries and noisy environments is presented, which combines a number of signal processing techniques to address the current limitations of AE monitoring. The technique was developed for monitoring metallic landing gear components during pre-flight certification testing and results are presented from a full-scale steel landing gear component undergoing fatigue loading. Fracture onset was successfully identify automatically at 49,000 fatigue cycles prior to final failure (validated by the use of dye penetrant inspection) and the fracture position was located to within 10. mm of the actual location

Effet de l'angle de flèche sur le bruit à large bande de ventilateur

Cette étude vise à comprendre la mécanique de réduction de bruit afin de mitiger le bruit large bande en utilisant l’angle de flèche tout en préservant le rendement aérodynamique. Nous avons choisi des modèles et outils de calculs afin de comprendre le comportement aérodynamique ainsi que le bruit généré par l’angle de flèche. En premier lieu, une simulation Reynolds Averaged Navier Stokes (RANS) est utilisée afin d’évaluer le champ d’écoulement. Ensuite, une méthode Lattice Boltzmann (LBM) haute-fidélité est utilisée afin de prédire la radiation sonore. LBM nous permet de déterminer la source des bruits combinés. Finalement, afin de séparer le bruit large bande généré par les turbulences, nous avons adapté le modèle d’Amiet's leading-edge afin de représenter l’angle de flèche d’un ventilateur axial. Nos résultats indiquent que le dévers de pale avant surpasse le dévers de pale arrière pour la région décrochage, la radiation sonore et la consommation énergétique lorsque les performances aérodynamique est restaurée.Le bruit produit par le ventilateur de radiateur devient une préoccupation croissante. En effet, les véhicules électriques modernes ne produisent pas le bruit engendré par les groupes motopropulseurs et moteurs traditionnels. Fondé sur une revue de littérature, nous avons classé les différentes sources de bruit ainsi que leur contribution sur le spectre acoustique. Les concepts de dévers de pale avant et arrière ont démontré un potentiel avantage de réduction de bruit large bande aux détriments du rendement aérodynamique. Par conséquent, cette approche est très peu utilisée dans l'industrie. Cette étude vise à comprendre la mécanique de réduction de bruit afin de mitiger le bruit large bande en utilisant l'angle de flèche tout en préservant le rendement aérodynamique. Nous avons choisi des modèles et outils de calculs afin de comprendre le comportement aérodynamique ainsi que le bruit généré par l'angle de flèche. En premier lieu, une simulation Reynolds Averaged Navier Stokes (RANS) est utilisée afin d'évaluer le champ d'écoulement. Ensuite, une méthode Lattice Boltzmann (LBM) haute-fidélité est utilisée afin de prédire la radiation sonore. LBM nous permet de déterminer la source des bruits combinés. Finalement, afin de séparer le bruit large bande généré par les turbulences, nous avons adapté le modèle d'Amiet's leading edge afin de représenter l'angle de flèche d'un ventilateur axial. Nos résultats indiquent que le dévers de pale avant surpasse le dévers de pale arrière pour la région décrochage, la radiation sonore et la consommation énergétique lorsque les performances aérodynamique est restaurée. Nous recommandons le dévers de pale avant afin de réduire le bruit de large bande émis par le ventilateur du radiateur. Cependant, des recherches additionnelles seront nécessaires afin d'évaluer le bruit tonal. Ces recherches pourront renforcer l'utilisation de l'angle de flèche dans la conception de pales.Abstract : The radiator fan noise is becoming a growing concern since other noise sources radiated from traditional powertrains and combustion engines are omitted in modern electric vehicles. Based on a literature review, we classified the noise sources and their contribution in noise spectra. The forward sweep and backward sweep showed a strong potential in broadband noise reduction but at the cost of loss in aerodynamic efficiency. Hence, this skepticism restrained from its wide usage in fan design. Therefore, this study aims at understanding the noise reduction mechanism so that to mitigate broadband noise using blade sweep by preserving its aerodynamic performance. The various computational tools are used to investigate the aerodynamic behavior and its associated noise in swept blades. First, an industry-friendly steady Reynolds Averaged Navier Stokes (RANS) simulation technique is assessed to investigate the flow field and later a high-fidelity, unsteady Lattice Boltzmann method (LBM) is evaluated to predict the noise radiation. LBM provides the combined knowledge of all noise sources. So, finally, to segregate broadband noise generated due to turbulence interaction, we adapted Amiet's leading-edge noise prediction tool to the swept blade of an axial fan. The results indicate that forward sweep has improved pressure rise by almost 25% than backward sweep and unswept blade when designed for similar loadings. In addition, the forward sweep has reduced noise levels by 12 dB than unswept blade. We recommend using a forward sweep to reduce broadband noise emitted by the radiator fan based on our findings. However, further research is needed to investigate tonal noise that could strengthen the usage of sweep in blade design

A Computational Modeling Approach of Fracture-Induced Acoustic Emission

Acoustic Emission (AE) has become a prominent Nondestructive Testing (NDT) technique with capabilities to be used for Structural Health Monitoring (SHM) applications that entail in-service monitoring, detecting damage-prone areas, and establishing damage prognostics of structures. The next generation of acoustics-based techniques for SHM will rely upon the reliable and quantitative characterization of AE signals related to dominant damage mechanisms. In this context, the forward problem of simulating AE activity is addressed herein by proposing advanced finite element models for damage-induced stress wave generation and propagation. Acoustic emission for this purpose is viewed as part of the dynamic process of energy release caused by damage initiation. To form the computational approach, full field experimental information obtained from monitoring the damage initiation process using digital image correlation is used to construct constitutive laws, e.g. traction-separation law, and to define other damage related parameters. Subsequently, 3D FE simulations based on such experimental data are implemented using cohesive zone modeling and extended finite element method to create an initial failure. Numerically simulated AE signals from the dynamic response due to the onset of damage are evaluated in the context of the inverse problem of source identification and localization. The results successfully demonstrate material and geometry effects of the propagating source and describe completely the AE process from crack-induced isolated source to transient and steady-state dynamic response. Furthermore, the computational model is used to provide quantified measures of the energy release associated with crack. In addition, the effect of plasticity on simulated traveling waves ahead of the crack tip was investigated and revealed nonlinear interactions that had been postulated to exist. Ultimately, the forward AE methodology is applied to an aerospace structural component to recreate the debonding process and associated stress release propagation. All damage-induced wave propagation simulations presented in this dissertation create a pathway for the quantitative comparison between experimental and theoretical predictions of AE.Ph.D., Mechanical Engineering and Mechanics -- Drexel University, 201

Propogation Of Wind Turbine Noise Through Wakes And Turbulent Atmosphere

It is well known that the atmospheric inhomogeneities have great impact on sound propagation over long ranges. For the application of predicting wind turbine noise, either the flow wakes generated by rotating turbine blades or small-scale atmospheric turbulence can affect the propagation of sound over ground surfaces from individual turbines. In this thesis, the effects of wake and atmospheric turbulence on the propagation of wind turbine noise are investigated. By introducing the Parabolic Equation (PE) method, the effects of atmospheric changes in sound speed can be incorporated at each marching step as the prediction of sound field advances in the horizontal ranges. With a simulated wake profile near the wind turbine, more accurate predictions in the sound field can be achieved for realistic atmospheric conditions. This work aims to improve current prediction schemes for assessing the impact of wind turbine noise on the neighborhood communities

Embedding of fine features in multi-scale electromagnetic models

Modelling detailed electromagnetic interactions in Electromagnetic Compatibility predictions is an extremely demanding task, made more difficult by the increasing complexity of modem engineering problems. Over the last decade major innovations in numerical models and methods have been introduced to reduce demands on computational resources or render the simulations of large systems containing a diverse range of physical features possible. This thesis presents one of the methods of dealing with large systems which utilises the concept of sub-cells containing fine geometrical objects. A general approach to embedding fine features into a coarse numerical time-domain techniques such as the Transmission Line Modelling (TLM) method is proposed. A non-standard node has been developed that mimics the electro- magnetic behaviour of virtually any object or group of small objects wholly or partially enclosed by a volume of space represented by the numerical cell. The core of this scheme is to identify a suitable set of local field solutions to Maxwell's equations within the vicinity of the enclosed objects and, by correctly sampling the fields on the boundary of the cell, to integrate these with field solutions represented by the neighbouring nodes, ensuring both field continuity and power conservation. The idea whilst simple leads to an algorithm that is both explicitly stable and conservative as well as only incurring a minor computational overhead compared to a conventional TLM algorithm. It is noted that, as the required identification and evaluation of the local field solutions occurs as a pre-processing stage prior to the main TLM run and that the non-standard nodes are a small proportion of the coarse grid, a significant overall reduction in computational requirements is achieved in comparison to direct fine meshing of the features. Another advantage of this approach lies in the fact that the local solutions to Maxwell's equations calculated in the pre-run process can be obtained by any suitable means. Analytical formulations, numerical results of another simulation or simply experimental measurements are some of the possibilities. The approach is employed to investigate a variety of EMC problems. An analysis of the field scattered from multiple cylindrical geometries embedded within a single two-dimensional cell is presented. Multiple conducting and lossy wires, dielectric rods and dielectric coated wires, conducting strips and slots are also studied. Three-dimensional simulations are shown for an arbitrarily orientated wires, small dielectric and conducting spheres and other canonical shapes. The approach is also successfully applied to other disciplines where modelling plays an important role. The flexibility of the algorithm is demonstrated for simulations of photonic structures with the primary focus placed upon photonic band-gap materials