Littérature scientifique sur le sujet « Aeroacustic »

This Special Issue highlights the latest enhancements in the abatement of noise and vibrations of aerospace and automotive systems. The reduction of acoustic emissions and the improvement of cabin interior comfort are on the path of all major transportation industries, having a direct impact on customer satisfaction and, consequently, the commercial success of new products. Topics covered in this Special Issue deal with computational, instrumentation and data analysis of noise and vibrations of fixed wing aircrafts, satellites, spacecrafts, automotives and trains, ranging from aerodynamically generated noise to engine noise, sound absorption, cabin acoustic treatments, duct acoustics and vibroacoustic properties of materials. The focus of this Special Issue is also related to industrial aspects, e.g.,: numerical and experimental studies have been performed for an existing and commercialized engine to enable design improvements aimed at reducing noise and vibrations; moreover, an optimization is provided for the design of low vibroacoustic volute centrifugal compressors and fans whose fluids should be strictly kept in the system without any leakage. Existing procedures and algorithms useful to reach the abovementioned objectives in the most efficient way are illustrated in the collected papers.

In this work, a new noise suppressing airfoil trailing-edge design, termed "finned serrations", is presented and numerically evaluated. This brand-new approach consists of the superposition of two different noise suppressing morphological features inspired by the wings of the owl. Embedded Large Eddy Simulations are employed in tandem with the Ffowcs WilliamsHawkings model to predict and analyze the design aerodynamics and aeroacoustics and compare the obtained output to that of a flat trailing-edge airfoil. Finned serrations are shown to combine the effects of having finlets and serrations. Because of the bluntness of the serration roots, the airfoil is subject to vortex shedding, while the flow is generally decorrelated in the spanwise direction, thanks to the channeling effect of the finlets. The turbulent kinetic energy distribution close to the airfoil trailing-edge surface is also significantly altered, as the more energetic eddies are convected away from the airfoil surface. Lastly, mixing across the airfoil surface is improved, and the average size of the turbulent coherent structures near the airfoil trailing-edge is reduced. The presented results suggest that the coupling of different noise-suppressing mechanisms is a promising path to explore, with the goal of coming up with new, quieter trailing-edge configurations.

The present work collects the results of research conducted on thermo-fluid dynamics and aero-acoustic noise sources, especially on the characteristic noise of the condensing boilers combustion and centrifugal fans noise. The confined combustion cause instability phenomena which produce pressure fluctuations inside the combustion chamber, resulting in generation of noise. The instability phenomena of condensing boilers can be divided in two main types: the first one is called "Rumbling," where an interaction among flame, acoustic field and system fluid dynamics occurs; the second one is called "Hooting", which is a thermo-kinetic instability. On the first part of this paper is shown the experimental and numerical analysis of the "Hooting" in order to find: causes, characteristics, and a methodology that allows the prediction of this phenomenon. For this purpose has been made a simplified prototype of a condensing boiler to study: the main acoustic characteristics of the instability phenomenon, the fluid dynamic parameters which affect its generation and find a simple geometry to perform 3D simulation analysis. In order to evaluate the influence on the emitted spectrum of combustion chamber geometry, it has been performed an acoustic modes FEM analysis of the combustion chamber prototyped cavity. To study the reproducibility and therefore the prediction of the “Hooting” spectra sound pressure level a hybrid CFD/CAA model has been performed. The above numerical model combining a thermo-fluid dynamics simulation of combustion chemical reaction, with a frequency domain acoustic simulation considering the Lighthill’s acoustic analogy extension proposed by Curle. Beyond to this numerical method, CFD transient simulations have been performed to calculate the spectra of sound pressure levels of combustion chamber, analyzing the obtained pressure fluctuation. The last part of this work described two numerical methodologies to predict the noise level emitted from centrifugal fan outlet. The first one requires the combination of a transient CFD simulation with a CAA acoustic simulation using two different acoustic analogies. The Ffowcs Williams-Hawkins analogy estimates the emitted sound pressure level with the definition of a dipoles distribution placed on fixed and mobile hard surfaces present in a fluid domain. This numerical method enables to calculate centrifugal fans blade passing frequency noise and broadband noise as well. The Lowson theory defines an equivalent single sound source considering the pressure distribution on blades and rotation speed of the impeller. This numerical method enables to calculate only the noise contribution of blade passing frequency and its harmonics. The second numerical method considered enable to calculate the sound pressure level spectra directly from relative pressure fluctuations from CFD transient simulations. The simulation results have been compared with the experimental measurements performed on a test rig carried out according to UNI EN ISO 5136.

Desde a última década, as autoridades aeronáuticas dos países membros da ICAO vem, gradativamente, aumentando as restrições nos níveis de ruído externo de aeronaves, principalmente nas proximidades dos aeroportos. Por isso os novos motores aeronáuticos precisam ter projetos mais silenciosos, tornando as técnicas de predição de ruído de motores cada vez mais importantes. Diferente das técnicas semi-analíticas, que vêm evoluindo nas últimas décadas, as técnicas semiempíricas possuem suas bases lastreadas em técnicas e dados que remontam à década de 70, como as desenvolvidas no projeto ANOPP. Uma bancada de estudos aeroacústicos para um conjunto rotor/estator foi construída no departamento de Engenharia Aeronáutica da Escola de Engenharia de São Carlos, permitindo desenvolver uma metodologia capaz de gerar uma técnica semi-empírica utilizando métodos e dados novos. Tal bancada é capaz de variar a rotação, o espaçamento rotor/estator e controlar a vazão mássica, resultando em 71 configurações avaliadas. Para isso, uma antena de parede com 14 microfones foi usada. O espectro do ruído de banda larga é modelado como um ruído rosa e o ruído tonal é modelado por um comportamento exponencial, resultando em 5 parâmetros: nível do ruído, decaimento linear e fator de forma da banda larga, nível do primeiro tonal e o decaimento exponencial de seus harmônicos. Uma regressão superficial Kriging é utilizada para aproximar os 5 parâmetros utilizando as variáveis do experimento e o estudo mostrou que Mach Tip e RSS são as principais variáveis que definem o ruído, assim como utilizado pelo projeto ANOPP. Assim, um modelo de previsão é definido para o conjunto rotor/estator estudado na bancada, o que permite prever o espectro em condições não ensaiadas. A análise do modelo resultou em uma ferramenta de interpretação dos resultados. Ao modelo são aplicadas 3 técnicas de validação cruzada: leave one out, monte carlo e repeated k-folds e mostrou que o modelo desenvolvido possui um erro médio, do nível do ruído total do espectro, de 2.35 dBs e desvio padrão de 0.91.
Since the last decade, the countries members of ICAO, via its aeronautical authorities, has been gradually increasing the restrictions on external aircraft noise levels, especially in the vicinity of airports. Because that, the new aero-engines need quieter designs, so noise prediction techniques for aero-engines are getting even more important. Semi-analytical techniques have undergone a major evolution since the 70th until nowadays, but semi-empirical techniques still have their bases pegged in techniques and data defined on the 70th, developed in the ANOPP project. An Aeroacoustics Fan Rig to investigate a Rotor/Stator assembly was developed at Aeronautical Engineering Department of São Carlos School of Engineering, allowing the development of a methodology capable of defining a semi-empirical technique based on new data and methods. Such rig is able to vary the rotation, the rotor/stator spacing and mass flow rate, resulting in a set of 71 configurations tested. To measure the noise, a microphone wall antenna with 14 sensors were used. The broadband noise was modeled by a pink noise and the tonal with exponential behavior, resulting in 5 parameters: broadband noise level, decay and form factor and the level and decay of tonal noise. A superficial kriging regression were used to approach the parameters using the experimental variables and the investigation has shown that Mach Tip and RSS are the most important variables that defines the noise, as well on ANOPP. A prediction model for the rotor/stator noise are defined with the 5 approximation of the parameters, that allow to predict the spectra at operations points not measured. The model analyses of the model resulted on a tool for results interpretation. Tree different cross validation techniques are applied to model: leave ou out, Monte Carlo and repeated k-folds. That analysis shows that the model developed has average error of 2.35 dBs and standard deviation of 0.91 for the spectrum level predicted.

I temi affrontati nella ricerca in ambito termofluidodinamico richiedono spesso un approccio multidisciplinare e tecniche di indagine non tradizionali. Una delle strategie più utilizzate, vista anche la sempre crescente disponibilità di risorse computazionali, è quella di operare con modelli numerici per la simulazione di fenomeni complessi, multifisici e multiscala. Una tematica di forte interesse ingegneristico è lo studio delle fluttuazioni di pressione derivanti dall’interazione tra un corpo e una corrente d’aria che lo investe: tale disturbo può ricadere nel campo dell’udibile e la sua diffusione può contribuire all’aumento dell’inquinamento acustico. Ne è un esempio il rumore prodotto dalle turbine eoliche di grande taglia per le quali sono state già state adottate tecniche di abbattimento del rumore, come l’impiego di bordi di uscita dentellati (trailing edge serration). Un altro tema di estrema rilevanza è quello del trasporto di fluidi organici veicolanti virus o batteri: la recente pandemia da SARS–CoV–2 ha messo in evidenza quanto sia importante valutare accuratamente la dinamica delle micro–gocce di saliva e la loro interazione termofluidodinamica con l’ambiente al fine di fornire corrette linee guida sulla distanza sociale e sulle buone pratiche da seguire nella quotidianità all’interno del contesto pandemico. In questo lavoro di tesi viene utilizzato un approccio numerico per lo studio dell’emissione aeroacustica prodotta da oggetti investiti da un flusso d’aria e della diffusione aerea di micro–particelle di fluido organico veicolanti virus. Viene sviluppato un solutore in grado di condurre simulazioni dirette (Direct Numerical Simulation – DNS) del campo aeroacustico, utilizzando condizioni al contorno non riflettive e schemi di integrazione temporale Runge–Kutta espliciti di alto ordine, e indagata la possibilità di adottare il riscaldamento localizzato quale tecnica di smorzamento delle fluttuazioni di pressione caratteristiche di un’onda sonora. Viene, inoltre, presentato un modello con approccio Euleriano–Lagrangiano multiscala, che permetta di valutare la diffusione in ambiente di particelle di fluido muco–salivare, nonché il processo di cristallizzazione della quota–parte salina delle droplet accoppiando il metodo Particle–Source–In–cell (PSI–cell) alla Population Balance Equation (PBE). Viene indagata anche la possibilità di ridurre la trasmissione di SARS–CoV–2 utilizzando la radiazione ultravioletta di tipo C quale tecnica di disinfezione real–time. I modelli sono sviluppati adottando il metodo di discretizzazione ai volumi finiti non strutturati e co–locati disponibile all’interno della libreria OpenFOAM.
The new issues addressed in scientific research in the thermal and fluid dynamic field often require a multidisciplinary approach and non–traditional investigation techniques. One of the most used strategies, also because of increasing availability of computational resources, is to operate with numerical models that allow the simulation of complex, multiphysics and multiscale phenomena. A cutting-edge topic is certainly the study of fluctuating pressure resulting from a body and air interaction: this disturbance can be such that to be in the hearing range and its diffusion can contribute to the increase in noise pollution and have a significant impact on our daily life. As an example, we can refer to the noise produced by multi–megawatt wind turbines that are often equipped with trailing edge serration in order to reduce the aeroacouistic emission. Another crucial topic in this moment is related to the organic fluids, carrying viruses or bacteria, diffusion: SARS–CoV–2 pandemic has highlighted how important is to understand and rigorous study saliva droplets dynamics and their interaction with the environment in order to provide guidelines on social distance and good practices to be followed in daily life. In this PhD thesis a numerical approach is used to study the aeroacoustic emission radiated by objects in a flow as well as to investigate airborne diffusion of organic fluid micro - particles carrying viruses. A new solver is developed in order to perform Direct Numerical Simulation of the aeroacoustic fields. Explicit high–order Runge–Kutta schemes are employed for time integration and non–reflective boundary conditions are adopted. The local wall heating effect fluctuating pressure is also investigated, in order to give an insight on a new method for active controlling the noise emission. Furthermore, a new computational model, developed in a multiscale Eulerian - Lagrangian framework, is presented. This approach allows to evaluate the spreading of micro–droplets emitted in respiratory activities, as well as their thermal and fluid dynamic interaction with the surrounding environment, taking also into account the droplet dry nuclei formation. Saliva sodium chloride crystallization kinetics is modelled by coupling Particle–Source–In–cell (PSI–cell) method with Population Balance Equation (PBE). Moreover, a real–time disinfection strategy is studied: biological inactivation of SARS–CoV–2 using ultraviolet–C radiation is addressed. The aforementioned models are developed adopting the unstructured, co–located, finite volume method available in the well-known OpenFOAM library.

One of the major concerns in the urban areas surrounding airports is the noise pollution caused by the acoustic emissions coming from aircraft operations. It has been demonstrated that a continuous and prolonged exposure to noise pollution, not only results in a lower quality of life, but has harmful effects on human health as well. Definitely, noise may be the cause of psychological disorders, such as insomnia, anxiety, aggressiveness, and physiological disturbances, such as hearing impairment, hypertension and ischemic heart diseases. For these reasons, since the 1960s, when turbojet engines spread in the aviation industry, regulations on aircraft noise emissions have been introduced. Afterwards, due to the continuous growth in air traffic, these legislations have become more and more restrictive. This led to the introduction of the noise certification process for commercial aircrafts at the operating points critical in terms of noise perceived on the ground. Due to the introduction of noise regulations, the goal of reducing the acoustic emissions produced by aircrafts has been a constant challenge for aeronautical industry and engine designers continuously strives hard to to develop quieter components. In this context experimental and numerical investigations on noise generation and propagation mechanisms have become a main research field for aeronautical manufacturers. Such studies are aimed at gaining a deeper insight on the physical phenomena in order to develop effective low noise strategy. In particular, well-calibrated and reliable numerical tools for noise prediction can be used within the engine design loop allowing the evaluation of the acoustic emissions caused by each component before it is tested in a demonstrator. As a consequence, these tools are of great importance in the reduction of aircraft noise. Aeroacoustic simulations within an engine environment can be performed by means of different techniques, ranging from analytical models to 3D numerical solvers: fast and robust methods have primary importance at the beginning of the design loop, when it is essential to gain a general understanding of the effect of primary design parameters on the noise emissions. On the other hand, three-dimensional aeroacoustic solvers, more accurate and reliable, are able to to provide detailed knowledge of the acoustic phenomena during the advanced design verifications. Aircrafts acoustic emissions can be divided into either external and internal noise. The former contribution arises from the interaction between the airflow and the aircraft itself (fuselage, wings, control surfaces, etc...), the latter instead is generated inside the engines as a result of various mechanisms and then radiates outside. At the dawn of civil aviation, the most critical noise source was the exhaust jet, but with the introduction of modern high-bypass ratio turbofan engines jet noise has been strongly reduced. Also fan noise has been drastically lowered over the years taking advantage of low-noise design criteria and acoustic treatments in the bypass duct. Hence, the other engine noise sources, such as the low-pressure turbine (LPT), have become significant in the overall aircraft emissions, first and foremost at some critical operating conditions. For this reason, nowadays a wide range of components need to be investigated during the engine design in order to apply low-noise criteria required to meet the increasingly restrictive noise regulations. In this context, the main topic of this PhD thesis is the development of two different methods to be used when evaluating the tone noise emissions of low pressure turbines. The first one is a two-dimensional model that computes the transmission of an acoustic wave across a blade row. This method has been included in a preliminary noise prediction procedure capable of quickly estimating the noise emissions of a multistage turbine. The second method allow the extraction of the acoustic waves from a 3D unsteady CFD solution obtained by Traf code. Traf code is commonly used to perform URANS simulations of a pair of neighbour blade rows. During this PhD activity, a post processing technique of the unsteady solution, based on the discrete Fourier transform, has been implemented in order to extract the acoustic components from the time depending solution at each BPF. Finally, after the DFT in time domain, tone noise levels can be calculated in terms of SPL and PWL values by means of radial mode analysis. The key aspects of this procedure are the capability to provide detailed results in terms of acoustic emissions for rotor/stator interactions within the pair of rows with a single simulation and to account for non-linear aeroacoustic effects, which can be relevant in modern low-pressure turbine environments (high pressure ratio, low axial spacing). The validation of these methods has been carried out by comparing their results both with the results of simulations coming from a previously validated approach based on Lars code and acoustic experimental data measured at a cold-flow rig. Finally, although these methods have been developed specifically for low-pressure turbines, they can be actually used in any axial turbo-machinery environment where rotor-stator interactions are relevant sources in the overall emissions.