Acoustic Analogies and Large-Eddy Simulations of Incompressible and Cavitating Flows Around Bluff Bodies

L'attivit\ue0 di ricerca riportata in questa tesi riguarda lo studio numerico della generazione e propagazione del rumore idroacustico. Le onde sonore possono essere emesse ogni volta che esiste un movimento relativo tra due fluidi o tra un fluido e una superficie. Nei decenni passati \ue8 stata prestata molta attenzione ai problemi di rumore aeroacustico. Nel corso degli anni, modelli teorici e numerici adatti per flussi transonici o super-sonici sono stati sviluppati e la loro efficacia \ue8 stata testata. La principale fonte di rumore \ue8 stata identificata nei termini detti di thickness e loading. Per rilevare questo tipo di fonte di rumore \ue8 sufficiente considerare i termini lineari dell'equazione di Ffowcs-Williams e Hawkings. Nell'ambiente subacqueo le onde acustiche, quindi i disturbi della pressione, viaggiano a velocit\ue0 molto pi\uf9 alta di quella del moto del flusso, cos\uec che la maggior parte dei fenomeni idrodinamici si trovano in un regime incomprimibile. La lunghezza d'onda \ue8 comunemente molto pi\uf9 grande della scala di lunghezza del problema considerato. Inoltre, il vortice che si sviluppa nella parte posteriore di un corpo immerso persiste sulla scia dando contributo considerevole al campo acustico. In queste condizioni, i meccanismi di produzione e propagazione del rumore necessitano di modelli diverso. In questo lavoro, vengono analizzate e discusse diverse metodologie di soluzione dell'equazione FW-H per tenere conto dei termini non lineari. In particolare, la forma avvettiva dei termini di volume viene derivata. Il campo fluidodinamico, considerato come una raccolta di impulsi di rumore, deve essere riprodotto accuratamente. Una simulazione Large-Eddy (LES) \ue8 qui considerata come lo strumento pi\uf9 vantaggioso per riprodurre flussi turbolenti e, allo stesso tempo, affrontare casi di interesse pratico. La prima parte dello studio \ue8 dedicata alla valutazione del modello, in seguito viene eseguita una LES di un flusso turbolento attorno a un cilindro quadrato di lunghezza finita. Si confronta il contributo dei diversi termini dell'equazione FW-H con la pressione dinamica del fluido. Attraverso l'analisi dimensionale si osserva che per problemi idrodinamici, dove la velocit\ue0 di un corpo \ue8 piccola rispetto alla velocit\ue0 del suono, l'integrazione diretta del termine del volume \ue8 lecita e pratica. Il calcolo diretto dei termini non lineari, sulla regione del volume che circonda il corpo immerso, viene quindi impiegato, nella seconda parte della tesi, per lo studio del rumore generato da un flusso attorno a tre diverse geometrie: sfera, cubo ed ellissoide. L'ultima parte della tesi \ue8 dedicata ad uno studio preliminare del campo acustico emesso da un flusso cavitante. La cavitazione pu\uf2 essere interpretata come la formazione di bolle di vapore a causa di una forte variazione di pressione, scendendo questa al di sotto della pressione di saturazione del liquido. L'importanza di studiare i flussi cavitanti \ue8 correlato alla loro presenza in varie applicazioni tecniche, come pompe, turbine, eliche di navi e sistemi di iniezione di carburante, nonch\ue9 in scienze mediche. Esistono diversi tipi di cavitazione, tra le pi\uf9 importanti ci sono: sheet cavitation, bubble cavitation and vortex cavitation. La sheet cavitation pu\uf2 verificarsi sui profili alari, su pale di pompe ed eliche, in particolare quando il carico \ue8 elevato. Questo tipo di cavitazione difficilmente pu\uf2 essere evitato, a causa dei requisiti di alta efficienza. La dinamica della sheet cavitation spesso genera forti fluttuazioni di pressione dovute a il collasso delle strutture del vapore del capannone, che potrebbe portare all'erosione del materiale superficiale e ad una emissione acustica intensa e complessa. In questa tesi viene proposto uno studio preliminare sul rumore di cavitazione, considerando prima una bolla isolata poi una nuvola di bolle e poi un hydrofoil.The research activity reported in this thesis concerns the numerical study of hydroacoustic noise generation and propagation. Sound waves may be emitted whenever a relative motion exists between two fluids or between a fluid and a surface. In the past decades much attention has been paid to aeroacoustic noise problems. Over the years, theoretical and numerical models suitable for transonic or super-sonic flows have been developed, and their effectiveness has been tested. The main source of noise has been identified with the thickness and loading noise terms. To detect this type of noise source is enough to consider the linear terms of the Ffowcs-Williams and Hawkings equation. In underwater environment the acoustic waves, thus the pressure disturbances, travel at speed much higher than that of the flow motion, such that most of hydrodynamic phenomena are in an incompressible regime. Wave length is commonly much larger than the length scale of the considered problem. Moreover, vortex developing at the rear of an immersed body, persists on the wake until braking downstream thus giving a considerable contribution to the noise signature. Under these conditions, the mechanisms of noise production and propagation need a different modeling. Thus, in this work, different solution methodologies of the FW-H equation are analyzed and discussed in order to account for the non-linear terms. In particular, the advective form of the non-linear terms, suitable for wind-tunnel type of problems, is derived. The flow field, regarded as a collection of noise impulses, needs to be reproduced accurately. A Large-Eddy Simulation is here considered as the most advantageous tool to reproduce turbulent flows and, at the same time, deal with cases of practical interest. The first part of the study is dedicated to the assessment of the model: we perform a LES of a flow around a finite-size square cylinder. We compare the contribution from different terms of the FW-H equation with the fluid dynamic pressure. Different methods which are proposed in literature were considered. The direct integration of the volume terms was found to give the most accurate results. Moreover, through dimensional analysis it is observed that for hydrodynamic problems, where velocity of a body is small compared the speed of sound, the direct integration of the volume term is licit and practical . The direct computation of non-linear terms, by integrating on the volume region surrounding the immersed body, is then employed, in the second part of the thesis, for the study of noise signature generated by a flow around three different geometries: sphere, cube and prolate spheroid. Last part of the thesis is devoted to a preliminary study of the acoustic field emitted by a cavitating flow. Cavitation may be interpreted as the rupture of the liquid continuum due to excessive stresses. It is the evaporation of a liquid in a flow when the pressure drops below the saturation pressure of that liquid. The importance of understanding cavitating flows is related to their occurrence in various technical applications, such as pumps, turbines, ship propellers and fuel injection systems, as well as in medical sciences. There are several types of cavitation, such as: sheet cavitation, bubble cavitation and vortex cavitation. Sheet cavitation may occur on hydrofoils, on blades of pumps and propellers, specifically when the loading is high. This type of cavitation can hardly be avoided, because of high efficiency requirements. The dynamics of sheet cavitation often generates strong pressure fluctuations due to the collapse of shed vapor structures, which might lead to erosion of surface material and intense and complex noise track. In this thesis a preliminary study on the cavitation noise is proposed, first considering an isolated bubble then a bubble cloud and then an hydrofoil

Full Acoustic Analogy of the fluid-dynamics noise of an immersed cube

The estimation of fluid dynamic noise generated by anthropogenic sources in realistic marine basins and on land is paramount for human safety and environmental protection. Classical acoustic analogies have limited capabilities when considering the natural variability and peculiarities of the acoustic propagation domain. The Full Acoustic Analogy (FAA), based on the combination of an acoustic analogy for source characterization and a propagation model for far-field transmission, allows the estimation of detailed soundmaps, practical when assessing the risk associated with exposure to fluid-dynamic noise, both impulsive and continuous. The verification of the methodology, consisting of comparing of the far-field acoustic pressure signal obtained with the FAA and with the Ffowcs-Williams and Hawkings equation (classical acoustic analogy for moving immersed bodies), is proven for the first time for the quadrupole non-linear terms. The latter may contribute significantly to the total noise field at small-to-medium distances from the source. In conjunction, the ability of the FAA method to predict the acoustic pressure distribution within the three-dimensional propagation domain is highlighted

Enclosure acoustics considerations for the study of the effect of noise on fish

Comunicación presentada en el 54º Congreso Español de Acústica – TECNIACÚSTICA 2023, Cuenca, 18-20 de octubre de 2023.El estudio del comportamiento de los peces resulta extremadamente complicado en un entorno de libertad, especialmente si hablamos de su exposición a diferentes fuentes sonoras. Por este motivo, de las investigaciones en marcha son llevadas a cabo en el seno de un laboratorio, bien en peceras o bien en tanques, teniendo así un entorno controlado donde monitorizar continuamente el comportamiento de las muestras. Sin embargo, un recinto confinado difiere considerablemente de un espacio abierto. Mientras que un pez en libertad estará sometido por norma general a un campo sonoro libre, cuando hablamos de un recinto cerrado las condiciones cambian notablemente.Studying the behaviour of fish is extremely difficult in a free environment, especially when it comes to their exposure to different sound sources. For this reason, existing research is carried out in a laboratory, either in fish tanks or in tanks, thus having a controlled environment in which the behaviour of the samples can be continuously monitored. However, a confined enclosure differs considerably from an open space. While a fish in the wild will generally be subjected to a free sound field, when we talk about an enclosed area the conditions change markedly.This research was financed by the European Union Next Generation EU and FEDER funds under the projects PCI2022-135081-2 and PID2021-127426OB-C22 of the Ministry of Science and Innovation of Spain, respectively

Marine propeller noise propagation within bounded domains

In the present work, we develop a new methodology to investigate the propagation of acoustic noise originating from a naval propeller, in a bounded marine environment. The propagation in a realistic environment is achieved by coupling the Ffowcs-Williams and Hawkings (FW-H) equation with the acoustic wave equation, overcoming some intrinsic limitations of the FW-H equation. First, the FW-H equation applied to the hydrodynamics field obtained from Large Eddy Simulation, accurately characterizes the propeller source in the acoustic near-field. Then, the propagation in the acoustic far-field is evaluated in an arbitrary domain by solving the acoustic wave equation. After validating the new proposed methodology, we investigate the propagation of the linear part of the noise generated by a naval propeller within a canal. The results show complex interaction between the noise source and the environment: the decay rate of the acoustic energy is strictly related to the distance of the source from the boundaries of the canal; local maxima and minima of the acoustic energy are observed resulting from the superposition of direct and reflected waves; the water–air interface introduces a peculiar asymmetry of the acoustic field associated with the interaction between the acoustic waves

Analysis of Performance of Cavitation Models with Analytically Calculated Coefficients

Cavitation is often simulated using a mixture model, which considers the transport of an active scalar, namely the vapor fraction αv. Source and sink terms of the transport equation of αv, namely vaporization and condensation terms, rule the dynamics of the cavity and are described through different models. These models contain empirical coefficients generally calibrated through optimization processes. The purpose of this paper is to propose an analytical approach for the calculation of the coefficients, based on the time scales of vaporization and condensation processes. Four different models are compared considering as a test-case a two-dimensional flow around a cylinder. Some relevant quantities are analyzed both for standard value of coefficients, as found in the literature, and the coefficients calculated through the analytical approach. The study shows that the analytical computation of the coefficients of the model substantially improve the results, and the models considered give similar results, both in terms of cavitation regime and mean vapor fraction produced

Hydrodynamic noise from a propeller in open sea condition

In the present work a hybrid methodology is used to evaluate the hydrodynamic noise generated by a marine propeller in open sea condition. The hydrodynamic field is computed using Large Eddy simulation under the assumption of incompressible flow field; the acoustic field is reconstructed by applying the advec-tive Ffowcs Williams and Hawkings equation. For the hydrodynamics, we use the dynamic Lagrangian model for the closure of the subgrid-scale stresses and a wall-layer model to skip the resolution of the viscous sub-layer. We consider a propeller well studied in literature for a single value of the advance ratio. A grid of about 6x106 cells is used for reproducing accurately both the stresses over the propeller and the wake, the latter responsible of quadrupole noise. The equations are solved in a fixed-to-the-body frame of reference. The different noise generation mechanisms are investigated separately. Thickness and loading terms related to the propeller shape and velocity, provide significant pressure disturbance in the near field. The quadrupole noise component is obtained by integrating over an external permeable surface. Its contribution is investigated in relation to the presence of vortex persisting in the wak

Non-reflective hard source method for multiple physically extended sources and scattering bodies

In this paper, we focus on methodologies to inject a noise source in a numerical model of noise propagation in confined domains. This is a problem of primary importance when dealing with propagation of fluid-dynamic induced noise in confined basins, like ships at sea or wind farms. We first assess the performance of the literature hard source (HS) and transparent source methods; successively, we propose a novel method named the non-reflective HS (NRHS) method. It takes advantage of the linearity of the equation governing the propagation of acoustic waves in fluids and is based on the decomposition of the total signal in the sum of direct and reflected signals. It presents the advantages of the hard source method removing the main drawback consisting of the well-known problem of spurious reflections. To check the reliability of the HS vs the NRHS, a non-dimensional parameter (the encumbrance) has been defined, which gives a measure of the extension of the generation domain with respect to the propagation domain in relation to the principal wavelength of the acoustic waves and the presence of reflecting surfaces. The method herein developed gives accurate results in the case of a single-point source, where the literature methods behave well; more importantly, the NRHS method maintains its own accuracy when a noise source needs to be represented by a large number of points in space, situations of very practical importance where the standard methods may exhibit inaccuracy. This is a point of importance since the use of large generation domains is in favor of the accuracy of the source characterization, which can exhibit a complex directivity. The new method has been tested in a number of archetypal situations characterized by the presence of a reflecting plane, a scattering body close to the source location, and two sources placed side by side. In all cases, the method has shown its own superiority with respect to the standard HS method, still preserving the flexibility and simplicity of the latter

A CONTRIBUTION TO THE CONSERVATION OF MARINE BIODIVERSITY: EXPERIMENTAL SETUP FOR THE STUDY OF THE EFFECT OF NOISE ON FISH

Recognising noise as one of the main pollutants in the marine system (Kunc, 2016; Weschke, 2019), it is important to determine the type of noise (amplitude and range of frequencies) and how it affects the marine ecosystem and the different species (MAURO, 2020; VAZZANA, 2020). The role sound plays in the live of fish has been established relatively recently and it affects many essential processes for their survival . Although it is known their sensitivity both to the sound pressure level (SPL) and to the particle motion (PM), and despite the fact that differences between sound pressure and acoustic PM field at low frequencies were reported years ago (Banner, 1968), most of the works limit their results to the impact of SPL. Recently, the relevance of PM in the impact of sound waves on fish and invertebrates has been claimed and the challenge to define standard protocols to measure it has been stressed (Popper and Hawkings, 2018; Nedelec 2016). Amongst others, the SONORA project (Filling the gap: Thresholds assessment and impact beyond acoustic pressure level linked to emerging blue-growth activities) aims to evaluate the effects at behavioural and biochemical, cellular and molecular levels of underwater anthropogenic noise on two commercial fish species in two different experimental condition. The behavioural effects will be evaluated on adults and/or juvenile fish. For this purpose, an experimental setup and methodology for the study of the effect of noise on fish in tanks has been implemented. The dimensions of the tanks are 2x2x0.75 m. The first step has been to carry out a vibro-acoustic study of these tanks taking into account the noise sources (airborne and vibration) existing in the laboratory. This experimental study was complemented with a finite element simulation to estimate the spatial distribution of sound pressure levels and particle velocity inside the tank as a function of the sound radiated by the sound source. Two types of sound sources have been developed to carry out the experiments. The first is a conventional electrodynamic loudspeaker with a carbon fibre diaphragm. The second is a DML (Distributed Mode Loudspeaker) type. An accelerometer has been installed on both diaphragms so that it is possible to determine the radiated power of the loudspeaker from the vibration measurements. One or two hydrophones and a PM sensor based on a miniature hydrophone array will be used