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.
L'attività 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 è 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 è stata testata. La principale fonte di rumore è stata identificata nei termini detti di thickness e loading. Per rilevare questo tipo di fonte di rumore è 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 velocità molto più alta di quella del moto del flusso, così che la maggior parte dei fenomeni idrodinamici si trovano in un regime incomprimibile. La lunghezza d'onda è comunemente molto più 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) è qui considerata come lo strumento più vantaggioso per riprodurre flussi turbolenti e, allo stesso tempo, affrontare casi di interesse pratico. La prima parte dello studio è 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 velocità di un corpo è piccola rispetto alla velocità del suono, l'integrazione diretta del termine del volume è 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 è dedicata ad uno studio preliminare del campo acustico emesso da un flusso cavitante. La cavitazione può 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 è correlato alla loro presenza in varie applicazioni tecniche, come pompe, turbine, eliche di navi e sistemi di iniezione di carburante, nonché in scienze mediche. Esistono diversi tipi di cavitazione, tra le più importanti ci sono: sheet cavitation, bubble cavitation and vortex cavitation. La sheet cavitation può verificarsi sui profili alari, su pale di pompe ed eliche, in particolare quando il carico è elevato. Questo tipo di cavitazione difficilmente può 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.
Acoustic Analogies and Large-Eddy Simulations of Incompressible and Cavitating Flows Around Bluff Bodies / Cianferra, Marta. - (2018 Mar 23).
Acoustic Analogies and Large-Eddy Simulations of Incompressible and Cavitating Flows Around Bluff Bodies
CIANFERRA, MARTA
2018-03-23
Abstract
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.File | Dimensione | Formato | |
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