440 research outputs found
Cavitation Induction by Projectile Impacting on a Water Jet
The present paper focuses on the simulation of the high-velocity impact of a projectile impacting on a water-jet, causing the onset, development and collapse of cavitation. The simulation of the fluid motion is carried out using an explicit, compressible, density-based solver developed by the authors using the OpenFOAM library. It employs a barotropic two-phase flow model that simulates the phase-change due to cavitation and considers the co-existence of non-condensable and immiscible air. The projectile is considered to be rigid while its motion through the computational domain is modelled through a direct-forcing Immersed Boundary Method. Model validation is performed against the experiments of Field et al. [Field, J., Camus, J. J., Tinguely, M., Obreschkow, D., Farhat, M., 2012. Cavitation in impacted drops and jets and the effect on erosion damage thresholds. Wear 290–291, 154–160. doi:10.1016/j.wear.2012.03.006. URL http://www.sciencedirect.com/science/article/pii/S0043164812000968 ], who visualised cavity formation and shock propagation in liquid impacts at high velocities. Simulations unveil the shock structures and capture the high-speed jetting forming at the impact location, in addition to the subsequent cavitation induction and vapour formation due to refraction waves. Moreover, model predictions provide quantitative information and a better insight on the flow physics that has not been identified from the reported experimental data, such as shock-wave propagation, vapour formation quantity and induced pressures. Furthermore, evidence of the Richtmyer-Meshkov instability developing on the liquid-air interface are predicted when sufficient dense grid resolution is utilised
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Numerical simulation of three-phase flow in an external gear pump using immersed boundary approach
This paper presents a three-phase fully compressible model applied along with an immersed boundary model for predicting cavitation occurring in a two dimensional gear pump in the presence of non-condensable gas (NCG). Combination of these models is capable of overcoming numerical challenges such as modelling the contact between the gears and simulating the effect of NCG in cavitation. The model accounting for the effect of NCG also has broader applicability, since gas dissolved in liquids can come out of the solution when exposed to low pressures; this plays a significant role in the pump performance and cavitation erosion. Here the simulation results are presented for the gear pump at different operating conditions including the contact between gear, gear RPM and % of NCG; their effects on performance and cavitation is demonstrated. The results suggest that modelling the contact between the gears play a role in the cavitation prediction inside the gear pump. An increase in cavitation is observed when the contact is modelled even for the small pressure difference considered between the inlet and outlet. An increase in the RPM of the gears also results in increased cavitation within the pump, whereas an increase in the percentage of NCG content by a small amount can reduce the cavitation to a greater extent. This reduction is due to the expansion of the gas at a lower pressure which recovers the pressure and prevents or delays the phase-change process of the working fluid. The fluctuations in the outflow rate is also found to increase when the gears are in contact and also with increasing gas content
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Cavitation Induction by Projectile Impacting on a Water Jet
Following the work of Field et al. [4], who experimentally visualised cavity formation and shock propagation in impacted liquids at high velocities, the present study focuses on the simulation of the high velocity impact of a solid projectile on a water jet. The undeformable solid projectile is modelled through a direct forcing Immersed Boundary Method. The simulation is carried out using an explicit density based compressible solver, developed by Kyriazis et al. [6], which employs a two-phase flow model and includes phase change. This study gives a better insight on the phenomena following the impact of solids on liquids, including shock propagation and vapour formation, and demonstrates the capabilities of the presented Immersed Boundary Method to handle compressible cavitating flows
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Immersed boundary method for cavitating and biological flows
The aim of the present work is the development of a computational tool to ease the numerical simulation of cavitating flows in domains of complex topology or with arbitrary moving boundaries. Within the framework of Computational Fluid Dynamics(CFD), an Immersed Boundary (IB) Method has been developed. According to the IB methodology, the grid that discretises the computational domain does not need to conform to the geometry and the solid boundaries are modelled on a fixed canonical grid by alternations of the governing equations in their vicinity. This modelling strategy is beneficial in terms of both computational cost and numerical solution. The grid generation, which is a complex and time consuming process, is simplified as a regular canonical grid, non-conformal to the boundaries, can be used. In addition, when moving boundaries are present, a conformal grid would need to adapt or deform following the motion of boundaries, which would increase the computational cost of the simulations in the first case and affect the solution in the latter case; the use of IB method alleviates these issues. The developed method follows the direct-forcing approach, which simply adds to the governing equations a source term to account for the body force acting on the fluid. The simplicity of the method makes it suitable for complex flow regimes, including phase change, strong shocks and compressibility effects, as well as Fluid Structure Interaction (FSI). Since cavitation dynamics regard a wide range of applications of engineering interest, from hydraulic machines to novel therapeutic techniques, the method is designed to be applicable in a wide range of flow regimes. Turbulent modelling and flow induced motion has been taken into account. The method has been successfully applied to cavitating and incompressible cases where conventional techniques are not easily or at all applicable. The shock-wave interaction with material interfaces is studied via the high-speed impact of a solid projectile on a water jet, which has been studied only experimentally before and only qualitative observations existed. The numerical investigation with the proposed methodology unveiled rich information regarding the physics of the impact, the resulting shock formation, cavitation development and interface instabilities initiation. Moreover, the methodology was applied on the thoroughly studied pulsatile flow through a bi leaflet Mechanical Heart Valve, to provide additional information regarding shear stress development. The methodology aids an experimental campaign employing novel shear stress measuring techniques, carried out by our collaborators. The research work and the developed method described in the present Thesis, intend to set the foundations for more elaborate numerical investigations of highly complex problems of Fluid Dynamics
A compressible Lagrangian framework for the simulation of underwater implosion problems
The development of efficient algorithms to understand implosion dynamics presents a number of challenges. The foremost challenge is to efficiently represent the coupled compressible fluid dynamics of internal air and surrounding water. Secondly, the method must allow one to accurately detect or follow the interface between the phases. Finally, it must be capable of resolving any shock waves which may be created in air or water during the final stage of the collapse. We present a fully Lagrangian compressible numerical framework for the simulation of underwater implosion. Both air and water are considered compressible and the equations for the Lagrangian shock hydrodynamics are stabilized via a variationally consistent multiscale method [109]. A nodally perfect matched definition of the interface is used [57, 25] and then the kinetic variables, pressure and density, are duplicated at the interface level. An adaptive mesh generation procedure, which respects the interface connectivities, is applied to provide enough refinement at the interface level. This framework is then used to simulate the underwater implosion of a large cylindrical bubble, with a size in the order of cm. Rapid collapse and growth of the bubble occurred on very small spatial (0.3mm), and time (0.1ms) scales followed by Rayleigh-Taylor instabilities at the interface, in addition to the shock waves traveling in the fluid domains are among the phenomena that are observed in the simulation. We then extend our framework to model the underwater implosion of a cylindrical aluminum container considering a monolithic fluid-structure interaction (FSI). The aluminum cylinder, which separates the internal atmospheric-pressure air from the external high-pressure water, is modeled by a three
node rotation-free shell element. The cylinder undergoes fast transient deformations, large enough to produce self-contact along it. A novel elastic frictionless contact model is used to detect contact and compute the non-penetrating forces in the discretized domain between the mid-planes of the shell. Two schemes are tested, implicit using the predictor/multi-corrector Bossak scheme, and explicit, using the forward Euler scheme. The results of the two simulations are compared with experimental data.El desarrollo de métodos eficientes para modelar la dinámica de implosión presenta varios desafÃos. El primero es una representación eficaz de la dinámica del sistema acoplado de aire-agua. El segundo es que el método tiene que permitir una detección exacta o un seguimiento adecuado de la interfase entre ambas fases. Por último el método tiene que ser capaz de resolver cualquier choque que podrÃa generar en el
aire o en el agua, sobre todo en la última fase del colapso.
Nosotros presentamos un método numérico compresible y totalmente Lagrangiano para simular la implosión bajo el agua. Tanto el aire como el agua se consideran compresibles y las ecuaciones Lagrangianos para la hidrodinámica del choque se estabilizan mediante un método multiescala que es variacionalmente consistente [109]. Se utiliza una definición de interfase que coincide perfectamente con los nodos [57, 25]. Ésta, nos facilita duplicar eficazmente las variables cinéticas como la presión y la densidad en los nodos de la interfase. Con el fin de obtener suficiente resolución alrededor de la interfase, la malla se genera de forma adaptativa y respetando la posición de la interfase. A continuación el método desarrollado se utiliza para simular la implosión bajo el agua de una burbuja cilÃndrica del tamaño de un centÃmetro. Varios fenómenos se han capturado durante el colapso: un ciclo inmediato de colapso-crecimiento de la burbuja que ocurre en un espacio (0.3mm) y tiempo (0.1ms) bastante limitado, aparición de inestabilidades de tipo Rayleigh-Taylor en la interfase y formaron de varias ondas de choque que viajan tanto en el agua como en el aire. Después, seguimos el desarrollo del método para modelar la implosión bajo el agua de un contenedor metálico considerando una interacción monolÃtica de fluido y estructura. El cilindro de aluminio, que a su vez contiene aire a presión atmosférica y está rodeada de agua en alta presión, se modelando con elementos de lámina de tres nodos y sin grados de libertad de rotación. El cilindro se somete a deformaciones transitorias suficientemente rápidos y enormes hasta llegar a colapsar. Un nuevo modelo elástico de contacto sin considerar la fricción se ha desarrollado para detectar el contacto y calcular las fuerzas en el dominio discretizado entre las superficies medianas de las laminas. Dos esquemas temporales están considerados, uno es implÃcito utilizando el método de Bossak y otro es explÃcito utilizando Forward Euler. Al final los resultados de ambos casos se comparan con los resultados experimentales
A compressible Lagrangian framework for the simulation of underwater implosion problems
The development of efficient algorithms to understand implosion dynamics presents
a number of challenges. The foremost challenge is to efficiently represent the coupled
compressible fluid dynamics of internal air and surrounding water. Secondly,
the method must allow one to accurately detect or follow the interface between the
phases. Finally, it must be capable of resolving any shock waves which may be created
in air or water during the final stage of the collapse. We present a fully Lagrangian
compressible numerical framework for the simulation of underwater implosion. Both
air and water are considered compressible and the equations for the Lagrangian shock
hydrodynamics are stabilized via a variationally consistent multiscale method.
A nodally perfect matched definition of the interface is used and then the kinetic
variables, pressure and density, are duplicated at the interface level. An adaptive
mesh generation procedure, which respects the interface connectivities, is applied to
provide enough refinement at the interface level. This framework is then used to simulate
the underwater implosion of a large cylindrical bubble, with a size in the order of
cm. Rapid collapse and growth of the bubble occurred on very small spatial (0.3mm),
and time (0.1ms) scales followed by Rayleigh-Taylor instabilities at the interface, in
addition to the shock waves traveling in the fluid domains are among the phenomena
that are observed in the simulation. We then extend our framework to model the
underwater implosion of a cylindrical aluminum container considering a monolithic
fluid-structure interaction (FSI). The aluminum cylinder, which separates the internal
atmospheric-pressure air from the external high-pressure water, is modeled by a three
node rotation-free shell element. The cylinder undergoes fast transient deformations,
large enough to produce self-contact along it. A novel elastic frictionless contact model
is used to detect contact and compute the non-penetrating forces in the discretized
domain between the mid-planes of the shell. Two schemes are tested, implicit using
the predictor/multi-corrector Bossak scheme, and explicit, using the forward Euler
scheme. The results of the two simulations are compared with experimental data
An immersogeometric formulation for free-surface flows with application to marine engineering problems
An immersogeometric formulation is proposed to simulate free-surface flows around structures with complex geometry. The fluid–fluid interface (air–water interface) is handled by the level set method, while the fluid–structure interface is handled through an immersogeometric approach by immersing structures into non-boundary-fitted meshes and enforcing Dirichlet boundary conditions weakly. Residual-based variational multiscale method (RBVMS) is employed to stabilize the coupled Navier–Stokes equations of incompressible flows and level set convection equation. Other level set techniques, including re-distancing and mass balancing, are also incorporated into the immersed formulation. Adaptive quadrature rule is used to better capture the geometry of the immersed structure boundary by accurately integrating the intersected background elements. Generalized-α role= presentation style= box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; line-height: normal; font-size: 16.2px; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative; \u3eα method is adopted for time integration, which results in a two-stage predictor multi-corrector algorithm. GMRES solver preconditioned with block Jacobian matrices of individual fluid and level set subproblems is used for solving the coupled linear systems arising from the multi-corrector stage. The capability and accuracy of the proposed method are assessed by simulating three challenging marine engineering problems, which are a solitary wave impacting a stationary platform, dam break with an obstacle, and planing of a DTMB 5415 ship model. A refinement study is performed. The predictions of key quantities of interest by the proposed formulation are in good agreement with experimental results and boundary-fitted simulation results from others. The proposed formulation has great potential for wide applications in marine engineering problems
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
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