32 research outputs found

    DNS of multiphase flows: study of atomization and free-surface phenomena

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    The present thesis focuses on the numerical analysis of some diverse physical set-up that involve the interaction of two -or three immiscible and incompressible phases. The simulations are carried out by means of finite-volume algorithms developed on the in-house Computational Fluid Dynamics platform TermoFluids, developed by the Heat and Mass Transfer Technological Center (CTTC). They are intended to give detailed insights on the physics of the analyzed phenomena by carrying out Direct Numerical Simulations (DNS). In the context of multiphase flows, namely, Computational Multi-Fluid Dynamics (CMFD) field, DNS means that all the interfacial and turbulent scales of the phenomenon must be fully resolved. In the Introduction, a general overview of the engineering applications and the computational methods related to multiphase flows is proposed. The various types of physics analyzed in this work and the numerical approaches applied here to carry out efficient simulations are introduced. In Chapter 2, a low-dissipation convection scheme for the stable discretization of multiphase flow by means of interface-capturing schemes is analyzed. The hybrid form of the convective operator proposed incorporates localized low-dispersion characteristics to limit the growth of spurious flow solutions. Moreover, in comparison to pure-dissipative schemes, the discretization aims at minimizing the differences in kinetic energy preservation with respect to the continuous governing equations. This property plays a fundamental role in the case of flows presenting significant levels of turbulence. The simulation of a turbulent 2D coaxial jet with the low-dissipation convection scheme demonstrates its capability of solving correctly the two-phase turbulent problems. In Chapter 3, all the work carried out on the simulation of two-phase flow with the aid of Adaptive Mesh Refinement (AMR) strategies is described. The model is globally addressed at improving the representation of interfacial and turbulent scales in general multiphase flows. It is first applied to the simulation of simple multiphase phenomena, as 2D and 3D rising bubbles, to demonstrate the convergence of the method and the important computational savings in comparison to static mesh computations. However, its adoption becames essential in the simulation of instability and break-up phenomena, where the necessity of representing accurately the complex structures that appear at the interface, as ligaments and droplets, make the simulation particularly expensive in terms of computational cost. In Chapter 4, we analyze in detail the simulations of 3-D atomizing phenomena, including the coaxial jet case, characterized by the parallel injection of high speed liquid and gas fluxes, and the liquid spray case, characterized by the injection of a high speed liquid inside a still air chamber. In Chapter 5, an original single-phase scheme for the DNS of free-surface problems on 3-D unstructured meshes is presented. The scheme is based on a novel treatment of the interface for the deactivation of the light-phase, allowing an optimization of the classic two-phase model for the cases in which the influence of the lighter phase is negligible. Consequently, the model is particularly addressed at analyzing problems involving the movement of free-surfaces, as the evolution of waves on the sea, and their interaction with fixed and moving obstacles. Some practical cases of application are proposed, as the evaluation of stresses on an object due to the action of a dam-break event, and the interaction of linear waves with an oscillating water column device. In the same Chapter we describe the procedure to couple the single-phase model to the Immersed Boundary Method. The method is aimed at representing the interaction of a solid moving with prescribed velocity and the two-phase flow. The most significant example consists in the simulation of a sliding wedge into a liquid basin.Esta tesis se focaliza en la simulaci贸n num茅rica de algunos set-up f铆sicos que involucran la interacci贸n entre dos o tres fluidos incompresibles y immiscibles. Las simulaciones se realizan por medio de algoritmos de vol煤menes-finitos desarrollados en la plataforma propia de Fluido-Din谩mica Computacional (CFD) denominada TermoFluids, desarrollada en el Centro Tecnologico de Trasferencia de Calor (CTTC). Las simulaciones quieren estudiar en detalle la f铆sica de los fen贸menos analizados, realizando su Simulaci贸n Num茅rica Directa (DNS). En el contexto de los flujos multifase, DNS significa que todas las escalas interfaciales y turbulentas del fen贸meno han de ser totalmente resueltas. En la Introducci贸n, se propone una panor谩mica general de las aplicaciones de ingenier铆a y de los m茅todos computacionales relacionados con flujos multifases. Se introducen los varios tipos de f铆sica analizados en este trabajo y las estrategias num茅ricas aplicadas aqu铆 para efectuar su simulaci贸n de manera eficiente. En el Capitulo 2 se analiza un esquema convectivo de baja-disipaci贸n para la discretization de flujo multifase por medio de m茅todos de interface-capturing. La forma h铆brida del operador convectivo propuesto incorpora la caracter铆stica de una baja dispersi贸n localizada, focalizada en limitar el crecimiento de soluciones num茅ricas espurias. Adem谩s, en comparaci贸n con m茅todos disipativos puros, la discretizaci贸n apunta a minimizar las diferencias en la conservaci贸n de energ铆a cin茅tica en respeto a las ecuaciones continuas que gobiernan el flujo. Esta propiedad juega un papel fundamental en el caso de flujo caracterizado por un alto nivel de turbulencia. La simulaci贸n de un jet 2D coaxial turbulento con el m茅todo convectivo de baja disipaci贸n demuestra su capacidad de resolver correctamente un flujo de dos fases turbulentos. En el Capitulo 3 se reporta todo el trabajo realizado sobre la simulaci贸n de flujo multifase con el auxilio de t茅cnicas de refinamiento adaptativo de malla (AMR). El modelo es globalmente dirigido a la mejora de la representaci贸n de las escalas turbulentas y interfaciales en flujos multifases en general. Se aplica inicialmente a la simulaci贸n de flujos sencillos, como unos casos de burbujas flotantes 2D y 3D, demostrando la convergencia del m茅todo y los importantes ahorros computacional en comparaci贸n con los c谩lculos de mallas est谩ticas. La adopci贸n de la t茅cnica se hace esencial en la simulaci贸n de fen贸menos de inestabilidad y de ruptura, donde la necesidad de representar sacramentalmente las estructuras complejas que aparecen en la interfaz, como ligamentos o peque帽as gotas, hacen que la simulaci贸n sea particularmente pesada en t茅rminos de coste computacional. En el Capitulo 4 se reportan en detalle las simulaciones de fen贸menos de atomizaci贸n 3D. Esas incluyen el caso del jet coaxial, caracterizado por la inyecci贸n paralela de flujos de aire y liquido de altas velocidades, y el caso del spray liquido, que consiste en la inyecci贸n de un liquido dentro de una c谩mara de aire. En el Capitulo 5 se presenta un esquema de single-phase original, para el DNS de problemas de superficie libre en mallas 3D no-estructuradas. El esquema se basa en un nuevo tratamiento de la interfase para la desactivaci贸n de la fase ligera, permitiendo la optimizaci贸n del solver cl谩sico de dos fases para los casos en que la influencia de la fase mas ligera sea despreciable. En consecuencia, el modelo es particularmente indicado para la an谩lisis de problemas que involucran el movimiento de superficies libres, como la evoluci贸n de olas en la superficie marina y su interacci贸n con obst谩culos fijos o muebles. Se proponen algunos casos pr谩cticos de aplicaci贸n, como la evaluaci贸n de las fuerzas sobre un objeto debidos a un episodio de dam-break, o el estudio de las olas generadas por el impacto de un solido deslizante (representado integrando la tecnica de Immersed Boundary con el presente metodo de single-phase) con un embalse de agua.Postprint (published version

    Simulation of fluid-structure interaction and impact force on a reed valve

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    The cyclic impact force between a reed valve and the seat plate is the main reason of the valve failure in many thermo-technical devices as compressors, engines, etc. According to experimental observations the latter is due to fatigue and usually occurs in the leading part of the valve 鈥榥eck鈥. In this work, a complex numerical analysis is presented aimed to studying the external forces and internal stresses suffered by the valve. In particular, the impact force between the valve and the seat is studied. The numerical analysis relies on the coupled synergy of two different simulation concepts. In order to do so, two codes are used: (1) first, the in-house Computational Fluid Dynamics (CFD) code presented in [1] is employed to simulate the Fluid-Structure Interaction (FSI) between gas and valve, extracting reference data for valve displacement and external gas pressures; (2) second, the analysis of the internal structure stresses, together with the impact forces with the plate is implemented in a Computational Solid Dynamics (CSD) code developed in FreeFEM++ [2]. The impact force representation is based on the formulation presented in [3] where a conserving algorithm for frictionless dynamic contact/impact is developed. Due to the importance of obtaining an adequate impact force, an exhaustive study is carried out on its characterization in terms of numerical parameters, such as the penalty stiffness. Under this framework, the valve displacement and impact velocities are verified. Hence, impact forces are analysed in different scenarios, obtaining interesting observations about stresses distribution, with a particular focus on the points where failure is experienced.The authors acknowledge Voestalpine Precision Strip AB company for the previous research collaboration project that allowed to validate experimentally the presented numerical methods. P. Castrillo gratefully acknowledges the Universitat Politecnica de Catalunya and Banco Santander for the financial ` support of his predoctoral grant FPI-UPC (109 FPI-UPC 2018). E. Schillaci acknowledges the financial support of the Programa Torres Quevedo (PTQ2018-010060). This work has also been financially supported by a competitive R+D project (ENE2017-88697-R) by the Spanish Research Agency.Postprint (published version

    Numerical Analysis of Viscoelastic Fluid Injection Processes

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    In this work, a numerical framework aimed at simulating high-viscous and viscoelastic liquid injection processes is presented. The study of fluid injection with viscoelastic properties, such as synthetic polymers, is of great importance in several industrial sectors, such as metallurgy, automotive, food and pharmaceutical products, ink jets and 3D printers. In this work, a Geometrical Volume-of-Fluid (VOF) method is used to represent the interface, while finite-volume discretizations of Navier-Stokes equations on collocated unstructured meshes are solved through a fractional step method. Viscoelastic constitutive models are used to resolve the non-Newtonian behaviours. The employed implementations allow integrating different types of constitutive equations and stabilization approaches. The test case proposed in the current work consist of the simulation of the discharge of a polymeric jet from an upper nozzle into an air-containing cavity with solid bottom. The behavior of the fluids under analysis is validated by observing the onset of fluid-buckling s tructures. This phenomenon consists in the appearance of toroidal oscillation (as coiling and folding patterns) after the jet hits the solid surface and begins to accumulate. First, low-Reynolds number (Re<1.0) Newtonian Jets are analyzed, with the objective of validating the appearance of buckling as a function of Re number, and comparing the results with reference works. Finally, the potentialities of the proposed numerical methods are shown by simulating buckling phenomena in viscoelastic Jets described with the Oldroyd-B constitutive model.This work was developed in the context of a research project (BASE3D 001-P-001646) co-financed by the European Union Regional Development Fund within the framework of the ERDF Operational Program of Catalonia 2014-2020 with a grant of 50% of total cost eligible. The work has also been financially supported by a competitive R+D project (ENE2017-88697-R) of the Spanish Research Agency. The author E.Schillaci acknowledges the financial support of the Programa Torres Quevedo (PTQ2018- 010060).Postprint (published version

    A numerical set-up for the simulation of infection probability from SARS- CoV-2 in public transport vehicles

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    In this work, a numerical framework aimed at simulating the transport of contaminants and infectious agents within a closed domain is presented. The method employs mature CFD algorithms to calculate air fields with reasonable computational costs. The main objective is to give fast response to stakeholders about air quality indicators in the design phase of HVAC systems. A discussion regarding the size and characteristics of different contaminants is proposed, highlighting the most appropriate methods and coefficients needed to simulate their transport. Next, the methodology employed to evaluate the risk of infection is presented. The numerical set-up, based on the buoyantBoussinesqPimpleFoam solver in OpenFOAM, was tuned by simulating the well-known case of the heated floor cavity, providing accurate results. Hence, the case study of a transport vehicle of generic shape is presented, in order to show possible results in terms of air-age distribution, PM2.5 distribution, and global infection risk matrix.This work has been developed in the context of the Rolen Purifica Bus R&D project, partially financed by INNOTEC (ACCIO - Ag猫ncia per la Competitivitat de l鈥橢mpresa, Generalitat de 麓 Catalunya). J. Vera has been financially supported by the Ministerio de Educaci贸n y Ciencia (MEC), Spain, (FPI grant PRE2018-084017). N. Morozova is supported by the by the Ministerio de Econom铆a y Competitividad, Spain [FPU16/06333 predoctoral contract].Peer ReviewedPostprint (published version

    Analysis of high-order interpolation schemes for the finite-volume resolution of linear problems on unstructured meshes

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    Finite-volume strategies in fluid-structure interaction problems would be of crucial importance in many engineering applications such as in the analysis of reed valves in reciprocating compressors. The efficient implementation of this strategy passes from the formulation of reliable high-order schemes on 3D unstructured meshes. The development of high-order models is essential in bending-dominant problems, where the phenomenon of shear blocking appears. In order to solve this problem, it is possible to either increase the number of elements or increase the interpolation order of the main variable. Increasing the number of elements does not always yield good results and implies a very high computational cost that, in real problems, is inadmissible. Using unstructured meshes is also vital because they are necessary for real problems where the geometries are complex and depart from canonical rectangular or regular shapes. This work presents a series of tests to demonstrate the feasibility of a high-order model using finite volumes for linear elasticity on unstructured and structured meshes. The high-order interpolation will be performed using two different schemes such as the Moving Least Squares (MLS) and the Local Regression Estimators (LRE). The reliability of the method for solving 2D and 3D problems will be verified by solving some known test cases with an analytical solution such as a thin beam or problems where stress concentrations appear

    Numerical study of an impulse wave generated by a sliding mass

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    漏 2018 WIT PressIn this work, a numerical framework for the direct numerical simulation of tsunami waves generated by landslide events is proposed. The method, implemented on the TermoFluids numerical platform, adopts a free surface model for the simulation of momentum equations; thus, considering the effect of air on the flow physics negligible. The effect of the solid motion on the flow is taken into account by means of a direct forcing immersed boundary method (IBM). The method is available for 3-D unstructured meshes; however, it can be integrated with an adaptive mesh refinement (AMR) tool to dynamically increase the local definition of the mesh in the vicinity of the interfaces, which separate the phases or in the presence of vortical structures. The method is firstly validated by simulating the entrance of objects into still water surfaces for 2-D and 3-D configurations. Next, the case of tsunami generation from a subaerial landslide is studied and the results are validated by comparison to experimental and numerical measurements. Overall, the model demonstrates its efficiency in the simulation of this type of physics, and a wide versatility in the choice of the domain discretization.Peer ReviewedPostprint (published version

    High Sensitivity C-Reactive Protein Increases the Risk of Carotid Plaque Instability in Male Dyslipidemic Patients

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    Background: The aim of this study was to evaluate how the high sensitivity C-reactive protein (hs-CRP) values influence the risk of carotid plaque instability in association with other cardiovascular risk factors. Methods: One hundred and fifty-six carotid plaques from both symptomatic and asymptomatic patients requiring surgical carotid endarterectomy were retrospectively collected. According to the modified American Heart Association, atherosclerosis plaques have been histologically distinguished into unstable and stable. The following anamnestic and hematochemical data were also considered: age, gender, hypertension, diabetes mellitus, smoking habit, therapy, low-density lipoprotein (LDL)-C, kidney failure and hs-CRP. Results: The results of our study clearly show that high levels of hs-CRP significantly increase the carotid plaque instability in dyslipidemic patients. Specifically, a 67% increase of the risk of carotid plaque instability was observed in patients with high LDL-C. Therefore, the highest risk was observed in male dyslipidemic patients 2333 (95% CI 0.73鈥7.48) and in aged female patients 2713 (95% CI 0.14鈥53.27). Discussion: These data strongly suggest a biological relationship between the hs-CRP values and the alteration of lipidic metabolism mostly in male patients affected by carotid atherosclerosis. The measurement of hs-CRP might be useful as a potential screening tool in the prevention of atheroscletotic disease
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