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

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 simulación numérica de algunos set-up físicos que involucran la interacción entre dos o tres fluidos incompresibles y immiscibles. Las simulaciones se realizan por medio de algoritmos de volúmenes-finitos desarrollados en la plataforma propia de Fluido-Dinámica Computacional (CFD) denominada TermoFluids, desarrollada en el Centro Tecnologico de Trasferencia de Calor (CTTC). Las simulaciones quieren estudiar en detalle la física de los fenómenos analizados, realizando su Simulación Numérica Directa (DNS). En el contexto de los flujos multifase, DNS significa que todas las escalas interfaciales y turbulentas del fenómeno han de ser totalmente resueltas. En la Introducción, se propone una panorámica general de las aplicaciones de ingeniería y de los métodos computacionales relacionados con flujos multifases. Se introducen los varios tipos de física analizados en este trabajo y las estrategias numéricas aplicadas aquí para efectuar su simulación de manera eficiente. En el Capitulo 2 se analiza un esquema convectivo de baja-disipación para la discretization de flujo multifase por medio de métodos de interface-capturing. La forma híbrida del operador convectivo propuesto incorpora la característica de una baja dispersión localizada, focalizada en limitar el crecimiento de soluciones numéricas espurias. Además, en comparación con métodos disipativos puros, la discretización apunta a minimizar las diferencias en la conservación de energía cinética 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 simulación de un jet 2D coaxial turbulento con el método convectivo de baja disipación demuestra su capacidad de resolver correctamente un flujo de dos fases turbulentos. En el Capitulo 3 se reporta todo el trabajo realizado sobre la simulación de flujo multifase con el auxilio de técnicas de refinamiento adaptativo de malla (AMR). El modelo es globalmente dirigido a la mejora de la representación de las escalas turbulentas y interfaciales en flujos multifases en general. Se aplica inicialmente a la simulación de flujos sencillos, como unos casos de burbujas flotantes 2D y 3D, demostrando la convergencia del método y los importantes ahorros computacional en comparación con los cálculos de mallas estáticas. La adopción de la técnica se hace esencial en la simulación de fenómenos de inestabilidad y de ruptura, donde la necesidad de representar sacramentalmente las estructuras complejas que aparecen en la interfaz, como ligamentos o pequeñas gotas, hacen que la simulación sea particularmente pesada en términos de coste computacional. En el Capitulo 4 se reportan en detalle las simulaciones de fenómenos de atomización 3D. Esas incluyen el caso del jet coaxial, caracterizado por la inyección paralela de flujos de aire y liquido de altas velocidades, y el caso del spray liquido, que consiste en la inyección de un liquido dentro de una cámara 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 desactivación de la fase ligera, permitiendo la optimización del solver clásico 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 análisis de problemas que involucran el movimiento de superficies libres, como la evolución de olas en la superficie marina y su interacción con obstáculos fijos o muebles. Se proponen algunos casos prácticos de aplicación, como la evaluación 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

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

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 simulación numérica de algunos set-up físicos que involucran la interacción entre dos o tres fluidos incompresibles y immiscibles. Las simulaciones se realizan por medio de algoritmos de volúmenes-finitos desarrollados en la plataforma propia de Fluido-Dinámica Computacional (CFD) denominada TermoFluids, desarrollada en el Centro Tecnologico de Trasferencia de Calor (CTTC). Las simulaciones quieren estudiar en detalle la física de los fenómenos analizados, realizando su Simulación Numérica Directa (DNS). En el contexto de los flujos multifase, DNS significa que todas las escalas interfaciales y turbulentas del fenómeno han de ser totalmente resueltas. En la Introducción, se propone una panorámica general de las aplicaciones de ingeniería y de los métodos computacionales relacionados con flujos multifases. Se introducen los varios tipos de física analizados en este trabajo y las estrategias numéricas aplicadas aquí para efectuar su simulación de manera eficiente. En el Capitulo 2 se analiza un esquema convectivo de baja-disipación para la discretization de flujo multifase por medio de métodos de interface-capturing. La forma híbrida del operador convectivo propuesto incorpora la característica de una baja dispersión localizada, focalizada en limitar el crecimiento de soluciones numéricas espurias. Además, en comparación con métodos disipativos puros, la discretización apunta a minimizar las diferencias en la conservación de energía cinética 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 simulación de un jet 2D coaxial turbulento con el método convectivo de baja disipación demuestra su capacidad de resolver correctamente un flujo de dos fases turbulentos. En el Capitulo 3 se reporta todo el trabajo realizado sobre la simulación de flujo multifase con el auxilio de técnicas de refinamiento adaptativo de malla (AMR). El modelo es globalmente dirigido a la mejora de la representación de las escalas turbulentas y interfaciales en flujos multifases en general. Se aplica inicialmente a la simulación de flujos sencillos, como unos casos de burbujas flotantes 2D y 3D, demostrando la convergencia del método y los importantes ahorros computacional en comparación con los cálculos de mallas estáticas. La adopción de la técnica se hace esencial en la simulación de fenómenos de inestabilidad y de ruptura, donde la necesidad de representar sacramentalmente las estructuras complejas que aparecen en la interfaz, como ligamentos o pequeñas gotas, hacen que la simulación sea particularmente pesada en términos de coste computacional. En el Capitulo 4 se reportan en detalle las simulaciones de fenómenos de atomización 3D. Esas incluyen el caso del jet coaxial, caracterizado por la inyección paralela de flujos de aire y liquido de altas velocidades, y el caso del spray liquido, que consiste en la inyección de un liquido dentro de una cámara 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 desactivación de la fase ligera, permitiendo la optimización del solver clásico 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 análisis de problemas que involucran el movimiento de superficies libres, como la evolución de olas en la superficie marina y su interacción con obstáculos fijos o muebles. Se proponen algunos casos prácticos de aplicación, como la evaluación 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

Direct numerical simulation of multiphase flows with unstable interfaces

Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.This paper presents a numerical model that intends to simulate efficiently the surface instability that arise in multiphase flows, typically liquid-gas, both for laminar or turbulent regimes. The model is developed on the in-house computing platform TermoFluids , and operates the finite-volume, direct numerical simulation (DNS) of multiphase flows by means of a conservative level-set method for the interface-capturing. The mesh size is optimized by means of an adaptive mesh refinement (AMR) strategy, that allows the dynamic re-concentration of the mesh in the vicinity of the interfaces between fluids, in order to correctly represent the diverse structures (as ligaments and droplets) that may rise from unstable phenomena. In addition, special attention is given to the discretization of the various terms of the momentum equations, to ensure stability of the flow and correct representation of turbulent vortices. As shown, the method is capable of truthfully simulate the interface phenomena as the Kelvin-Helmholtz instability and the Plateau-Rayleigh instability, both in the case of 2-D and 3-D configurations. Therefore it is suitable for the simulation of complex phenomena such as simulation of air-blast atomization, with several important application in the field of automotive and aerospace engines. A prove is given by our preliminary study of the 3-D coaxial liquid-gas jet.Peer ReviewedPostprint (published version

Evaluation of the Lubrication Regime for Rotary Compressors. Influence of Thermal Expansions on the Minimum Film Thickness.

In this work, the lubrication regime of a rotary compressor is studied in the regions of the roller, cylinder, shaft, and bearings. A code is programmed so it can solve multiple cases with different suction-discharge pressures, angular velocity, viscosity, and geometrical conditions, among others. In order to study the lubrication regime, the balance of the different forces that act on the roller is computed. To calculate the oil pressure forces, the Reynolds equation coupled with an iterative method is used to find the minimal distance and pressure distribution that balances the external forces. This process is repeated for a discrete number of rotation angles to evaluate the overall rotation of a cycle. In addition, the effect that the thermal expansion of the different materials can have on the lubrication regime is studied. If the materials get too hot, the distances between surfaces might increase or reduce with respect to the distances at room temperature. These changes might lead to a point where the lubrication regime changes. The objective is to include this thermal expansion effect in the model and evaluate how it can affect the lubrication regime.The author Jordi Vera has been financially supported by the Ministerio de Educación y Ciencia (MEC), Spain, (FPI grant PRE2018-084017). The author E.Schillaci acknowledges the financial support of the Programa Torres Quevedo (PTQ2018-010060).Postprint (published version

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

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 ‘neck’. 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

Analysis of high-order interpolation schemes for solving linear problems in unstructured meshes using the finite volume method

Finite-volume strategies in fluid-structure interaction problems would be of crucialvimportance 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.P. Castrillo gratefully acknowledges the Universitat Politècnica de Catalunya and Banco Santander for the financial support of his predoctoral grant FPI-UPC (109 FPI-UPC 2018). The authors are supported by the Ministerio de Economía y Competitividad, Spain, RETOtwin project (PDC2021-120970-I00).Peer ReviewedPostprint (published version

Numerical simulation of fluid structure interaction in free-surface flows: the WEC case

In this work we present a numerical framework to carry-out numerical simulations of fluid-structure interaction phenomena in free-surface flows. The framework employs a singlephase method to solve momentum equations and interface advection without solving the gas phase, an immersed boundary method (IBM) to represent the moving solid within the fluid matrix and a fluid structure interaction (FSI) algorithm to couple liquid and solid phases. The method is employed to study the case of a single point wave energy converter (WEC) device, studying its free decay and its response to progressive linear waves.E.Schillaci acknowledges the financial support of the Programa Torres Quevedo (PTQ2018- 010060). The work has also been supported by a competitive R+D project (PID2020-115837RBI00) of the Spanish Research Agency.Peer ReviewedPostprint (published version

On estimating the interface normal and curvature in PLIC-VOF approach for 3D arbitrary meshes

Volume of fluid (VOF) method with its Piecewise Linear Interface Calculation (PLIC) reconstruction algorithm is one of the most popular approaches in numerical simulation of interfacial flows with a wide range of applications in different areas. In an effort to evaluate the similarity of the PLIC-generated planes in comparison with the exact interface, a point-cloud, based on the polygon centers of PLIC planes is extracted, which later is used to form a triangular grid that represents the estimated interface. The main objective of this article is to evaluate the interface geometrical properties based on the extracted triangular grid of the interface. The methods presented in this article, characterized by a higher spatially convergence ratio, are compared with the commonly used methods. The proposed methods are tested for two 3-dimensional general test cases, where an evident improvement is seen in calculation accuracy and spatial convergence of the errors of interface normal vector and curvature.This work has been financially supported by MCIN/AEI/10.13039/ 501100011033 Spain, project PID2020-115837RBI00. E. Schillaci acknowledges the financial support of the Programa Torres Quevedo (PTQ2018-010060).Peer ReviewedPostprint (author's final draft