Dissertations / Theses on the topic 'One Dimensional Turbulence'

We investigate one-dimensional (1D) wave turbulence (WT) systems that are characterised by six-wave interactions. We begin by presenting a brief introduction to WT theory - the study of the non-equilibrium statistical mechanics of nonlinear random waves, by giving a short historical review followed by a discussion on the physical applications. We implement the WT description to a general six-wave Hamiltonian system that contains two invariants, namely, energy and wave action. This enables the subsequent derivations for the evolutions equations of the one-mode amplitude probability density function (PDF) and kinetic equation (KE). Analysis of the stationary solutions of these equations are made with additional checks on their underlying assumptions for validity. Moreover, we derive a differential approximation model (DAM) to the KE for super-local wave interactions and investigate the possible occurrence of a fluctuation relation. We then consider these results in the context of two physical systems - Kelvin waves in quantum turbulence (QT) and optical wave turbulence (OWT). We discuss the role of Kelvin waves in decaying QT, and show that they can be described by six-wave interactions. We explicitly compute the interaction coefficients for the Biot-Savart equation (BSE) Hamiltonian and represent the Kelvin wave dynamics in the form of a KE. The resulting non-equilibrium Kolmogorov-Zakharov (KZ) solutions to the KE are shown to be non-local, thus a new non-local theory for Kelvin wave interactions is discussed. A local equation for the dynamics of Kelvin waves, the local nonlinear equation (LNE), is derived from the BSE in the asymptotic limit of one long Kelvin wave. Numerical computation of the LNE leads to an agreement with the nonlocal Kelvin wave theory. Finally, we consider 1D OWT. We present the first experimental implementation of OWT and provide a comparable decaying numerical simulation for verification. We show that 1D OWT is described by a six-wave process and that the inverse cascade state leads to the development of coherent solitons at large scales. Further investigation is conducted into the behaviour of solitons and their impact to the WT description. Analysis of the fluxes and intensity PDFs lead to the development of a wave turbulence life cycle (WTLC), explaining the coexistence between coherent solitons and incoherent waves. Additional numerical simulations are performed in non-equilibrium stationary regimes to determine if a pure KZ state can be realised.

In the incineration of liquid hazardous wastes there exist "rogue" droplets (>300 μm diameter) which penetrate past the flame zone and burn as isolated droplets in the postflame gasses. Detailed knowledge of the droplet burnout points are essential to keeping the destruction removal efficiency in excess of the 99.99% required. The spread in trajectory endpoints of individual evaporating droplet streams injected into a turbulent combustor was investigated numerically. Results are in good agreement with the measurements. Correlation between the spread in the burnout points and initial droplet size, initial droplet velocity, interdroplet spacing, and droplet injection angle were investigated. The numerical investigation utilizes the novel One Dimensional Turbulence (ODT) {Kerstein (1999)} for the time developing fluid velocity and temperature fields with a new two phase flow model for predicting particle trajectories. The droplet heating/burning model used by Mulholland et al. (1991) is modified for application to this thesis.

This dissertation presents the development of a method for integrating two-phase flow into the vector formulation of the One Dimensional Turbulence model (ODT). The novel ODT model is an unsteady turbulent flow simulation model implemented on a one-dimensional domain, representing flow evolution as observed along a line of sight through a 3D turbulent flow. Overturning motions representing individual eddies are implemented as instantaneous rearrangement events. They obey applicable conservation laws and emulate the multiplicative increase of strain and decrease of length scales associated with the turbulent cascade. Eddy occurrences are random, with likelihoods proportional to a local measure of shear kinetic energy. These events punctuate conventional time advancement of viscous transport. In the present study, the ODT configuration used to simulate turbulent channel flow is augmented by a representation of particles coupled to the fluid by a drag law, with one-way coupling. It is straightforward to implement this drag coupling using the vector wall-normal fluid velocity profile evolved by ODT, but motion (displacement by eddy events) and velocity are distinct in ODT, so this procedure violates physical requirements such as correct representation of the marker-particle limit. Instead, a particle-eddy interaction mechanism is introduced. ODT eddies are instantaneous, so this interaction is defined by integrating the drag law over the lifetime of the corresponding physical eddy, but applying the resulting particle location and velocity change at the instant of eddy occurrence. A subtraction procedure is used to prevent double-counting of particle-eddy interaction due to subsequent viscous time advancement over the same time interval. The net outcome is a particle-eddy interaction that obeys correct limiting behaviors and transitions smoothly between these limits. This formulation introduces a free parameter that multiplies a scaling estimate of the eddy lifetime. Numerical simulations were run with turbulent friction Reynolds numbers ranging from 180 to 1395. Validation was achieved by comparing (1) wall-normal profiles of particle statistics with DNS, LES, and experiments; (2) wall deposition for particles from the inertial range of (Stokes number) 0.3 <= Tau+ <= 55,000 to DNS, LES, and experiments; (3) the non-inertial, Brownian Motion, regime was demonstrated by comparison with experiments and DNS.

Each year fires destroy millions of acres of woodland, lives, and property, and significantly contribute to air pollution. Increased knowledge of the physics and properties of the flame propagation is necessary to broaden the fundamental understanding and modeling capabilities of fires. Modeling flame propagation in fires is challenging because of the various modes of heat transfer with diverse fuels, multi-scale turbulence, and complex chemical kinetics. Standard physical models of turbulence like RANS and LES have been used to understand the flame behavior, but these models are limited by computational cost and their inability to resolve sub-grid scales. Application of several other models and empirical studies in fire modeling are usually limited to fire spread rate only. In some fires, flame propagation often occurs through convective heating by direct flame contact as opposed to radiative preheating alone. Under these conditions, resolution of the flame front can provide the detailed physics and insights into the flame propagation. The One Dimensional Turbulence (ODT) model is extended to turbulent flame propagation in biomass fuel beds representative of those in wild land fires. ODT is a stochastic model that is computationally affordable and can resolve both large and fine scales. ODT has been widely applied to many reacting and non-reacting flows like jet flames and pool fires. A detailed particle combustion model has been developed and implemented in the ODT model to investigate the fluctuating flame-fuel interface and to study flame propagation properties. The particle reaction is modeled as a single global decomposition reaction model. Radiative, convective, and internal particle conductive heat transfer are included. Gaseous combustion is modeled with a lookup table parameterized by mixture fraction and fractional heat loss using steady laminar flame let solutions. Results are presented from simulations of flame propagation in buoyantly driven flows. Particle size and loading are varied to study their effects in flame spread. A timescale analysis is performed to compare radiative, convective, conductive, and reactive particle time scales to the turbulent fluctuations. The flame propagation in homogeneous turbulence is also studied which better represents the wildland fire. The time scales involved in the wildland fire are overlapped using LEM model to study their effects on the flame properties and flame spread.

The sub-Antarctic Zone (SAZ) is a zone of vigorous vertical mixing in the Southern Ocean where it is difficult to obtain data for model validation on the turbulence conditions. In this study, a onedimensional configuration from the Nucleus for the European Modelling of the Ocean (NEMO) model was implemented in order to determine the sensitivity and turbulence response of an idealized SAZ water column. Various turbulence scheme parameterizations that are available for ocean models were tested. Furthermore, the number of vertical levels were varied in order to ascertain the sensitivity of the grid. The forcing data were obtained from various reanalyses (ERA-Interim, NASA, NCEP and JRA55) and were likewise tested. Different turbulence diagnostics and univariate indicators were chosen to ascertain the turbulence response and to analyse the energetics of the water column. It was found that using different reanalyses produced different tracer (salinity and temperature) results. Even though the results varied considerably, very high correlations were found for the potential energy anomaly between reanalyses and insignificant correlations were found for the other indicators. This suggested that it was a valuable descriptor which captured the buoyancy fluxes and wind stress information and can be efficiently used to assess the vertical turbulent state with data such as ARGO profiles. It was further found that for a single reanalysis, the turbulence schemes had produced similar results (with small variability and not to the extent as changing the reanalysis) for the turbulence diagnostics and univariate indicators. An important finding of an entrapped warm water parcel beneath cooler waters was found in simulation outputs as well as ARGO validation data. For realistic conditions observed from the ARGO floats, as the season progressed, there were no more instances of a warm water parcel. There was no reason however, to why there should not have been eddies passing by the region. In simulations, the warm water parcel persisted throughout the season for simulated data, likely causing the early stratification that affects ocean models in the SAZ. The stratification was found to have an approximate one month early onset observed from comparing the ARGO data profiles to simulated profiles. The Brunt Väisälä frequency, potential energy anomaly as well as the buoyancy flux were analysed and these diagnostics indicated that an approximate one month early stratification was found during November. It was likely that this false stratification signal may have influenced the summer stratification leading to a poor representation of the Mixed Layer Depth (MLD) and various other indicators. It was found that during the austral winter months, the model simulated comparable MLD's to the ARGO float data as well as theWinter Cruise data (obtained from the SA Agulhas II), capturing the winter dynamics well.

With strong diurnal warming, the temperature at the air-sea interface could be several degrees warmer than the temperature at one or two metres depth. To correctly interpret SST measurements during conditions of diurnal-warming of SST, the diurnal response to environmental conditions must be understood. This thesis is a study of the response of diurnal-warming of SST to the primary environmental conditions that cause it. A one-dimensional ocean turbulence model is used to simulate the diurnal-cycle of warming of SST. The model is developed and enhanced to enable accurate predictions of amplitudes of the night to day difference in SST and the stratification associated with strong warming events. The enhanced model is validated with data from in-situ instrumented moorings. The model is used to investigate the shape and timing of the warming response to environmental causes, including the timing of those causes. The one-dimensional turbulence model must be ‘forced’ with air-sea fluxes. Available data sets for these fluxes have various temporal resolutions, from just a few minutes (high resolution) to daily averages. The performance of the model is tested against temporal resolution of the air-sea fluxes. This allows for a realistic interpretation of the modelled SST for applications where data is only available at low temporal resolution. SSTs from the Meteosat Second Generation (MSG) satellite have recently become available. SSTs (at the air-sea interface) from the new model are compared with the satellite SSTs at buoy locations in the Atlantic and show useful agreement with the shape and amplitude of the diurnal cycle for several events, (within the limits imposed by the low-resolution forcing data presently available for the satellite/buoy match-ups).

The mechanism of flame propagation in fuel beds of wildland fires is important to understand and quantify fire spread rates. Fires spread by radiative and convective heating and often require direct flame contact to achieve ignition. The flame interface in an advancing fire is unsteady and turbulent, making study of intermittent flames in complex fuels difficult. This thesis applies the one-dimensional turbulence (ODT) model to a study of flame propagation by simulating a lab-scale fire representative of the flame interface in a fuel bed and incorporating solid fuel particles into the ODT code. The ODT model is able to resolve individual flames (a unique property of this model) and provide realistic turbulent statistics. ODT solves diffusion-reaction equations on a line-of-sight that is advanced either in time or in one spatial direction (perpendicular to the line-of-sight). Turbulent advection is modeled through stochastic domain mapping processes. A vertical wall fire, in which ethylene fuel is slowly fed through a porous ceramic, is modeled to investigate an unsteady turbulent flame front in a controlled environment. Simulations of this configuration are performed using a spatial formulation of the ODT model, where the ODT line is perpendicular to the wall and is advanced up the wall. Simulations include radiation and soot effects and are compared to experimental temperature data taken over a range of fuel flow rates. Flame structure, velocities, and temperature statistics are reported. The ODT model is shown to capture the evolution of the flame and describe the intermittent properties at the flame edge, though temperature fluctuations are somewhat over predicted. A solid particle devolatilization model was included in the ODT code to study the convective heating of unburnt solid fuels through direct flame contact. Here the particles are treated as sweet gum hardwood and a single-reaction, first order decomposition model is used to simulate the devolatilization rates. Only preliminary results were presented for a simple case, but this extension of the ODT model presents new opportunities for future research.

Thesis (Ph. D.)--Physics, Georgia Institute of Technology, 2005.
Jean Bellissard, Committee Member ; Turgay Uzer, Committee Member ; Roman Grigoriev, Committee Member ; Konstantin Mischaikow, Committee Member ; Predrag Cvitanovic, Committee Chair. Vita. Includes bibliographical references.

This dissertation presents the development and validation of the One Dimensional Turbulence (ODT) multiphase model in the Lagrangian reference frame. ODT is a stochastic model that captures the full range of length and time scales and provides statistical information on fine-scale turbulent-particle mixing and transport at low computational cost. The flow evolution is governed by a deterministic solution of the viscous processes and a stochastic representation of advection through stochastic domain mapping processes. The three algorithms for Lagrangian particle transport are presented within the context of the ODT approach. The Type-I and -C models consider the particle-eddy interaction as instantaneous and continuous change of the particle position and velocity, respectively. The Type-IC model combines the features of the Type-I and -C models. The models are applied to the multiphase flows in the homogeneous decaying turbulence and turbulent round jet. Particle dispersion, dispersion coefficients, and velocity statistics are predicted and compared with experimental data. The models accurately reproduces the experimental data sets and capture particle inertial effects and trajectory crossing effect. A new adjustable particle parameter is introduced into the ODT model, and sensitivity analysis is performed to facilitate parameter estimation and selection. A novel algorithm of the two-way momentum coupling between the particle and carrier phases is developed in the ODT multiphase model. Momentum exchange between the phases is accounted for through particle source terms in the viscous diffusion. The source term is implemented in eddy events through a new kernel transformation and an iterative procedure is required for eddy selection. This model is applied to a particle-laden turbulent jet flow, and simulation results are compared with experimental measurements. The effect of particle addition on the velocities of the gas phase is investigated. The development of particle velocity and particle number distribution are illustrated. The simulation results indicate that the model qualitatively captures the turbulent modulation with the presence of difference particle classes with different solid loadings. The model is then extended to simulate temperature evolution of the particles in a nonisothermal hot jet, in which heat transfer between the particles and gas is considered. The flow is bounded by a wall on the one side of the domain. The simulations are performed over a range of particle inertia and thermal relaxation time scales and different initial particle locations. The present study investigates the post-blast-phase mixing between the particles, the environment that is intended to heat them up, and the ambient environment that dilutes the jet flow. The results indicate that the model can qualitatively predict the important particle statistics in jet flame.

Les observations du vent solaire, du soleil et du milieu interstellaire nous montrent des milieux complexes ou le champ magnetique a un role dynamique et ou de multiples structures interagissent. Cette these contribue a une etude theorique, phenomenologique et numerique de la turbulence mhd en se placant dans ce contexte observationnel. On montre, grace a un modele mhd unidimensionel, que c'est la superposition d'une multitude d'evenements disruptifs, lies a l'intermittence de la dissipation joule, qui permet d'expliquer les histogrammes en lois de puissance des pics de luminosite observes en rayons x. Une interpretation de la structure thermique des boucles coronales solaires en reponse au chauffage dissipatif issu de ce modele mhd est aussi proposee. Enfin, une etude des solutions exactes d'un systeme mhd de dimension un offre une meilleure comprehension des chocs, nombreux dans l'heliosphere, en identifiant precisement les structures qui emergent. D'autre part, on developpe un formalisme de turbulence faible mhd en presence d'un fort champ magnetique uniforme permettant de deriver les lois d'echelles suivies par les spectres d'energie dans le cas incompressible et en presence de correlation entre champ de vitesse et champ magnetique. Le calcul analytique des constantes apparaissant devant les spectres est aussi realise. Enfin, une phenomenologie de la decroissance temporelle de l'energie en mhd est proposee en s'appuyant sur la persistence des grands tourbillons et sur le modele d'iroshnikov-kraichnan. Il apparait que les lois de declin different significativement suivant le type de fluide considere, neutre ou conducteur, mettant ainsi en evidence le role dynamique important des ondes d'alfven.

The study and modelling of two-phase flow, even the simplest ones such as the bubbly flow, remains a challenge that requires exploring the physical phenomena from different spatial and temporal resolution levels. CFD (Computational Fluid Dynamics) is a widespread and promising tool for modelling, but nowadays, there is no single approach or method to predict the dynamics of these systems at the different resolution levels providing enough precision of the results. The inherent difficulties of the events occurring in this flow, mainly those related with the interface between phases, makes that low or intermediate resolution level approaches as system codes (RELAP, TRACE, ...) or 3D TFM (Two-Fluid Model) have significant issues to reproduce acceptable results, unless well-known scenarios and global values are considered. Instead, methods based on high resolution level such as Interfacial Tracking Method (ITM) or Volume Of Fluid (VOF) require a high computational effort that makes unfeasible its use in complex systems. In this thesis, an open-source simulation framework has been designed and developed using the OpenFOAM library to analyze the cases from microescale to macroscale levels. The different approaches and the information that is required in each one of them have been studied for bubbly flow. In the first part, the dynamics of single bubbles at a high resolution level have been examined through VOF. This technique has allowed to obtain accurate results related to the bubble formation, terminal velocity, path, wake and instabilities produced by the wake. However, this approach has been impractical for real scenarios with more than dozens of bubbles. Alternatively, this thesis proposes a CFD Discrete Element Method (CFD-DEM) technique, where each bubble is represented discretely. A novel solver for bubbly flow has been developed in this thesis. This includes a large number of improvements necessary to reproduce the bubble-bubble and bubble-wall interactions, turbulence, velocity seen by the bubbles, momentum and mass exchange term over the cells or bubble expansion, among others. But also new implementations as an algorithm to seed the bubbles in the system have been incorporated. As a result, this new solver gives more accurate results as the provided up to date. Following the decrease on resolution level, and therefore the required computational resources, a 3D TFM have been developed with a population balance equation solved with an implementation of the Quadrature Method Of Moments (QMOM). The solver is implemented with the same closure models as the CFD-DEM to analyze the effects involved with the lost of information due to the averaging of the instantaneous Navier-Stokes equation. The analysis of the results with CFD-DEM reveals the discrepancies found by considering averaged values and homogeneous flow in the models of the classical TFM formulation. Finally, for the lowest resolution level approach, the system code RELAP5/MOD3 is used for modelling the bubbly flow regime. The code has been modified to reproduce properly the two-phase flow characteristics in vertical pipes, comparing the performance of the calculation of the drag term based on drift-velocity and drag coefficient approaches.
El estudio y modelado de flujos bifásicos, incluso los más simples como el bubbly flow, sigue siendo un reto que conlleva aproximarse a los fenómenos físicos que lo rigen desde diferentes niveles de resolución espacial y temporal. El uso de códigos CFD (Computational Fluid Dynamics) como herramienta de modelado está muy extendida y resulta prometedora, pero hoy por hoy, no existe una única aproximación o técnica de resolución que permita predecir la dinámica de estos sistemas en los diferentes niveles de resolución, y que ofrezca suficiente precisión en sus resultados. La dificultad intrínseca de los fenómenos que allí ocurren, sobre todo los ligados a la interfase entre ambas fases, hace que los códigos de bajo o medio nivel de resolución, como pueden ser los códigos de sistema (RELAP, TRACE, etc.) o los basados en aproximaciones 3D TFM (Two-Fluid Model) tengan serios problemas para ofrecer resultados aceptables, a no ser que se trate de escenarios muy conocidos y se busquen resultados globales. En cambio, códigos basados en alto nivel de resolución, como los que utilizan VOF (Volume Of Fluid), requirieren de un esfuerzo computacional tan elevado que no pueden ser aplicados a sistemas complejos. En esta tesis, mediante el uso de la librería OpenFOAM se ha creado un marco de simulación de código abierto para analizar los escenarios desde niveles de resolución de microescala a macroescala, analizando las diferentes aproximaciones, así como la información que es necesaria aportar en cada una de ellas, para el estudio del régimen de bubbly flow. En la primera parte se estudia la dinámica de burbujas individuales a un alto nivel de resolución mediante el uso del método VOF (Volume Of Fluid). Esta técnica ha permitido obtener resultados precisos como la formación de la burbuja, velocidad terminal, camino recorrido, estela producida por la burbuja e inestabilidades que produce en su camino. Pero esta aproximación resulta inviable para entornos reales con la participación de más de unas pocas decenas de burbujas. Como alternativa, se propone el uso de técnicas CFD-DEM (Discrete Element Methods) en la que se representa a las burbujas como partículas discretas. En esta tesis se ha desarrollado un nuevo solver para bubbly flow en el que se han añadido un gran número de nuevos modelos, como los necesarios para contemplar los choques entre burbujas o con las paredes, la turbulencia, la velocidad vista por las burbujas, la distribución del intercambio de momento y masas con el fluido en las diferentes celdas por cada una de las burbujas o la expansión de la fase gaseosa entre otros. Pero también se han tenido que incluir nuevos algoritmos como el necesario para inyectar de forma adecuada la fase gaseosa en el sistema. Este nuevo solver ofrece resultados con un nivel de resolución superior a los desarrollados hasta la fecha. Siguiendo con la reducción del nivel de resolución, y por tanto los recursos computacionales necesarios, se efectúa el desarrollo de un solver tridimensional de TFM en el que se ha implementado el método QMOM (Quadrature Method Of Moments) para resolver la ecuación de balance poblacional. El solver se desarrolla con los mismos modelos de cierre que el CFD-DEM para analizar los efectos relacionados con la pérdida de información debido al promediado de las ecuaciones instantáneas de Navier-Stokes. El análisis de resultados de CFD-DEM permite determinar las discrepancias encontradas por considerar los valores promediados y el flujo homogéneo de los modelos clásicos de TFM. Por último, como aproximación de nivel de resolución más bajo, se investiga el uso uso de códigos de sistema, utilizando el código RELAP5/MOD3 para analizar el modelado del flujo en condiciones de bubbly flow. El código es modificado para reproducir correctamente el flujo bifásico en tuberías verticales, comparando el comportamiento de aproximaciones para el cálculo del término d
L'estudi i modelatge de fluxos bifàsics, fins i tot els més simples com bubbly flow, segueix sent un repte que comporta aproximar-se als fenòmens físics que ho regeixen des de diferents nivells de resolució espacial i temporal. L'ús de codis CFD (Computational Fluid Dynamics) com a eina de modelatge està molt estesa i resulta prometedora, però ara per ara, no existeix una única aproximació o tècnica de resolució que permeta predir la dinàmica d'aquests sistemes en els diferents nivells de resolució, i que oferisca suficient precisió en els seus resultats. Les dificultat intrínseques dels fenòmens que allí ocorren, sobre tots els lligats a la interfase entre les dues fases, fa que els codis de baix o mig nivell de resolució, com poden ser els codis de sistema (RELAP,TRACE, etc.) o els basats en aproximacions 3D TFM (Two-Fluid Model) tinguen seriosos problemes per a oferir resultats acceptables , llevat que es tracte d'escenaris molt coneguts i se persegueixen resultats globals. En canvi, codis basats en alt nivell de resolució, com els que utilitzen VOF (Volume Of Fluid), requereixen d'un esforç computacional tan elevat que no poden ser aplicats a sistemes complexos. En aquesta tesi, mitjançant l'ús de la llibreria OpenFOAM s'ha creat un marc de simulació de codi obert per a analitzar els escenaris des de nivells de resolució de microescala a macroescala, analitzant les diferents aproximacions, així com la informació que és necessària aportar en cadascuna d'elles, per a l'estudi del règim de bubbly flow. En la primera part s'estudia la dinàmica de bambolles individuals a un alt nivell de resolució mitjançant l'ús del mètode VOF. Aquesta tècnica ha permès obtenir resultats precisos com la formació de la bambolla, velocitat terminal, camí recorregut, estela produida per la bambolla i inestabilitats que produeix en el seu camí. Però aquesta aproximació resulta inviable per a entorns reals amb la participació de més d'unes poques desenes de bambolles. Com a alternativa en aqueix cas es proposa l'ús de tècniques CFD-DEM (Discrete Element Methods) en la qual es representa a les bambolles com a partícules discretes. En aquesta tesi s'ha desenvolupat un nou solver per a bubbly flow en el qual s'han afegit un gran nombre de nous models, com els necessaris per a contemplar els xocs entre bambolles o amb les parets, la turbulència, la velocitat vista per les bambolles, la distribució de l'intercanvi de moment i masses amb el fluid en les diferents cel·les per cadascuna de les bambolles o els models d'expansió de la fase gasosa entre uns altres. Però també s'ha hagut d'incloure nous algoritmes com el necessari per a injectar de forma adequada la fase gasosa en el sistema. Aquest nou solver ofereix resultats amb un nivell de resolució superior als desenvolupat fins la data. Seguint amb la reducció del nivell de resolució, i per tant els recursos computacionals necessaris, s'efectua el desenvolupament d'un solver tridimensional de TFM en el qual s'ha implementat el mètode QMOM (Quadrature Method Of Moments) per a resoldre l'equació de balanç poblacional. El solver es desenvolupa amb els mateixos models de tancament que el CFD-DEM per a analitzar els efectes relacionats amb la pèrdua d'informació a causa del promitjat de les equacions instantànies de Navier-Stokes. L'anàlisi de resultats de CFD-DEM permet determinar les discrepàncies ocasionades per considerar els valors promitjats i el flux homogeni dels models clàssics de TFM. Finalment, com a aproximació de nivell de resolució més baix, s'analitza l'ús de codis de sistema, utilitzant el codi RELAP5/MOD3 per a analitzar el modelatge del fluxos en règim de bubbly flow. El codi és modificat per a reproduir correctament les característiques del flux bifàsic en canonades verticals, comparant el comportament d'aproximacions per al càlcul del terme de drag basades en velocitat de drift flux model i de les basades en coe
Peña Monferrer, C. (2017). Computational fluid dynamics multiscale modelling of bubbly flow. A critical study and new developments on volume of fluid, discrete element and two-fluid methods [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90493
TESIS

Etude hydrodynamique du phenomene d'accretion sur un objet compact et stabilite dynamique d'un tel ecoulement. On effectue une simulation numerique d'un flot bidimensionnel keplerien pour permettre d'estimer l'echelle de temps dynamique en fonction de la turbulence

Symmetry analysis of the evolution equation of the two-point correlation tensor Rij (xk, rl, t) in the case of planar generation of turbulence in an otherwise quiescent semi-infinite body of fluid has revealed some interesting solutions concerning the statistical properties of turbulence and how they develop with distance from the generation source. The first solution concerns the classical case of shear-free turbulent diffusion. Here, the turbulent kinetic energy is distributed according to a power law x−n where n is a constant larger than one, and x is the normal distance to the forcing plane. The integral length scales of turbulence increase linearly with x. A second case is considered when the symmetry of scaling of space is broken by introducing confinement to the flow. The turbulent kinetic energy decays with x as exp (−x) and the integral length scales remain constant along x. A third case treated is turbulent diffusion in a rotating frame, where symmetry of scaling of time is broken. Turbulent kinetic energy is distributed according to x−2 and there is an upper limit to turbulence propagation. The purpose of the present work is to investigate characteristics of this type of flow by means of large eddy simulation. Turbulent fields are generated in a box of isotropic turbulence using standard procedures. Planar samples of the generated fields are fed as a series of unsteady and nonuniform boundary conditions to the zero initial fields in an elongated turbulence box and turbulence propagation is monitored. The three cases are distinguished in simulations by imposing periodic and slip boundary conditions on lateral sides of the simulation box for the cases of free and confined turbulent diffusion respectively, and by solving LES equations in the rotating frame of reference for the third case. Specifically, the present work discusses identification criteria of turbulent front from filtered fields of LES turbulence. Furthermore, propagation of the front and associated profiles of turbulent kinetic energy and vorticity are discussed and compared to experimental and direct numerical simulation results. Complementing the main results, principles of symmetry analysis of two-point correlation equations and a description of the algorithm used for generation of the isotropic and rotating homogeneous turbulence fields are given. Finally, the performance of the presently popular Reynolds-averaged models in the three cases is evaluated.

Indiana University-Purdue University Indianapolis (IUPUI)
Testing of a wave-rotor constant-volume combustor (WRCVC) showed the viability of the application of wave rotors as a pressure gain combustor. The aero-thermal design of the WRCVC rig had originally been performed with a time-dependent, one-dimensional model which applies a single-step reaction model for the combustion process of the air-fuel mixture. That numerical model was validated with experimental data with respect of matching the flame propagation speed and the pressure traces inside the passages of the WRCVC. However, the numerical model utilized a single progress variable representing the air-fuel mixture, which assumes that fuel and air are perfectly mixed with a uniform concentration; thus, limiting the validity of the model. In the present work, a two-step reaction model is implemented in the combustion model with four species variables: fuel, oxidant, intermediate and product. This combustion model is developed for a more detailed representation for the combustion process inside the wave rotor. A two-step reaction model presented a more realistic representation for the stratified air-fuel mixture charges in the WRCVC; additionally it shows more realistic modeling for the partial combustion process for rich fuel-air mixtures. The combustion model also accounts for flammability limits to exert flame extinction for non-flammable mixtures. The combustion model applies the eddy-breakup model where the reaction rate is influenced by the turbulence time scale. The experimental data currently available from the initial testing of the WRCVC rig is utilized to calibrate the model to determine the parameters, which are not directly measured and no directly related practice available in the literature. A prediction of the apparent ignition the location inside the passage is estimated by examination of measurements from the on-rotor instrumentations. The incorporation of circumferential leakage (passage-to-passage), and stand-off ignition models in the numerical model, contributed towards a better match between predictions and experimental data. The thesis also includes a comprehensive discussion of the governing equations used in the numerical model. The predictions from the two-step reaction model are validated using experimental data from the WRCVC for deflagrative combustion tests. The predictions matched the experimental data well. The predicted pressure traces are compared with the experimentally measured pressures in the passages. The flame propagation along the passage is also evaluated with ion probes data and the predicted reaction zone.