Download or read book A Novel Diffuse interface Model and Numerical Methods for Compressible Turbulent Two phase Flows and Scalar Transport written by Suhas Jain Suresh and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Compressible two-phase flows and the transport of scalars in two-phase flows have a wide range of applications in natural and engineering processes. In this thesis we first present a novel diffuse-interface model for the simulation of compressible two-phase flows. We start with the baseline five-equation model that consists of equations for the transport of volume fraction, mass of each phase, momentum, and total energy. It is known that this model cannot be used with a non-dissipative scheme as is, and the direct solution of these equations with a dissipative scheme results in artificial diffusion of the interface which results in poor accuracy. We, therefore, propose interface-regularization (diffusion--sharpening) terms to this five-equation model in such a way that the resulting model can now be used with a non-dissipative central scheme, and the model also maintains the discrete conservation of mass of each phase, momentum, and total energy of the system. For stable numerical simulations of compressible flows, it is known that a discrete entropy condition needs to be satisfied, and the discrete conservation of kinetic energy alone is not a sufficient condition, unlike in incompressible flows. To achieve this, we first propose discrete consistency conditions between the numerical fluxes, and then present a set of numerical fluxes--which satisfies these consistency conditions--that results in an exact discrete conservation of kinetic energy and approximate conservation of entropy (a KEEP scheme) in the absence of pressure work, viscosity, and thermal diffusion effects. Next, we also propose a novel scalar-transport model for the simulation of scalar quantities in two-phase flows. In a two-phase flow, the scalar quantities typically have very different diffusivities in the two phases, which results in effective confinement of the scalar quantities in one of the phases, in the time scales of interest. This confinement of the scalars leads to the formation of sharp gradients of the scalar concentration values at the interface and could result in artificial numerical leakage of the scalar and negative values for the scalar concentration values, presenting a serious challenge for its numerical simulations. To overcome this challenge, we propose a new consistent scalar-transport model that prevents artificial numerical leakage of the scalar at the interface. The proposed model also maintains the positivity property of the scalar concentration field, a physical realizability requirement for the simulation of scalars. Finally, we present numerical simulations to verify and validate the models presented in this work in a wide range of settings and regimes, spanning laminar to turbulent flows; and assess the accuracy, robustness, and scalability of the models.

Download or read book A Novel Diffuse Interface Method for Two phase Flows and Application in Simulation of Micro bubble Entrainment written by Seyedshahabaddin Mirjalili and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Micro-bubbles have been observed in various contexts such as in raindrops impacting liquid pools, boiling heat transfer, aerosol generation, breaking oceanic waves and ship wakes. Due to their long residence time under the free surface, micro-bubbles impact air-sea mass transport, formation of white caps and the signature of seafaring vessels. As such, understanding the mechanism involved in the formation of these bubbles is of significant interest. Based on evidence from drop-pool impact experiments, the leading hypothesis for micro-bubble formation is that when liquid interfaces collide, a thin gas film is entrapped, which leaves behind hundreds of micro-bubbles as it retracts. Due to the wide range of length and time scales involved, there is a lack of understanding of this mechanism, in addition to quantitative data regarding the generated micro-bubbles. This physical understanding has significant practical value, as one seeks to model micro-bubbles in large scale turbulent two-phase flow simulations. In the first portion of this thesis, we present a novel diffuse interface method for simulation of incompressible, immiscible two-phase flows. The boundedness of this mass-conserving interface-capturing method is proven, and a comparison of the fully coupled solver with a state-of-the-art two-phase flow solver is provided. Then, we show how the momentum transport equation must be modified to achieve consistency with mass transport and conservation of momentum and kinetic energy. The practical importance of this correction, and our method's capability in modeling turbulent two-phase flows, is demonstrated via simulations of a liquid jet in cross-flow. Finally, we present a robust method for representation of surface tension forces that utilizes discrete surface energy definition. Overall, the desirable conservation properties, robustness, mass-momentum consistency, simplicity and parallel scalability render our method a promising and viable option for realistic two-phase flow simulations. In the second portion of the thesis, we study the impact of a drop on a deep liquid pool as an appropriate model problem for understanding how collisions between two arbitrarily-curved interfaces may lead to micro-bubble entrainment. Using numerical simulations with a boundary integral method, we explain the physics of thin gas film entrapment and the stages of its evolution. These numerical simulations, in addition to theoretical arguments, lead to the discovery of a transition in the dynamics of the thin gas film that is necessary for entrapment of high aspect ratio films that can shed micro-bubbles. After presenting our study on thin gas film entrapment, we employ our diffuse interface method to numerically simulate thin retracting gas films. A new scaling law for gas film retraction velocity is found. Moreover, using high-fidelity 3D simulations, we find that a transverse instability on the edge of the film is responsible for micro-bubble generation. By combining our findings from the drop-pool impact simulations and the thin gas film retraction simulations, we provide the means for subgrid-scale modeling of micro-bubbles in large scale two-phase flows.

Download or read book A Simple and Efficient Diffuse Interface Method for Compressible Two Phase Flows written by and published by . This book was released on 2009 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: In nuclear reactor safety and optimization there are key issues that rely on in-depth understanding of basic two-phase flow phenomena with heat and mass transfer. For many reasons, to be discussed, there is growing interest in the application of two-phase flow models to provide diffuse, but nevertheless resolved, simulation of interfaces between two immiscible compressible fluids - diffuse interface method (DIM). Because of its ability to dynamically create interfaces and to solve interfaces separating pure media and mixtures for DNS-like (Direct Numerical Simulation) simulations of interfacial flows, we examine the construction of a simple, robust, fast, and accurate numerical formulation for the 5-equation Kapila et al. [1] reduced two-phase model. Though apparently simple, the Kapila et al. model contains a volume fraction differential transport equation containing a nonlinear, non-conservative term which poses serious computational challenges. To circumvent the difficulties encountered with the single velocity and single pressure Kapila et al. [1] multiphase flow model, a 6-equation relaxation hyperbolic model is built to solve interface problems with compressible fluids. In this approach, pressure non-equilibrium is first restored, followed by a relaxation to an asymptotic solution which is convergent to the solutions of the Kapila et al. reduced model. The apparent complexity introduced with this extended hyperbolic model actually leads to considerable simplifications regarding numerical resolution, and the various ingredients used by this method are general enough to consider future extensions to problems involving complex physics.

Download or read book Mathematical Modeling of Disperse Two Phase Flows written by Christophe Morel and published by Springer. This book was released on 2015-07-17 with total page 365 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book develops the theoretical foundations of disperse two-phase flows, which are characterized by the existence of bubbles, droplets or solid particles finely dispersed in a carrier fluid, which can be a liquid or a gas. Chapters clarify many difficult subjects, including modeling of the interfacial area concentration. Basic knowledge of the subjects treated in this book is essential to practitioners of Computational Fluid Dynamics for two-phase flows in a variety of industrial and environmental settings. The author provides a complete derivation of the basic equations, followed by more advanced subjects like turbulence equations for the two phases (continuous and disperse) and multi-size particulate flow modeling. As well as theoretical material, readers will discover chapters concerned with closure relations and numerical issues. Many physical models are presented, covering key subjects including heat and mass transfers between phases, interfacial forces and fluid particles coalescence and breakup, amongst others. This book is highly suitable for students in the subject area, but may also be a useful reference text for more advanced scientists and engineers.

Download or read book Turbulent Flows written by Jean Piquet and published by Springer Science & Business Media. This book was released on 2001-03-26 with total page 778 pages. Available in PDF, EPUB and Kindle. Book excerpt: obtained are still severely limited to low Reynolds numbers (about only one decade better than direct numerical simulations), and the interpretation of such calculations for complex, curved geometries is still unclear. It is evident that a lot of work (and a very significant increase in available computing power) is required before such methods can be adopted in daily's engineering practice. I hope to l"Cport on all these topics in a near future. The book is divided into six chapters, each· chapter in subchapters, sections and subsections. The first part is introduced by Chapter 1 which summarizes the equations of fluid mechanies, it is developed in C~apters 2 to 4 devoted to the construction of turbulence models. What has been called "engineering methods" is considered in Chapter 2 where the Reynolds averaged equations al"C established and the closure problem studied (§1-3). A first detailed study of homogeneous turbulent flows follows (§4). It includes a review of available experimental data and their modeling. The eddy viscosity concept is analyzed in §5 with the l"Csulting ~alar-transport equation models such as the famous K-e model. Reynolds stl"Css models (Chapter 4) require a preliminary consideration of two-point turbulence concepts which are developed in Chapter 3 devoted to homogeneous turbulence. We review the two-point moments of velocity fields and their spectral transforms (§ 1), their general dynamics (§2) with the particular case of homogeneous, isotropie turbulence (§3) whel"C the so-called Kolmogorov's assumptions are discussed at length.

Download or read book Advanced Turbulent Flow Computations written by Roger Peyret and published by Springer. This book was released on 2014-05-04 with total page 320 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book collects the lecture notes concerning the IUTAM School on Advanced Turbulent Flow Computations held at CISM in Udine September 7–11, 1998. The course was intended for scientists, engineers and post-graduate students interested in the application of advanced numerical techniques for simulating turbulent flows. The topic comprises two closely connected main subjects: modelling and computation, mesh pionts necessary to simulate complex turbulent flow.

Download or read book Modeling Complex Turbulent Flows written by Manuel D. Salas and published by Springer Science & Business Media. This book was released on 1999-04-30 with total page 402 pages. Available in PDF, EPUB and Kindle. Book excerpt: Turbulence modeling both addresses a fundamental problem in physics, 'the last great unsolved problem of classical physics,' and has far-reaching importance in the solution of difficult practical problems from aeronautical engineering to dynamic meteorology. However, the growth of supercom puter facilities has recently caused an apparent shift in the focus of tur bulence research from modeling to direct numerical simulation (DNS) and large eddy simulation (LES). This shift in emphasis comes at a time when claims are being made in the world around us that scientific analysis itself will shortly be transformed or replaced by a more powerful 'paradigm' based on massive computations and sophisticated visualization. Although this viewpoint has not lacked ar ticulate and influential advocates, these claims can at best only be judged premature. After all, as one computational researcher lamented, 'the com puter only does what I tell it to do, and not what I want it to do. ' In turbulence research, the initial speculation that computational meth ods would replace not only model-based computations but even experimen tal measurements, have not come close to fulfillment. It is becoming clear that computational methods and model development are equal partners in turbulence research: DNS and LES remain valuable tools for suggesting and validating models, while turbulence models continue to be the preferred tool for practical computations. We believed that a symposium which would reaffirm the practical and scientific importance of turbulence modeling was both necessary and timely.

Download or read book Numerical Simulation of Turbulent Flows and Noise Generation written by Christophe Brun and published by Springer Science & Business Media. This book was released on 2009-03-07 with total page 344 pages. Available in PDF, EPUB and Kindle. Book excerpt: Large Eddy Simulation (LES) is a high-fidelity approach to the numerical simulation of turbulent flows. Recent developments have shown LES to be able to predict aerodynamic noise generation and propagation as well as the turbulent flow, by means of either a hybrid or a direct approach. This book is based on the results of two French/German research groups working on LES simulations in complex geometries and noise generation in turbulent flows. The results provide insights into modern prediction approaches for turbulent flows and noise generation mechanisms as well as their use for novel noise reduction concepts.

Download or read book Numerical Simulation of Two phase Turbulent Compressible Flows with a Multidomain Spectral Method written by Gustaaf Bernardus Jacobs and published by . This book was released on 2003 with total page 530 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Numerical Modeling of Compressible Two phase Flows with a Pressure based Method written by Markus Boger and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Numerical Methods for Fluid Dynamics written by Dale R. Durran and published by Springer Science & Business Media. This book was released on 2010-09-14 with total page 527 pages. Available in PDF, EPUB and Kindle. Book excerpt: This scholarly text provides an introduction to the numerical methods used to model partial differential equations, with focus on atmospheric and oceanic flows. The book covers both the essentials of building a numerical model and the more sophisticated techniques that are now available. Finite difference methods, spectral methods, finite element method, flux-corrected methods and TVC schemes are all discussed. Throughout, the author keeps to a middle ground between the theorem-proof formalism of a mathematical text and the highly empirical approach found in some engineering publications. The book establishes a concrete link between theory and practice using an extensive range of test problems to illustrate the theoretically derived properties of various methods. From the reviews: "...the books unquestionable advantage is the clarity and simplicity in presenting virtually all basic ideas and methods of numerical analysis currently actively used in geophysical fluid dynamics." Physics of Atmosphere and Ocean

Download or read book New Approaches in Modeling Multiphase Flows and Dispersion in Turbulence Fractal Methods and Synthetic Turbulence written by F.C.G.A. Nicolleau and published by Springer Science & Business Media. This book was released on 2011-10-29 with total page 159 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book contains a collection of the main contributions from the first five workshops held by Ercoftac Special Interest Group on Synthetic Turbulence Models (SIG42. It is intended as an illustration of the sig’s activities and of the latest developments in the field. This volume investigates the use of Kinematic Simulation (KS) and other synthetic turbulence models for the particular application to environmental flows. This volume offers the best syntheses on the research status in KS, which is widely used in various domains, including Lagrangian aspects in turbulence mixing/stirring, particle dispersion/clustering, and last but not least, aeroacoustics. Flow realizations with complete spatial, and sometime spatio-temporal, dependency, are generated via superposition of random modes (mostly spatial, and sometime spatial and temporal, Fourier modes), with prescribed constraints such as: strict incompressibility (divergence-free velocity field at each point), high Reynolds energy spectrum. Recent improvements consisted in incorporating linear dynamics, for instance in rotating and/or stably-stratified flows, with possible easy generalization to MHD flows, and perhaps to plasmas. KS for channel flows have also been validated. However, the absence of "sweeping effects" in present conventional KS versions is identified as a major drawback in very different applications: inertial particle clustering as well as in aeroacoustics. Nevertheless, this issue was addressed in some reference papers, and merits to be revisited in the light of new studies in progress.

Download or read book Simulation and Modeling of Compressible Turbulent Mixing Layer written by Seyed Navid Samadi Vaghefi and published by . This book was released on 2014 with total page 158 pages. Available in PDF, EPUB and Kindle. Book excerpt: Direct numerical simulations (DNS) of compressible turbulent mixing layer are performed for subsonic to supersonic Mach numbers. Each simulation achieves the self-similar state and it is shown that the turbulent statistics during this state agree well with previous numerical and experimental works. The DNS data is used to extract the physics of compressible turbulence and to perform a priori analysis for subgrid scale (SGS) closures. The flow dynamics in proximity of the turbulent/non-turbulent interface (TNTI), separating the turbulent and the irrotational regions, is analyzed using the DNS data. This interface is detected by using a certain threshold for the vorticity norm. The conditional flow statistics based on the normal distance from the TNTI are compared for different convective Mach numbers. It is shown that the thickness of the interface layer is approximately one Taylor length scale for both incompressible and compressible mixing layers, and the flow dynamics in this layer differs from deep inside the turbulent region. Various terms in the transport equations for total kinetic energy, turbulent kinetic energy, and vorticity are examined in order to better understand the transport mechanisms across the TNTI in compressible flows. The DNS data is also employed to analyze the local flow topology in compressible mixing layers using the invariants of the velocity gradient tensor. The topological and dissipating behaviors of the flow are analyzed in two different regions: near the TNTI, and inside the turbulent region. It is found that the distribution of various flow topologies in regions close to the TNTI differs from inside the turbulent region, and in these regions the most probable topologies are non-focal. The occurrence probability of different flow topologies conditioned by the dilatation level is presented and it is shown that the structures in the locally compressed regions tend to have stable topologies while in locally expanded regions the unstable topologies are prevalent. In order to better understand the behavior of different flow topologies, the probability distributions of vorticity norm, dissipation, and rate of stretching are analyzed in incompressible, compressed and expanded regions. The DNS data is also used to perform a priori analysis for subgrid scale (SGS) viscous and scalar closures. Several models for each closure are tested and effects of filter width, compressibility level, and Schmidt number on their performance are studied. A new model for SGS viscous dissipation is proposed based on the scaling of SGS kinetic energy. The proposed model yields the best prediction of SGS viscous dissipation among the considered models for filter widths corresponding to the inertial range. For the range of Mach numbers and Schmidt numbers studied in this work, the SGS scalar dissipation model based on proportionality of turbulent time scale and scalar mixing time scale produces the best results in the filter widths corresponding to the inertial subrange. For both viscous and scalar SGS dissipation models, two dynamic approaches are used to compute the model coefficient. It is shown that if the dynamic procedure based on global equilibrium of dissipation and production is employed, more accurate results are generated compared to the conventional dynamic method based on test-filtering.

Download or read book Analysis of Diffuse Interface Models for Two phase Flows with and Without Surfactants written by Josef Thomas Weber and published by . This book was released on 2016 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book On the Numerics of Diffuse interface Models for Two phase Flow with Species Transport written by Fabian Klingbeil and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Computation and Comparison of Efficient Turbulence Models for Aeronautics European Research Project ETMA written by Alain Dervieux and published by Springer Science & Business Media. This book was released on 1998-08-27 with total page 604 pages. Available in PDF, EPUB and Kindle. Book excerpt: This volume contains contributions to the BRITE-EURAM 3rd Framework Programme ETMA and extended articles of the TMA-Workshop. It focusses on turbulence modelling techniques suitable to use in typical flow configurations, with emphasis on compressibility effects and inherent unsteadiness. These methodologies are applied to the Navier-Stokes equations, involving various turbulence modelling levels from algebraic to RSM. Basic turbulent flows in aeronautics are considered; mixing layers, wall-flows (flat-plate, backward-facing step, ramp, bump), and more complex configurations (bump, aerofoil). A critical assessment of the turbulence modelling performances is offered, based on previous results and on the experimental data-base of this research programme. The ETMA results figure in the data-base constituted by all partners and organized by INRIA

Download or read book Direct Numerical Simulation of Two phase Flow with Application to Air Layer Drag Reduction written by Dokyun Kim and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: An accurate and robust numerical method has been developed to simulate turbulent two-phase flows. The phase interface is tracked by the level-set method to capture frequent topological changes due to breaking or merging. Because of the broad-band characteristics of length scales in two-phase flow, a Lagrangian drop breakup model has been developed, which is coupled to the level-set method. In this approach, small subgrid droplets produced from resolved ligaments are then transferred from the level-set representation to the Lagrangian particles. The further secondary atomization is handled by a stochastic breakup model. When pinching-off of ligaments is not resolved on the level-set grid, a capillary breakup model is used to predict the drop size distribution from the pinching off and inserted as Lagrangian drops. This method improves the mass conservation as well as reducing the computational cost. For a high-fidelity simulation of two-phase flow, a new numerical algorithm has been developed to improve the robustness of the numerical method. The conservative formulation of Navier-Stokes equations is solved with a density correction term in the present method. The density flux terms are calculated from the level-set field for accuracy. In addition, a constant coefficient Poisson system is solved for pressure to satisfy the continuity equation in the fractional-step method. In order to show the capability of the method as an efficient tool in the breakup process, the atomization of a round liquid jet surrounded by a coaxial gas is considered. The numerical results are consistent with the observed breakup mechanisms in the experiment and the stability analysis. The drop size distribution of the resulting spray after breakup is also compared with the experimental data. The subgrid drops are also predicted by the Lagrangian drop breakup model, which shows the applicability of our method for numerical simulation of the atomization process. Both theoretical and numerical approaches are employed to investigate the stability mechanisms of the air layer drag reduction (ALDR) phenomenon. A linear viscous stability analysis is performed by solving the Orr-Sommerfeld equations in a two-dimensional two-phase Couette-Poiseuille flow configuration that mimics the far-downstream region from an air injector. Air-layer stability is reduced as the free-stream velocity, Froude number, and velocity gradients at the air-liquid interface are increased, whereas the air-layer stability is enhanced as the gas flow rate and surface tension force are increased. Nonlinear stability characteristics are also studied using numerical simulations with the same Couette flow configuration as indicated in the linear stability analysis. The study shows that the Weber number has a significant effect on the breakup of the phase interface. As the Weber number increases, the liquid ligaments become thinner, requiring higher grid resolution. Therefore, for simulations of high Weber number flows, the use of a Lagrangian spray breakup model is essential to predict the dynamics of subgrid-scale liquid structures. Direct Numerical Simulation (DNS) of two-phase flow is also performed to investigate the air layer drag reduction (ALDR) phenomenon in turbulent water flow over a backward-facing step. The Reynolds and Weber numbers based on the water properties and step height are 22,800 and 560, respectively. The total number of grid points is about 271 million for DNS. Two different air-flow injection rates are examined to investigate the mechanism and stability of the air layer. For high air-flow rate, the stable air layer is formed on the plate and more than 90% drag reduction is obtained, whereas, in the case of low air-flow rate, the air layer breaks up and ALDR is not achieved. The initial Kelvin-Helmholtz instability causes the streamwise wave structure, while turbulence interaction forms the spawise waves and causes ligament breakups. However, overall rupture of the air layer is mainly determined by the stability of the streamwise wave. The stability of the streamwise wave can be predicted from the stability analysis in the far-downstream region.