Starting with two well-tested, one-dimensional models of non-evaporating, mixing-limited sprays, ... more Starting with two well-tested, one-dimensional models of non-evaporating, mixing-limited sprays, governing equations for liquid mass and two-phase momentum for each model can be manipulated to reveal the formal similarity between momentum and liquid volume fraction. The consequence of this similarity is that momentum, when properly non-dimensionalized, is equal to the liquid volume fraction at any time and at any axial location within a non-evaporating, mixing-limited spray with a constant rate of injection. An alternative, the more well-known similarity between mass fraction and velocity, is also mathematically evident. We compare predictions of this mathematical analysis to high-fidelity, first-principles simulation results of a non-evaporating spray to assess the validity of the theoretical similarity. The analysis of the simulation not only confirms the mathematical derivations but also points to subtlety in the definition of the spray velocity. In particular, the density-weight...
ABSTRACT Predicting the liquid film dynamics inside the injector cup of gas-centered swirl coaxia... more ABSTRACT Predicting the liquid film dynamics inside the injector cup of gas-centered swirl coaxial fuel injectors requires a general two-phase approach that is appropriate for all liquid volume fractions, high Weber number, and complex geometries. The rapid exchange of momentum at the highly convoluted interface requires tight numerical coupling between the gas and liquid phases. An Eulerian two-phase model is implemented to represent the liquid and gas interactions in the injector as well as the atomization processes at the rough interface. The model, originally proposed by Vallet et al, assumes that in the limit of infinite Reynolds and Weber number, features of the atomization process acting at large length scales are separable from small scale mechanisms. A transport equation for the liquid volume fraction represents the dispersion of the liquid into the gas via a traditional turbulent diffusion hypothesis. A model for the growth of mean interfacial surface area is then used to characterize the growth of instability at the interface, allowing a characterization of Sauter mean diameter. The model shows promise as a computationally inexpensive tool for characterizing spray quality in regions where optical experimental data are difficult to obtain and two-phase direct numerical simulation methods are too demanding.
The acceleration of microparticles to supersonic velocities is required for microscopic ballistic... more The acceleration of microparticles to supersonic velocities is required for microscopic ballistic testing, a method for understanding material characteristics under extreme dynamic conditions, and for projectile gene and drug delivery, a needle-free administration technique. However, precise aerodynamic effects upon supersonic microsphere motion at sub-300 Reynolds numbers have not been quantified. We derive drag coefficients for microspheres traveling in air at subsonic, transonic, and supersonic velocities from the measured trajectories of microspheres launched by laser-induced projectile acceleration. Moreover, the observed drag effects on microspheres in atmospheric (760 Torr) and reduced pressure (76 Torr) are compared with existing empirical data and drag coefficient models. We find that the existing models adequately predict the drag coefficient for subsonic microspheres, while rarefaction effects cause a discrepancy between the model and empirical data in the supersonic regi...
Fluid turbulence is characterized by strong coupling across a broad range of scales. Furthermore,... more Fluid turbulence is characterized by strong coupling across a broad range of scales. Furthermore, besides the usual local cascades, such coupling may extend to interactions that are non-local in scale-space. As such the computational demands associated with explicitly resolving the full set of scales and their interactions, as in the Direct Numerical Simulation (DNS) of the Navier-Stokes equations, in most problems of practical interest are so high that reduced modeling of scales and interactions is required before further progress can be made. While popular reduced models are typically based on phenomenological modeling of relevant turbulent processes, recent advances in machine learning techniques have energized efforts to further improve the accuracy of such reduced models. In contrast to such efforts that seek to improve an existing turbulence model, we propose a machine learning(ML) methodology that captures, de novo, underlying turbulence phenomenology without a pre-specified ...
Computational simulations in deforming domains often require complex mesh adaptation. The goal of... more Computational simulations in deforming domains often require complex mesh adaptation. The goal of the present work is to provide continuous and autonomous adaptation that does not require prior knowledge of the boundary motion. Even in cases where the boundary motions are pre-determined, this adaptation scheme can be helpful if the geometry or motion is very complex, since our scheme is innately unstructured. Our approach is limited to simplical meshes, namely triangles or tetrahedra, due to their convenient geometric properties. This abstract summarizes recent progress in parallelizing the algorithms for use in distributed calculations. Mesh motion is the preferred way of maintaining mesh quality, since it avoids interpolation errors and can be naturally included into the governing equations using relative
The effectiveness of gelled fuel jet impingement, food production, and polymer–based spray coatin... more The effectiveness of gelled fuel jet impingement, food production, and polymer–based spray coatings can depend on the atomization and droplet collision behavior of fluids with complex rheology. Non-Newtonian effects such as shear-thinning, shear thickening, and viscoelastic extensional hardening (usually a consequence of macromolecular interactions) could alter collision outcomes as well as drop-to-drop mixing rates. Extensional hardening specifically has been shown to promote the stability of liquid ligaments, a structure often formed during the transients of a droplet collision. In this study, direct numerical simulations of single phase viscoelastic droplet collisions are performed within a finite volume framework. The free surface is modeled with an explicit moving mesh interface tracking method, allowing highly accurate calculation of surface curvature, which is increasingly important with decreasing Weber number. In this method, the boundary of the computational domain acts as...
A computational study of oblique binary viscoelastic droplet collisions is performed. The free su... more A computational study of oblique binary viscoelastic droplet collisions is performed. The free surface is modeled by a Lagrangian moving-mesh interface tracking method which is validated against analytical solutions and experimental data for binary Newtonian droplet collisions. Interpolation error due to cell deformation is minimized with a local topological mesh adaptation algorithm. This method was then applied to the collision of viscoelastic droplets through a finite volume implementation of the linearized Phan-ThienTanner constitutive stress model. The extensional nature of an oblique droplet collision highlights the effects of the large Trouton ratios typically found in this class of fluids though increased stabilization of fluid ligament structures and expansion of the coalsence regime. By altering the spray kinematics in this manner, the spray characteristics as a whole are altered by the viscoelastic rheological effects. The fully three dimensional computational study inves...
Diesel spray numerical modeling is still a challenging task due to the complex phenomena taking p... more Diesel spray numerical modeling is still a challenging task due to the complex phenomena taking place and the wide range of spatial and time scales required to capture the physics involved. The aim of this work is to evaluate the so-called Σ-Y Eulerian atomization model, originally proposed by Vallet & Borghi, for CFD simulations of diesel engine sprays. This model has emerged as an alternative to the traditional discrete particle methods, widely employed on practical design applications for more than twenty years. Those Lagrangian liquid spray models are not well suited for the description of the primary atomization and the dense two-phase flow occurring at the near field of diesel sprays, where basic model hypothesis cannot be fully accomplished. The Σ-Y model was developed on the basis of an Eulerian representation of the liquid/gas mixture by means of a single-fluid variable density turbulent flow. It is assumed that under high Reynolds and Weber numbers, large scale flow featur...
The adoption of Machine Learning (ML) for building emulators for complex physical processes has s... more The adoption of Machine Learning (ML) for building emulators for complex physical processes has seen an exponential rise in the recent years. While neural networks are good function approximators, optimizing the hyper-parameters of the network to reach a global minimum is not trivial, and often needs human knowl- edge and expertise. In this light, automatic ML or autoML methods have gained large interest as they automate the process of network hyper-parameter tuning. In addition, Neural Architecture Search (NAS) has shown promising outcomes for improving model performance. While autoML methods have grown in popularity for image, text and other applications, their effectiveness for high-dimensional, complex scientific datasets remains to be investigated. In this work, a data driven emulator for turbulence closure terms in the context of Large Eddy Simulation (LES) models is trained using Artificial Neural Networks and an autoML frame- work based on Bayesian Optimization, incorporatin...
Starting with two well-tested, one-dimensional models of non-evaporating, mixing-limited sprays, ... more Starting with two well-tested, one-dimensional models of non-evaporating, mixing-limited sprays, governing equations for liquid mass and two-phase momentum for each model can be manipulated to reveal the formal similarity between momentum and liquid volume fraction. The consequence of this similarity is that momentum, when properly non-dimensionalized, is equal to the liquid volume fraction at any time and at any axial location within a non-evaporating, mixing-limited spray with a constant rate of injection. An alternative, the more well-known similarity between mass fraction and velocity, is also mathematically evident. We compare predictions of this mathematical analysis to high-fidelity, first-principles simulation results of a non-evaporating spray to assess the validity of the theoretical similarity. The analysis of the simulation not only confirms the mathematical derivations but also points to subtlety in the definition of the spray velocity. In particular, the density-weight...
ABSTRACT Predicting the liquid film dynamics inside the injector cup of gas-centered swirl coaxia... more ABSTRACT Predicting the liquid film dynamics inside the injector cup of gas-centered swirl coaxial fuel injectors requires a general two-phase approach that is appropriate for all liquid volume fractions, high Weber number, and complex geometries. The rapid exchange of momentum at the highly convoluted interface requires tight numerical coupling between the gas and liquid phases. An Eulerian two-phase model is implemented to represent the liquid and gas interactions in the injector as well as the atomization processes at the rough interface. The model, originally proposed by Vallet et al, assumes that in the limit of infinite Reynolds and Weber number, features of the atomization process acting at large length scales are separable from small scale mechanisms. A transport equation for the liquid volume fraction represents the dispersion of the liquid into the gas via a traditional turbulent diffusion hypothesis. A model for the growth of mean interfacial surface area is then used to characterize the growth of instability at the interface, allowing a characterization of Sauter mean diameter. The model shows promise as a computationally inexpensive tool for characterizing spray quality in regions where optical experimental data are difficult to obtain and two-phase direct numerical simulation methods are too demanding.
The acceleration of microparticles to supersonic velocities is required for microscopic ballistic... more The acceleration of microparticles to supersonic velocities is required for microscopic ballistic testing, a method for understanding material characteristics under extreme dynamic conditions, and for projectile gene and drug delivery, a needle-free administration technique. However, precise aerodynamic effects upon supersonic microsphere motion at sub-300 Reynolds numbers have not been quantified. We derive drag coefficients for microspheres traveling in air at subsonic, transonic, and supersonic velocities from the measured trajectories of microspheres launched by laser-induced projectile acceleration. Moreover, the observed drag effects on microspheres in atmospheric (760 Torr) and reduced pressure (76 Torr) are compared with existing empirical data and drag coefficient models. We find that the existing models adequately predict the drag coefficient for subsonic microspheres, while rarefaction effects cause a discrepancy between the model and empirical data in the supersonic regi...
Fluid turbulence is characterized by strong coupling across a broad range of scales. Furthermore,... more Fluid turbulence is characterized by strong coupling across a broad range of scales. Furthermore, besides the usual local cascades, such coupling may extend to interactions that are non-local in scale-space. As such the computational demands associated with explicitly resolving the full set of scales and their interactions, as in the Direct Numerical Simulation (DNS) of the Navier-Stokes equations, in most problems of practical interest are so high that reduced modeling of scales and interactions is required before further progress can be made. While popular reduced models are typically based on phenomenological modeling of relevant turbulent processes, recent advances in machine learning techniques have energized efforts to further improve the accuracy of such reduced models. In contrast to such efforts that seek to improve an existing turbulence model, we propose a machine learning(ML) methodology that captures, de novo, underlying turbulence phenomenology without a pre-specified ...
Computational simulations in deforming domains often require complex mesh adaptation. The goal of... more Computational simulations in deforming domains often require complex mesh adaptation. The goal of the present work is to provide continuous and autonomous adaptation that does not require prior knowledge of the boundary motion. Even in cases where the boundary motions are pre-determined, this adaptation scheme can be helpful if the geometry or motion is very complex, since our scheme is innately unstructured. Our approach is limited to simplical meshes, namely triangles or tetrahedra, due to their convenient geometric properties. This abstract summarizes recent progress in parallelizing the algorithms for use in distributed calculations. Mesh motion is the preferred way of maintaining mesh quality, since it avoids interpolation errors and can be naturally included into the governing equations using relative
The effectiveness of gelled fuel jet impingement, food production, and polymer–based spray coatin... more The effectiveness of gelled fuel jet impingement, food production, and polymer–based spray coatings can depend on the atomization and droplet collision behavior of fluids with complex rheology. Non-Newtonian effects such as shear-thinning, shear thickening, and viscoelastic extensional hardening (usually a consequence of macromolecular interactions) could alter collision outcomes as well as drop-to-drop mixing rates. Extensional hardening specifically has been shown to promote the stability of liquid ligaments, a structure often formed during the transients of a droplet collision. In this study, direct numerical simulations of single phase viscoelastic droplet collisions are performed within a finite volume framework. The free surface is modeled with an explicit moving mesh interface tracking method, allowing highly accurate calculation of surface curvature, which is increasingly important with decreasing Weber number. In this method, the boundary of the computational domain acts as...
A computational study of oblique binary viscoelastic droplet collisions is performed. The free su... more A computational study of oblique binary viscoelastic droplet collisions is performed. The free surface is modeled by a Lagrangian moving-mesh interface tracking method which is validated against analytical solutions and experimental data for binary Newtonian droplet collisions. Interpolation error due to cell deformation is minimized with a local topological mesh adaptation algorithm. This method was then applied to the collision of viscoelastic droplets through a finite volume implementation of the linearized Phan-ThienTanner constitutive stress model. The extensional nature of an oblique droplet collision highlights the effects of the large Trouton ratios typically found in this class of fluids though increased stabilization of fluid ligament structures and expansion of the coalsence regime. By altering the spray kinematics in this manner, the spray characteristics as a whole are altered by the viscoelastic rheological effects. The fully three dimensional computational study inves...
Diesel spray numerical modeling is still a challenging task due to the complex phenomena taking p... more Diesel spray numerical modeling is still a challenging task due to the complex phenomena taking place and the wide range of spatial and time scales required to capture the physics involved. The aim of this work is to evaluate the so-called Σ-Y Eulerian atomization model, originally proposed by Vallet & Borghi, for CFD simulations of diesel engine sprays. This model has emerged as an alternative to the traditional discrete particle methods, widely employed on practical design applications for more than twenty years. Those Lagrangian liquid spray models are not well suited for the description of the primary atomization and the dense two-phase flow occurring at the near field of diesel sprays, where basic model hypothesis cannot be fully accomplished. The Σ-Y model was developed on the basis of an Eulerian representation of the liquid/gas mixture by means of a single-fluid variable density turbulent flow. It is assumed that under high Reynolds and Weber numbers, large scale flow featur...
The adoption of Machine Learning (ML) for building emulators for complex physical processes has s... more The adoption of Machine Learning (ML) for building emulators for complex physical processes has seen an exponential rise in the recent years. While neural networks are good function approximators, optimizing the hyper-parameters of the network to reach a global minimum is not trivial, and often needs human knowl- edge and expertise. In this light, automatic ML or autoML methods have gained large interest as they automate the process of network hyper-parameter tuning. In addition, Neural Architecture Search (NAS) has shown promising outcomes for improving model performance. While autoML methods have grown in popularity for image, text and other applications, their effectiveness for high-dimensional, complex scientific datasets remains to be investigated. In this work, a data driven emulator for turbulence closure terms in the context of Large Eddy Simulation (LES) models is trained using Artificial Neural Networks and an autoML frame- work based on Bayesian Optimization, incorporatin...
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