Browsing by Subject "Simulations"

Now showing 1 - 6 of 6
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    Advanced simulation and modeling of turbulent sprays
    (2014-03) Liu, Wanjiao
    Sprays have wide applications in agriculture, pharmaceutical synthesis, engines, ink jet printing and so on. The successful spray applications and the control of spray param- eters require a thorough understanding towards the physical mechanisms. Numerical tools have been developed in the past few years for simulating the multiphase turbu- lent flows like sprays. Several researchers have successfully carried out direct numerical simulations (DNS) to investigate the primary breakup in such flows. DNS is accurate but requires extensive computational resources. In comparison, large eddy simulation (LES) is more practical, resolving only the large-scale flow structures and modeling the small-scale effects. The major difficulty with LES of multiphase turbulent flows is the need to model the interfacial subgrid-scale terms. Subgrid surface tension force, for ex- ample, plays an important role in the small droplet formation process. Subgrid surface tension force is, however, a highly non-linear term and can be difficult to model. In this research, we propose a new approach that combines the filtered density function (FDF) approach with the large eddy simulation. The major advantage of FDF is that the non-linear surface tension force appears in a closed form and thus needs no sub- grid modeling. The FDF transport equation is solved conveniently via a Lagrangian Monte-Carlo method. The Lagrangian approach is attractive in that it facilitates the transport of the liquid-gas interface without the diffusive or dispersive errors found in the Eulerian approaches. The surface tension source term in the momentum equation is closed using a Lagrangian volume of fluid (LVOF) approach. We utilize concepts from the smoothed particle hydrodynamics (SPH) in the LVOF approach to obtain the surface tension source term based on the Lagrangian particles. Several modifications have been made towards the original SPH formulation such that it is more suitable for the large-scale, turbulent multiphase flow simulations. Multiple particles are seeded in each Eulerian cell to achieve higher statistical accuracy, while the original SPH seeds one particle in each cell. What's more, a weighted SPH formula for the color function is adopted and is shown to be capable of handling variable particle number density. Performance assessment is via the rotation of Zalesak's disk and an oscillating elliptical droplet. Results show that the modified approach is able to handle the variable particle number density case appropriately. The simulations of multiphase turbulent flows are then performed with the proposed FDF-VOF methodology. At the same time, results from the simulations are compared with the DNS approach for validation and com- parison. Results show that the FDF-LES based approach can be a promising method, in that it models the flow with lower computational cost than DNS, yet maintaining accuracy in a model-free manor.
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    Markov Chains Meet Molecular Motors
    (2022-11) Shrivastava, Rachit
    Transportation of important cargoes inside the cells of living organism is critical for several cellular functions. Most form of cargo transportation inside the cells is accomplished by teams of nanometer sized proteins called molecular motors. These proteins walk on filamentous tracks inside the cells while carrying a common cargo from its source to destination. Malfunctions in this process could lead to several life-threatening maladies ranging from neurodegenerative diseases to cancers. Thus, fundamental understanding of how molecular motors coordinate the transport of a common cargo is of immense scientific importance. Due to small sizes and stochastic nature of these motor proteins, experiments often lack the spatial and temporal resolutions to investigate the intracellular cargo transport process with molecular details. Mathematical modeling provides a helping hand and can not only add to the information obtained experimentally, but also guide future experiment design, thereby helping us understand the genesis of diseases and aid to the discovery of cures. In this thesis, we have utilized the theory of Markov chains to mathematically model intracellular cargo transport process by teams of molecular motors. Backed by the mathematical theory, we developed a numerical framework which improves upon the existing methodologies and enables us to numerically compute the biologically important statistics of the cargo transport by teams of motor proteins in a more realistic setting on a regular desktop PC. In contrast, previous methodologies regularly employed supercomputing clusters to obtain these statistics. Thus our methods are more accessible to biophysicists studying molecular motors but lacking the access to appropriate computational infrastructure. Further, we develop toy models to mathematically analyze the cargo transport process by two molecular motors replicating the well known "tug of war" and "coordinated movement" type scenarios during multi-motor cargo transport. Our results show that the cargo transport velocities could display non-trivial characteristics and phase transitions depending on certain experimental parameters which is an unexpected phenomenon. The methods developed here are not only limited to modeling cargo transport by molecular motors but also serve as a stepping stone for modeling more general class of processes where a group of stochastic agents accomplishes a common goal.
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    A Mile in Their Shoes: An Immigrant Simulation (2019-09-17)
    (2019) University of Minnesota Duluth. College of Liberal Arts Change Team
    On September 17th the CLA Change Team will be running "A Mile in Their Shoes:  an Immigration Simulation" from 1:30 - 4:00 in Chester Park 105, allowing students and community members to go through the difficult experience of immigrating to the United States. Participants will take a citizenship test, cross the border, simulate ICE detention, go to court, and finally find a job in the United States.  The simulation should take about 1/2 hour to complete.  Participants are not expected to stay for the entire 2 1/2 hours.  As part of this event, we hope to share with participants the difficult journey to the United States that some members of the UMD community have faced
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    Nanophotonic Assemblies and Light Management for Increased Absorption and Photoluminescence
    (2020-10) Quan, Matthew
    This thesis explores three projects involving the interaction of incident light with nanocrystals: absorption enhancement in patterned quantum dot solids (pQDS), time dynamics of plasmonic nanorod assemblies, and the usage of luminescent solar concentrators (LSCs) to enhance upconversion. First, the effect of nanostructure shape on the absorption in pQDS is studied, and then these structures are integrated with plasmonic nanorings and bullseyes to enhance both absorption and photoluminescence (PL). In the next chapter, the time-dependent optical properties of a gold nanorod assembly is modeled following ultrafast laser illumination, accounting for both changes in the electron density and the nanostructure geometry. Finally, Monte Carlo modeling is used to study the effect of LSCs and mirrors on the absorption and upconverted PL from a hydrogel containing the upconverting donor/acceptor pair PtOEP/DPAS.
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    Simulation data for "Influence of charge sequence on the adsorption of polyelectrolytes to oppositely-charged polyelectrolyte brushes"
    (2019-06-20) Sethuraman, Vaidyanathan; McGovern, Michael; Morse, David C; Dorfman, Kevin D; vsethura@umn.edu; Sethuraman, Vaidyanathan; Dorfman Group
    When a solution of polyanionic chains is placed in contact with a polycationic brush, the polyanions adsorb into the brush. We investigate the influence of the charge sequences of the free and bound species on the thermodynamics of polyelectrolyte adsorption. As model systems, we consider free and brush polyelectrolytes with either block or alternating charge sequences, and study the adsorption process using coarse-grained Langevin dynamics with implicit solvent, explicit counterions, and excess salt. Free energy, internal energy, and entropy of adsorption are computed using umbrella sampling methods. When the number of polyanions exceed the number of polycations, the brush becomes overcharged. Free chains adsorb most strongly when both free and tethered chains have a block charge sequence, and most weakly when both species have an alternating sequence. Adsorption is stronger when the free polyanion has a block sequence and the tethered polycation is alternating than in the reverse case of an alternating free polymer and a tethered block copolymer. Sequence-dependent effects are shown to be largely energetic, rather than entropic, in origin.
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    Simulations and synthetic observations of active galactic nuclei jets in galaxy clusters: numerical tools and experiments.
    (2011-07) Mendygral, Peter John
    We present the results and analysis of three-dimensional magnetohydrodynamic (MHD) simulations of jets from active galactic nuclei (AGN) in galaxy cluster environments. The purpose of these simulations was to investigate the interaction of AGN outflows with their environments and to understand the observational consequences that interaction produces. Synthetic X-ray observations of a set of simulations with AGN jets embedded in an analytically defined galaxy cluster were used to examine the reliability of common observational techniques. We explored the accuracy of measuring the enthalpy of X-ray cavities produced by AGN jets and found that observational techniques are accurate to within a factor of 2. We also tested observational methods for estimating the cavity age and mechanical power of AGN. Despite the simplified nature of the models these techniques are based on, estimates of cavity ages and jet powers were also accurate to within a factor of two. To expand our ability to produce more realistic simulations of AGN outflows we developed a new MHD code for numerical astrophysics called WOMBAT. This code was specifically designed and optimized for high performance and scaling on modern supercomputers. It has several additional physics modules for including optically thin radiative cooling, the effects of gravity and the transport and evolution of cosmic rays. We also present a new total energy conserving method for including the effects of gravity that was implemented into WOMBAT. We demonstrate that a non-conservative gravity scheme negatively impacts the accuracy and convergence of numerical Riemann solvers. Finally, we present WOMBAT simulations of AGN outflows in a galaxy cluster extracted from a cosmological simulation. We explore the effects of cluster "weather" on AGN jets and lobes. Although we chose a relatively relaxed cluster for these simulations, we find that bulk flows in the intra-cluster medium (ICM) were sufficient to significantly deflect the jets and lobes. We present synthetic X-ray observations that show highly asymmetric X-ray cavity structures. Synthetic radio images reveal similarities between "double-double" radio galaxies and our intermittent jets as well as morphologies similar to wide-angle tail galaxies. We show that variations in ram pressure in the ICM dominated both magnetic stresses and pressure variations on large scales. On smaller scales, however, magnetic stresses were dominant in localized regions around the jets and lobes. Through this work we find that observational methods used to estimate critical characteristics of AGN outflows, such as the energy released by the central engine, are reliable to within a factor of a few. We also found that AGN outflows can be significantly influenced ICM "weather". These findings reenforce the reliability of our understanding of these systems, and demonstrate how AGN outflows might be used to probe many of the properties of their environments.