Browsing by Subject "Fluid dynamics"

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Computational Analysis of Energy Exchange Mechanisms in Turbulent Flows with Thermal Nonequilibrium
(2018-01) Neville, Aaron
The design of hypersonic vehicles is signiﬁcantly aﬀected by the state of boundary layer. Hypersonic boundary layers can be laminar or turbulent, and in chemical and vibrational nonequilibrium, each with diﬀerent length and time scales. In turbulent boundary layers, heating augmentation can be an order of magnitude or higher above laminar heating rates. The scales of the internal energy relaxation processes can be of the same order or greater than the turbulent ﬂow scales, and can interact with turbulent motion. Understanding how turbulent motion and internal energy relaxation interact is relevant to ﬂow control. Fundamental ﬂows are studied to understand how energy is exchanged between turbulent motion and internal energy relaxation. Speciﬁcally, high-ﬁdelity DNS of vibrational energy relaxation eﬀects in compressible isotropic and temporally evolving shear layers are presented. The energy exchange mechanisms are analyzed by decomposing the ﬂow into incompressible and compressible energy modes. By varying the vibrational relaxation rate, the tuning of the relaxation rate to the turbulent ﬂow is studied. Vibrational energy relaxation is demonstrated to be coupled to the turbulent ﬂow through the compressible modes of the gas. Compressions and expansions generate ﬂuctuations in the thermal state, and the vibrational energy lags behind these ﬂuctuations. Energy is then transferred to or from the vibrational energy mode at a rate proportional to the relaxation time, and the ﬂuctuations are damped. Damping of turbulent quantities are strongest when the vibrational relaxation rate is on the order of the turbulent large structure acoustic rate. Wavenumber speciﬁc damping is also observed in isotropic ﬂows when the relaxation time is on the order of the acoustic frequency of the wave. The eﬀects of vibrational relaxation are shown to increase with compressibility. However, the overall eﬀect on turbulent kinetic energy is weak due to the incompressible mode containing signiﬁcantly more energy than the compressible modes.
Data for DNA fragmentation in a steady shear flow
(2022-09-23) Qiao, Yiming; Ma, Zixue; Onyango, Clive; Cheng, Xiang; Dorfman, Kevin D; qiao0017@umn.edu; Qiao, Yiming; University of Minnesota Dorfman Research Lab
We have determined the susceptibility of T4 DNA (166 kilobase pairs, kbp) to fragmentation under steady shear in a cone-and-plate rheometer.
Dynamics and control of Newtonian and viscoelastic fluids
(2014-09) Lieu, Binh K.
Transition to turbulence represents one of the most intriguing natural phenomena. Flows that are smooth and ordered may become complex and disordered as the flow strength increases. This process is known as transition to turbulence. In this dissertation, we develop theoretical and computational tools for analysis and control of transition and turbulence in shear flows of Newtonian, such as air and water, and complex viscoelastic fluids, such as polymers and molten plastics.Part I of the dissertation is devoted to the design and verification of sensor-free and feedback-based strategies for controlling the onset of turbulence in channel flows of Newtonian fluids. We use high fidelity simulations of the nonlinear flow dynamics to demonstrate the effectiveness of our model-based approach to flow control design.In Part II, we utilize systems theoretic tools to study transition and turbulence in channel flows of viscoelastic fluids. For flows with strong elastic forces, we demonstrate that flow fluctuations can experience significant amplification even in the absence of inertia. We use our theoretical developments to uncover the underlying physical mechanism that leads to this high amplification. For turbulent flows with polymer additives, we develop a model-based method for analyzing the influence of polymers on drag reduction. We demonstrate that our approach predicts drag reducing trends observed in full-scale numerical simulations.In Part III, we develop mathematical framework and computational tools for calculating frequency responses of spatially distributed systems. Using state-of-the-art automatic spectral collocation techniques and new integral formulation, we show that our approach yields more reliable and accurate solutions than currently available methods.
Electrostatic Assist of Liquid Transfer in Printing Processes
(2018-07) Huang, Chung-Hsuan
Printing processes are being explored for the large-scale manufacture of electronic de- vices. Transfer of liquid from one surface to another plays a key role in most printing processes. During liquid transfer, a liquid bridge is formed and then undergoes sig- nificant extensional motion. Incomplete liquid transfer can produce defects that can be detrimental to device operation. One important printing process is gravure, which involves transfer of liquid from micron-scale cavities at high speeds. Electric fields are sometimes used to enhance liquid transfer, a technique known as electrostatic assist (ESA). However, its underlying physical mechanisms remain a mystery. This thesis uses a combination of theory and experiment to understand the fundamental mechanisms by which electrostatic forces influence liquid transfer. Liquid transfer without electric fields and cavities must be understood before study- ing the mechanism of ESA. We develop one-dimensional (1D) slender-jet and two- dimensional (2D) axisymmetric models of this phenomenon and compare the resulting predictions with previously published experimental data. At relatively low stretching speeds, predictions from both models of the amount of liquid transferred agree well with the experimental data. When the stretching speed is high enough, the models predict that each surface receives half the liquid, in agreement with experimental observations. For intermediate values of the stretching speed, predictions from each model can deviate substantially from the experimental data, which we speculate is due to the influence of surface defects that are not included in the models. The 1D and 2D model are modified to include electrostatic effects. The liquid be- haves like a perfect (non-conducting), or leaky dielectric (poorly conducting) material. The liquid is confined between two plates, with the top plate having a constant electro- static potential while the bottom plate is grounded. For perfect dielectrics, application of an electric field enhances liquid transfer to the more wettable surface because it slows the surface-tension-driven breakup of the bridge, thereby allowing more time for the con- tact line to retract on the less-wettable surface. For leaky dielectrics, application of an electric field can augment or oppose the influence of wettability differences, depending on the direction of the electric field and the sign of the interfacial charge. Experimental results confirm the enhancement of the amount of liquid transferred when the electric field is present, and the measured values are in good agreement with the predictions of the 1D perfect dielectric model. When one of the plate is replaced by a cavity, the presence of the cavity causes the contact line on the cavity wall to effectively pin and inhibits the liquid transfer. For perfect dielectrics, application of an electric field unpins the contact line on the cavity and leads to improvement of cavity emptying. For the leaky dielectrics, the presence of the surface charge does not further improve liquid transfer because of nearly zero electric tangential stress near the contact line on each surface.
Electrostatic effects in coating and printing processes
(2015-01) Ramkrishnan, Aruna
Coating and printing are interfacial processes that are highly relevant in industry. Precision coatings impart functionalities and boost the performance of products. On the other hand, high-resolution roll-to-roll printing is being increasingly explored for creating dense and flexible printed electronics at high speeds. Electrostatic effects often significantly influence both these processes. However, in industry, much of the current understanding of these effects is empirical and has not received a rigorous treatment. This thesis discusses how electrostatics and hydrodynamics couple in coating and printing applications, and presents different modes of investigation: simplified thin-film models and flow visualization experiments, to understand the underlying physics of these processes. Throughout this work, the electric response of liquids has been described by the perfect (non-conducting) and leaky dielectric (partially conducting) models, which are representative of many liquids used in industry. In coating processes, electrostatic charges are known to accumulate on the substrate due to various upstream operations (e.g. corona treatment, friction in roll-to-roll equipment). This leads to the buildup of an electric field in the subsequently coated film, which in turn causes the formation of defects due to electrostatically driven flows. Thus, in order to obtain high quality coatings, it is desirable to keep them resistant to electrostatic destabilization. We have carried out a systematic study via the construction of electrohydrodynamic lubrication models to understand the influence of charged substrates and charged interfaces on the leveling of liquid coatings. Based on our findings, we develop simple heuristics that can be used to design coatings that are stable to substrate charging and charge contamination. Electric fields are also present in some printing processes. Developed in the late1960s, electrostatic assist (ESA) has been long used to remove printing defects and enhance image quality in gravure printing, a high-resolution roll to-roll process. ESA involves the application of an electric field to pull ink out of cavities and transfer it onto the desired substrate. However, there is limited understanding of how this process works, which hinders its development as a tool for printed electronics. In order to address this issue, we develop a model for electrostatically assisted meniscus deformation near a cavity (this describes the first stage of electrostatic assist). Our calculations show that electric fields pull up the ink meniscus either at the edges or at the center of the cavity, depending on the ink conductivity. This suggests that ink contact with the substrate will be improved during ESA but air entrapment occurs for a certain range of conductivities, which would be detrimental to print quality. Our model also enables us to investigate the effect of cavity shape and spacing on the mode of deformation of the ink surface. In order to validate the findings from our electrohydrodynamic model, we have carried out flow visualization experiments to track the deformation of liquids contained in cavities, and these corroborate the qualitative trends of meniscus deformation predicted by the model.
Flow and Drying Dynamics in Gravity- and Capillary-Driven Coating Processes
(2017-06) Lade, Robert
Liquid-applied coatings are ubiquitous. Buildings, bridges, soda cans, compact discs, and newspapers make up a small fraction of everyday objects whose surfaces are enhanced by coatings. Typical processing steps for a liquid-applied coating include coating formulation, application, post-deposition flow, and solidification. This thesis focuses on the balance between the last two steps of this process and how this balance influences coating behavior and the ultimate quality of the final film. Specifically, post-deposition coating flows driven by gravity or capillarity are investigated in liquid systems that undergo evaporation-induced drying. In Chapter 2, coating defects caused by excessive gravity-driven flow (‘sag’) are studied. A novel particle tracking method is first developed to monitor sag in a model aqueous polymer system. A computational model is developed concurrently to validate the measurements made using particle tracking. This model is then used to generate a novel framework for predicting sag in liquid-applied coatings. Chapters 3–5 focus on capillary-driven flows in open microchannels. First, in Chapter 3, capillary flow dynamics of non-evaporating liquids are studied and compared against existing theoretical models. In Chapter 4, this work is extended to open microchannels fabricated using several three-dimensional (3D) printing technologies. 3D printed microchannels are found to confer unique flow dynamics to the capillary flow, including a distinct start–stop motion caused by surface roughness introduced by the 3D printing process. Finally, in Chapter 5, the influence of drying on capillary flow dynamics is investigated, again using a model aqueous polymer coating system. Drying is found to permanently pin the advancing contact line partway down the channel; three mechanisms of pinning are identified and characterized. Post-pinning flows induced by the coffee ring effect are found to lead to highly non-uniform dry film morphologies. The influence of surfactant, drying rate, and channel width are investigated. Throughout all of this work, the goal is to better understand the balance between flow and drying to facilitate prediction and control of coating behavior during relevant coating processes. As part of this goal, case studies are conducted throughout this thesis, investigating flow and drying behavior in real systems used in commercial coating processes, including latex paints and functional inks used in the manufacture of printed electronic devices.
Hypersonic Boundary Layer Stability Analysis Using Momentum Potential Theory
(2020-09) Houston, Mary
Linear Stability Theory (LST) and the Parabolized Stability Equations (PSE) have provided valuable tools for analysis and prediction of laminar to turbulent transition for plates, sharp cones, and geometries for which parallel-flow or a slowly-varying boundary layer can be assumed. However, these techniques struggle to capture the complex flow-physics present near the tip of blunt-cones. Input-output analysis has been used in conjunction with direct numerical simulation to capture the effects of nose bluntness on downstream stability. Using the results of the input-output analysis we apply momentum potential theory (MPT) to preform fluid-thermodynamic (FT) decomposition, separating disturbances into their vortical, thermal and acoustic components. A reference case of Mach 6 flow over a flat-plate is computed and output responses are compared to the results for Mach 6 flow over a blunt-cone of $7^{o}$ half angle. Perturbation eigenfunctions and structures are examined in the areas of second-mode amplification. For both the flat-plate and blunt-cone the vortical components are the largest, followed by the thermal then acoustic components. Fluid-thermodynamic structures in the second-mode amplification region of blunt-cone show wall-normal stretching above the critical layer. Fluid-thermodynamic decomposition of full-domain input and output results for the blunt-cone geometry are considered. It is found that input sensitivity is highest at the top of the entropy layer and along the boundary layer edge for the fore-half of the cone. Output response in the streamwise direction is highest in the regions between the generalized inflection point (GIP) and the boundary layer edge and dissipates near the surface, whereas wall-normal response extends to the surface and shows a local minimum between the GIP and boundary layer edge. To compliment existing studies on hypersonic boundary layer response to surface roughness/ vibration we look at input sensitivity and output response at the surface. It is found that there is greater sensitivity to wall-normal forcing than streamwise forcing at the surface and among the three FT components in this direction the vortical had the highest relative output amplitude. Finally, total fluctuating enthalpy (TFE) is considered for both the flat-plate and blunt-cone, in both cases the thermal terms provides the strongest source of TFE.
Liquid-film coating on rotating discrete objects
(2018-01) Li, Weihua
The flow of liquid films on discrete objects is encountered in coating processes for a wide range of products such as biomedical devices, automobiles, and food. Describing the shape of liquid films as they flow over discrete objects is a challenging task due to the large number of forces at play. These include gravitational, inertial, viscous, surface-tension, and centrifugal forces, and the complex interplay among them may lead to the growth of instabilities that degrade the quality of the final product. Motivated by the need to improve fundamental understanding of coating flows on discrete objects, we pick cylinders that rotate about their horizontal axes as model discrete objects and investigate four model problems highly relevant to industrial coating processes for rotating discrete objects. In each model problem, the interplay among all the forces is systematically examined to reveal the critical conditions for which a smooth coating can be obtained. For coating of surfactant-laden liquids on rotating cylinders, we applied lubrication theory to derive coupled nonlinear evolution equations to describe the variation of the film thickness and surfactant concentration as a function of time, the angular coordinate, and the axial coordinate. In the absence of gravitational effects, linear stability analysis reveals that surfactant-induced Marangoni stresses suppress the growth rate of instabilities driven by centrifugal effects and hinder the leveling of perturbations to the film thickness in both the angular and axial directions. When gravitational effects are present, Marangoni stresses lower the critical rotation rate needed to cause a liquid lobe to form and rotate in the angular direction. These stresses also lead to faster damping of this lobe, giving rise to a more axisymmetric coating. With the growth of axial instabilities at long times, Marangoni stresses significantly weaken the stabilizing effect of surface-tension forces, which are found to be responsible for keeping the coating axially uniform in a stable speed window. In addition, Marangoni stresses tend to reduce the spacing between droplets that form at low rotation rates, and suppress the growth rate of rings that form at high rotation grates. Flow visualization experiments yield observations that are qualitatively consistent with our simulation results. For cylinders with complex surface geometries (i.e., topographically patterned cylinders and elliptical cylinders), the Galerkin finite-element method is used to solve the Stokes equations, augmented with a term accounting for centrifugal forces, in a rotating frame of reference. For rapidly rotating cylinders where gravitational forces are negligible, surface-tension forces tend to drive liquid to the low-surface-curvature areas (e.g., pattern troughs) leading to the formation of liquid pools, while centrifugal forces tend to drive liquid in the opposite direction, giving rise to liquid droplets. The number of droplets or pools at steady state depends on the rotation rate, strength of surface tension, pattern frequency, and cylinder aspect ratio. When gravitational forces become significant, it is possible to obtain a coating that closely conforms to the cylinder surface in the patterned-cylinder case. With an increase in the pattern amplitude, recirculation regions start to form inside the troughs, which may strongly influence mixing, mass transport, and heat transport. These reciprocation regions can appear and vanish as the cylinder rotates due to the variation of gravitational forces around the cylinder surface. In the elliptical-cylinder case, simulation results show that smaller aspect ratio corresponds to less liquid that can be supported on the cylinder and also larger gradients in film thickness. A suitably chosen time-dependent rotation rate can greatly improve coating smoothness relative to the constant-rotation-rate case. For cylinders with sufficiently small aspect ratio, film rupture and liquid shedding may occur over the cylinder tips, so simultaneous drying and rotation along with the introduction of Marangoni stresses will likely be especially important for obtaining a smooth coating.
Methods for the Modeling and Simulation of Sprays and Other Interfacial Flows
(2019-09) Wenzel, Everett
Interfacial multiphase flows involve the motion of at least two fluids separated by surface tension. Atomizing interfacial flows, colloquially known as sprays, are among the most important fluid dynamic systems because of their ubiquity; power generation, delivery of aerosolized medicines, and productive produce farming all depend fundamentally on the detailed control of sprays. Atomization remains poorly understood because of a historical and persisting inability to accurately and affordably measure the dynamics inside and near the spray orifice outlet -- it is therefore desirable to be able to numerically simulate sprays with high fidelity. This dissertation presents computational methods that aim to improve current shortcomings in the modeling and simulation of sprays. Accurately characterizing the interfacial curvature of poorly-resolved liquid structures is addressed by deriving a series of finite particle methods for computing curvature. The methods are verified in analytical curvature tests, and validated against the oscillation frequency of ethanol droplets in air. The finite particle method, leveraging dynamic length scale modification, is demonstrated to out-perform the widely-used height function approach. Tracking the location of interfaces is also addressed, for which a coupled Eulerian-Lagrangian point mass particle scheme is introduced that preserves a well-distributed particle field, can be applied to an arbitrary number of fluids, and does not limit the simulation time step. The Eulerian-Lagrangian method is demonstrated to out-perform contemporary geometric volume of fluid methods at resolutions relevant to spray simulation in a variety of analytical phase tracking tests, and is dynamically evaluated by simulating extending three-phase elliptical regions, droplet dynamics, and Rayleigh-Taylor instabilities. The Eulerian-Lagrangian method is then extended to an approach for consistently and conservatively solving multiphase convection-diffusion problems -- this extension is verified via two analytical heat transfer problems, and robustness is demonstrated by simulating heated air blast atomization. Each of these tests conserves thermal energy and preserves boundedness of the temperature field. This dissertation concludes by outlining paths for consistently and conservatively solving the multiphase Navier-Stokes equations and the multiphase large eddy simulation equations in the coupled Eulerian-Lagrangian point mass particle framework.
Multiscale and Multiphysics of Blood Flow and Arterial Mechanics Growth and Remodeling
(2024-01) Schmidt Bazzi, Marisa
The circulatory system, resembling a complex network of pipes (blood vessels) and a ceaseless pumping system (heart), orchestrates the delivery of oxygen and nutrients to every cell and tissue in the human body. Unlike conventional engineering pipes, vascular tissue exhibits the remarkable ability to adapt its physical and mechanical properties in response to its environment, a phenomenon known as growth and remodeling (G&R). This process aims to maintain a balanced stress level, termed homeostatic stress.In healthy arteries, maintaining mechanical equilibrium involves a clever negative feedback loop that restores the system to its preferred state after any disturbances. However, when this delicate balance is disrupted, it can lead to a phenomenon called pathological G&R, characterized by a positive feedback loop. Aortic and intracranial aneurysms are prominent examples of this disrupted G&R. Characterized by the enlargement of vessels, aneurysms pose significant health risks, contributing to numerous annual fatalities. Moreover, blood disorders such as sickle cell disease can disrupt mechanical equilibrium by altering blood flow dynamics and creating localized hypoxia, especially in small arteries, such as the one found in our brain. Therefore, recognizing the connection between blood disorders and tissue-related diseases underscores the importance of exploring the interplay between fluid dynamics and tissue mechanics. This thesis investigates the interplay between computational fluid dynamics, mathematical modeling, and finite element analysis in the context of cardiovascular diseases. It primarily focuses on ascending thoracic and intracranial aneurysms related to sickle cell disease. We aim to enhance our understanding of the intricate mechanisms underlying vascular diseases. This heightened insight will be central in developing more holistic diagnostic and therapeutic approaches to effectively lessen their significant impact on individuals' health.

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