Browsing by Subject "Fluid mechanics"

Now showing 1 - 9 of 9

Analysis of tensioned-web-over-slot die coating.
(2009-12) Nam, Jaewook
In tensioned-web-over-slot die (TWOSD) coating, web is sustained only by the ten- sion of the web wrapped around the coating die. The distance between the die and web is set by the interaction between the hydrodynamic force of coating liquid and the normal stress resultant from the curved tensioned web, in what is called elaso- hydrodynamic interaction. In order to analyze this particular the coating flow, several tools are developed and tested on other coating flows relatively simpler than TWOSD flow. The theoretical conditions for the onset of vortex that degrades product qualities are proposed and the critical vortex birth trajectories inside the parameter space are tracked automatically by a direct tracking method. To detect a defect-causing flow feature in multi-layer coating, mid-gap invasion, the position of an interlayer separation point was tracked by theoretical model. The results are verified by flow visualization experiment for two- layer fixed-gap slot coating. Also linear stability analysis was performed, in order to detect unstable interlayer that signals coating defects in the two-layer slot coating. The purpose of the research is to understand the complicated flow characteristic inside the coating bead by solving the two-dimensional Navier-Stokes theory using finite element method and visualizing the coating bead flow on a lab-scale TWOSD coater. Using the tools described above, the flow features that leads to coating defects, such as, bead breakup, weeping, mid-gap invasion and feed slot vortex, are identified and mathematical forms of the onset condition for the features are presented. The onset conditions are combined into the direct tracking method that was used to construct the vortex-free operating window for the given die lip configurations. Furthermore, the tracking method can be used to shows the effect of die lip design on the critical parameters for the onset of the flow features.
Devising superconvergent HDG methods for partial differential equations
(2012-08) Shi, Ke
The DG methods are ideally suited for numerically solving hyperbolic problems. However this is not the case for diffusion problems,even though they are ideally suited for hp-adaptivity. Indeed, when compared with the classical continuous Galerkin methods on the same mesh, they have many more global degrees of freedom and they are not easy to implement. When compared with the mixed methods, they do not provide optimally convergent approximations to the flux and do not display superconvergence properties of the scalar variable. As a response to these disadvantages, the HDG methods were introduced in [6]. Therein, it was shown that HDG methods can be implemented as efficiently as the mixed methods. Later in [7] it was proven that the HDG methods do share with mixed methods their superior convergence properties while retaining the advantages typical of the DG methods. Inspired by these results, in this Thesis we are trying to explore HDG methods in a wider circumstance.
Fluid mechanic phenomena relating to flow control in conduits and pumps
(2014-08) Bayazit, Yilmaz
The attainment of controlled homogenized fluid flow is a major issue in the efficient utilization of internal flows for applications as diverse as heat exchange, electrostatic filtration, water purification, particle conveyance, swirl control, and waste disposal. Among the candidate methodologies for accomplishing the homogenization task, perforated plates provide exceptional versatility and adaptability. The principle that underlies perforated plate flow control is the tendency of a flowing fluid to seek the path of least resistance. This tendency is coupled with the capability of the fluid to "see" what lies ahead, enabling it to adjust its trajectory. That capability is due to streamwise diffusion, which transfers information both upstream and downstream. In contrast, advection is a one-way information transfer mechanism, the direction of transfer coinciding with the direction of fluid motion.The degree of homogenization afforded by perforated plates depends on several geometrical and operating parameters. The geometrical parameters include: (a) plate porosity, (b) plate thickness, (c) aperture diameter, (d) pattern of aperture deployment, and (e) distance between apertures. With respect to operating parameters, those investigated here encompass (f) fluid velocity, (g) flow regime, and (h) angle of attack. Nondimensionalization diminished the total number of parameters to five. Numerical simulation was employed to solve the three-dimensional flow covering a Reynolds number range from 0.01 to 25,000. Results extracted from the solutions included dimensionless pressure drop, downstream distance for disturbance decay, vector diagrams and streamlines, and flow regime boundaries. A paradox where the pressure drop for a thin plate exceeded that for a thick plate was rationalized.The pressure drop characteristics of a perforated plate are akin to those for a porous medium. The Darcy-Forchheimer pressure drop model was extended into the turbulent flow regime for the first time, thereby contradicting the prior limitation to inertial laminar flow.The Taguchi method was applied to pinpoint the most important among the independent variables with respect to the dimensionless pressure drop, highlighting the importance of the porosity.Control of the liquid flow produced by a pump was analyzed as a problem of fluid-structural interaction.
On solutions to Navier-Stokes equations in critical spaces.
(2010-07) Rusin, Walter Mieczyslaw
In this thesis, we consider solutions to the incompressible Navier-Stokes equa- tions in three spatial dimensions in the critical homogenous Sobolev space. We attempt to unify the theory of mild solutions and the theory of suitable weak solutions to show that assuming the existence of initial data leading to a finite time singularity the set of such initial is closed in the weak topology and sequen- tially compact modulo translations and dilations. This result is motivated by a theorem of Gallagher, Iftimie and Planchon which states that this set is closed in the strong topology. We present two proofs of our result. The first one is based on the theory of suitable weak solutions and partial regularity for the Navier-Stokes equaitons. The second approach is rooted in the profile decomposition developed by Gallagher.
Onset of dynamic wetting failure: the mechanics of high-speed fluid displacement
(2013-07) Vandre, Eric Allen
Dynamic wetting is crucial to processes where a liquid displaces another fluid along a solid surface, such as the deposition of a coating liquid onto a moving substrate. Numerous studies report the failure of dynamic wetting when process speed exceeds some critical value. Typically, wetting failure is a precursor to air entrainment, which produces catastrophic defects in coatings. However, the hydrodynamic factors that influence the transition to wetting failure remain poorly understood from empirical and theoretical perspectives. This work investigates the fundamentals of wetting failure in a variety of systems that are relevant to industrial coating flows. A hydrodynamic model is developed for planar and axisymmetric geometries where an advancing fluid displaces a receding fluid along a smooth, moving substrate. Numerical solutions predict the onset of wetting failure at a critical substrate speed, which coincides with a turning point in the steady-state solution path for a given set of system parameters. Flow-field analysis reveals a physical mechanism where wetting failure results when capillary forces can no longer support the pressure gradients necessary to steadily displace the receding fluid.Novel experimental systems are used to measure the substrate speeds and meniscus shapes associated with the onset of air entrainment during wetting failure. Using high-speed visualization techniques, air entrainment is identified by the elongation of triangular air films with system-dependent size. Air films become unstable to thickness perturbations and ultimately rupture, leading to the entrainment of air bubbles. Meniscus confinement in a narrow gap between the substrate and a stationary plate is shown to delay air entrainment to higher speeds for a variety of water/glycerol solutions. In addition, liquid pressurization (relative to ambient air) further postpones air entrainment when the meniscus is located near a sharp corner along the plate. Recorded critical speeds compare well to predictions from the model, supporting the hydrodynamic mechanism for the onset of wetting failure. Lastly, the common practice of curtain coating is investigated using the hydrodynamic model. Due to the complexity of this system, a new hybrid method is developed to reduce computational cost associated with the numerical analysis. Results show that the onset of wetting failure varies strongly with the operating conditions of this system. In addition, stresses from the air flow dramatically affect the steady wetting behavior of curtain coating. Ultimately, these findings emphasize the important role of two-fluid displacement mechanics during high-speed wetting. Although this work was motivated by coating flows, it is also relevant to a number of other applications such as microfluidic devices, oil-recovery systems, and splashing droplets.
Stretching and slipping liquid bridges: liquid transfer in industrial printing.
(2011-08) Dodds, Shawn
Liquid bridges with moving contact lines are found in a variety of settings, such as capillary feeders and high-speed printing processes. Despite this relevance, studies on liquid bridges often assume that the contact lines remain pinned in place during stretching. While this may be the case for some applications, contact line motion is \emph{desirable} in printing processes so that the amount of liquid transferred can be maximized. In this thesis we study several model problems to improve our understanding of how moving contact lines alter the dynamics of liquid bridges. We use the finite element method to study the stretching of a liquid bridge between either two flat plates or a flat plate and a cavity. For axisymmetric bridges we find that while the wettability of the two surfaces is a key factor in controlling liquid transfer between two flat plates, the presence of a cavity leads to fundamentally different bridge dynamics. This is due to the pinning of the contact line on the cavity wall, which leads to a decrease in the amount of liquid transferred to the flat plate. We find that the presence of inertia aids in cavity emptying by forcing the interface further into the cavity. However, this increase in emptying can be offset by an increased tendency for the production of satellite drops as the flat plate is made more wettable. To study non-axisymmetric flows we solve the Navier-Stokes equations in three dimensions. We find that when the stretching motion is asymmetric the liquid remains evenly distributed after breakup, so long as the two plates are not accelerating relative to each other. If the bridge shape is not initially cylindrical we find that the ability of the bridge to maintain its initial shape after breakup depends on the friction between the contact line and the solid. Finally, we use flow visualization to observe the stretching of liquid bridges both with and without small air bubbles. We find that while the breakup of wetting fluids between two identical surfaces is symmetric about the bridge midpoint, contact line pinning breaks this symmetry at slow stretching speeds for nonwetting fluids. We exploit this observation to force the bubbles selectively toward the least hydrophillic plate confining the bridge.
Swimming Despite Obstacles: Bacterial Swimming as an Evolution-selected Feature
(2022-08) Kamdar, Shashank
In the 1670s, Leeuwenhoek used a single-lens microscope to bring the unfamiliar microscopic world of bacteria to human attention. In this research work, we use biophysical tools of quantitative microscopy and fluid dynamics to revisit the same world of microbes and shed light on the intricate yet fascinating motion of microbes. In particular, this thesis details two fundamentally significant problems related to microbial locomotion: 1) motility of microbes in complex fluids, and 2) impact of multiflagellarity on bacterial motility. Locomotion of flagellated microorganisms is of great importance for a wide range of biological processes from disease infection, to reproduction, and to ecosystem health. Bacterial swimming in simple Newtonian fluids is well understood; however, our understanding of their motion in their natural habitats comprising of microscopic particles and polymers is still far from complete. Even after six decades of research, whether bacteria show motility enhancement in polymer solutions and what is the origin of this enhancement remain under debate. We tackled this problem from a new perspective: we studied bacterial locomotion in dilute colloidal suspensions, which do not exhibit complex rheological behaviors such as shear thinning, thickening, etc. Surprisingly, we found that all the measurable swimming features of bacteria in colloidal suspensions are quantitatively the same as those in polymer solutions. This suggests a common origin of bacterial motility enhancement in all complex fluids and challenges all the existing theories which exclusively used polymer dynamics to explain this behavior. We subsequently developed a simple hydrodynamic model considering the colloidal nature of complex fluids, which predicted bacterial motility enhancement in both colloidal suspensions and polymer solutions. We also propose a new mechanism of bacterial wobbling that shows the enhancement and also reproduced bacterial helical trajectories with large pitches—another puzzling behavior of bacterial locomotion. Thus, our study combining experiments and theory unambiguously resolved the long-standing controversy of two problems at once, i.e., the origin of bacterial motility enhancement in complex fluids and the mechanism of bacterial wobbling in Newtonian fluids. Bacterial species also show variations in their flagellar architecture and adapt two common arrangements: monotrichous or uniflagellar bacteria possess a single flagellum at the pole of their body and peritrichous bacteria grow multiple flagella over their body, which form a helical rotating bundle propelling bacterial swimming. Although the cellular features of bacteria are under strong evolutionary selective pressures, extensive studies suggest that multiflagellarity confers no noticeable benefit to bacterial motility. These findings pose a long-standing question: why does multiflagellarity emerge in bacteria given the tremendous metabolic cost of flagellar synthesis? Here, contrary to common views that seek the answer beyond the basic function of flagella in motility, we show that multiflagellarity indeed provides a significant selective advantage in bacterial motility, allowing bacteria to maintain a constant swimming speed over a wide range of body sizes. Through experiments of immense sample sizes and detailed hydrodynamic modeling and simulations, we quantitatively reveal how bacteria utilize the increasing number of flagella to regulate the flagellar motor load, which leads to faster flagellar rotational speeds balancing the higher hydrodynamic drag on the bacterial body of larger sizes. Without such an elegant mechanism, the swimming speeds of uniflagellar bacteria decrease with increasing body sizes. This stark difference between the two swimming modes provides a novel fluid dynamic insight into the crucial role of multiflagellarity in maintaining optimum motility for navigation and survival in their native habitats. Beyond, the ecological implications, results, and insights from this thesis serve as guidelines for devising artificial swimmers that efficiently navigate complex biological environments for drug delivery.
Synthetic jet flow and heat transfer for electronics cooling
(2014-05) Huang, Longzhong
The progressive increase of heat dissipation from modern electronics requires more and more powerful cooling systems. Various cooling technologies have been developed such as liquid cooling, micro-channel cooling, and active cooling. The present study focuses on applying a unique device called a synthetic jet to cool electronics. A synthetic jet is able to generate an unsteady flow with a simple structure that makes it effective in convective heat transfer. This study provides both practical and fundamental view of synthetic jets in the application of electronics cooling. A mock-up synthetic jet is fabricated to study heat transfer and fluid mechanics of synthetic jet cooling. The scaled synthetic jet is geometrically and dynamically similar to the actual jet. The heat transfer performance characteristics of a synthetic jet impinging on a fin are tested with different operating frequencies and with different orifice shapes. Flow visualizations and detail flow field measurements of the impinging synthetic jet flow are documented to support the heat transfer experiment. The optimized parameters obtained from the scaled experiment are applied to the actual synthetic jet design. The actual synthetic jet is realized using a piezoelectric stack and applied on a cooling system based on a full-sized heat sink module. The cooling performance of the whole system is documented. The noise characteristics of the actual synthetic jet is tested and analyzed. A muffler with optimized parameters is found and used for noise reduction. Numerical simulation is used to find the optimal design for the synthetic jets. The computation is realized by the commercial software ANSYS Fluent. The numerical model is verified by comparing the computational results with experimental results. A parametric study of heat transfer performance of synthetic jet cooling is documented.
Validated Numerical Simulations Investigating the Effects of Cross-sectional Asymmetry on Fluid Flow and Heat Transfer
(2017-11) Ghosh, Abhimanyu
The work documented in this thesis focuses on numerical analysis as the primary means of gaining insight into the behavior of fluid flow and associated heat transfer in a situation where some kind of cross-sectional asymmetry has been introduced in the flow. As such, results from numerical experiments validating experimental data in asymmetric flow around pipe bends are presented near the beginning of this work. Thereafter, the results of numerical investigation into three situations have been presented. The first involves analyzing fluid flow in a piezometer ring, a device widely used to measure pressure in pipes, where the flow is cross-sectionally asymmetric due to the ring being present right after a 90° bend. The results from this chapter include recommendations for tap-off angles in a piezometer ring and the relative diameters of the ring and the tubes connecting it to the main pipe. The second situation involves numerical analysis of a rectangular fluid jet switching axes before impinging on a flat plate. The switching of axes in rectangular jets is a commonly observed asymmetry in fluid flow that is seldom investigated numerically due to very high computational cost. The results from this chapter present correlations that accurately predict experimental data on heat transfer from plate center, and graphs and figures that demonstrate the off-centered position of the areas of highest heat transfer on the plate. The third and final situation involves numerically analyzing 90° pipe bends of varying radii of curvature, fitted with orifices of varying blockage positioned at the start of the bend, for effects on heat transfer from the portion of the pipe past the bend. The effect of both the bend and the orifice is to introduce asymmetry in the flow and the results presented demonstrate the relative influence of the orifice and the bend radii on fluid flow and heat transfer from the straight section of the pipe after the bend.

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