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Basnet, Keshav. "Flow around porous barriers: fundamental flow physics and applications." Diss., University of Iowa, 2015. https://ir.uiowa.edu/etd/1824.
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Investigating flow and turbulence structure around a barrier mounted on the ground or placed in its vicinity is a fundamental problem in wind engineering because of many practical applications related to protection against adverse effects induced by major wind storms (e.g., hurricanes) and snow events (e.g., snow fences used to reduce adverse effects of snow drifting on the roads). In this work the focus is on the case when the obstacle/barrier is porous and the shape of the obstacle is close to a high-aspect-ratio rectangular cylinder situated in the vicinity of the ground. The study employs a range of numerical and experimental techniques to achieve this goal that include 3D LES and 2D RANS numerical simulations, and RTK survey and 3D photogrammetry techniques to measure ground elevations and snow deposits in the field.
In the first part of the study, high-resolution large eddy simulations are used to understand the fundamental flow physics of flow past 2D solid and porous vertical plates with a special focus on describing the unsteady wind loads on the obstacle, vortical structure of the turbulent wake, spectral content of the wake, the separated shear layers and of the characteristics of the large-scale vortex shedding behind the plate, if present. Results show that LES can accurately predict mean flow and turbulence statistics around solid/porous cylinders. Then, a detailed parametric study of flow past vertical solid and porous plates situated in the vicinity of a horizontal bed is performed for the purpose of understanding changes in the mean flow structure, turbulence statistics and dynamics of large scale coherent structures as a function of the main nondimensional geometrical parameters (bottom gap for solid and porous plates, and porosity and average hole size of porous plates) and flow variables (e.g., bed roughness) that affect the wake flow. In particular, the LES flow fields allowed clarifying how the interactions between the bottom and the top separated shear layers change with increasing bottom gap and what is the effect of the bleeding flow on the interactions between the separated shear layers that determine the coherence of the large-scale eddies at large distances from the wake.
In the second part of the thesis, a novel methodology based on field monitoring of the snow deposits and RANS numerical simulations is proposed to improve the design of snow fences and in particular the design of lightweight plastic snow fences that are commonly used to protect roads in the US Midwest against the snow drifting. The goal of the design optimization procedure is to propose a snow fence design that can retain a considerable amount of snow within a shorter downwind distance compared to fences of standard design. A major contribution of the present thesis was the development of a novel non-intrusive image-based technique that can be used to quantitatively estimate the temporal evolution of the volume of snow trapped by a fence over long periods of time. This technique is based on 3-D close range photogrammetry. Results showed that this technique can produce estimations of the snow deposits of comparable accuracy to that given by commonly used methods. This is the first application of this type of techniques to measurements of the snow deposits.
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Paleo, Cageao Paloma. "Fluid-particle interaction in geophysical flows : debris flow." Thesis, University of Nottingham, 2014. http://eprints.nottingham.ac.uk/27808/.
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Small scale laboratory experiments were conducted to study the dynamic mor- phology and rheological behaviour of fluid-particle mixtures, such as snout-body architecture, levee formation, deposition and particle segregation effects. Debris flows consist of an agitated mixture of rock and sediment saturated with water. They are mobilized under the influence of gravity from hill slopes and channels and can reach long run-out distance and have extremely destructive power. Better understanding of the mechanisms that govern these flows is required to assess and mitigate the hazard of debris flows and similar geophysical flows. Debris flow models are required to accurately deal with evolving behaviours in space and time, to be able to predict flow height, velocity profiles and run-out distances and shapes. The evolution of laboratory debris flows, both dry glass beads and mixtures with water or glycerol, released from behind a lock gate to flow down an inclined flume, was observed through the channel side wall and captured with high speed video and PIV analysis to provide velocity profiles through out the flow depth. Pore pressure and the normal and shear stress at the base of the flow were also measured. Distinct regions were characterized by the non-fluctuating region and the in- termittent granular cloud surrounding the flows. The extent of these regions was shown to be related to flow properties. The separation of these two regions allowed the systematic definition of bulk flow characteristics such as characteristic height and flow front position. Laboratory flows showed variations in morphology and rheological characteristics under the influence of particle size, roughness element diameter, interstitial fluid viscosity and solid volume fraction. Mono-dispersed and poly-dispersed components mixed with liquids without fine sediments, reveal a head and body structure and an appearance similar to the classic anatomy of real debris flows. Unsaturated fronts were observed in mono-dispersed flows, suggesting that particle segregation is not the only mechanism. A numerical simulation of laboratory debris flows using the computer model RAMMS (RApid Mass Movements Simulation) was tested with dry laboratory flows, showing close similarity to calculated mean velocities.
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Pathmathas, Thirunavukkarasu. "Granular flow modelling of rotating drum flows using positron emission particle tracking." Doctoral thesis, University of Cape Town, 2015. http://hdl.handle.net/11427/15707.
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Tumbling mills are characterized by a flowing granular mixture comprising slurry, ore and grinding media. Akin to fluid flow, a rheological description underpinning granular flow has long been expected and pursued by many researchers. Unfortunately, no single theory has hitherto been able to successfully describe all the peculiar features and flow phases of granular systems. Tumbling mills exhibit a rich coexistence of all known flow phases and is arguably the most complicated of the granular flow geometries. Not surprisingly, current comminution models are almost entirely empirical with limited predictive capability beyond their window of design. Using Positron Emission Particle Tracking (PEPT) data we recover the key ingredients (velocity, shear rate, volume concentration, bed depth) for developing, testing and calibrating granular flow models. In this regard, 5 mm and 8 mm glass beads are rotated within a 476 mm diameter mill, fitted with angled lifter bars along the inner azimuthal walls and operated in batch mode across a range of drum rotation speeds that span cascading and cataracting Froude regimes. After averaging the PEPT outputs into representative volume elements, subsequent continuum analysis of the flowing layer revealed a rich coexistence of flow regimes - (i) quasi-static, (ii) dense (liquid-like), and (iii) inertial - that are consistent with the measured volume concentrations spanning these regimes in rotating drums. Appropriately matched constitutive choices for the shear stresses then facilitated the derivation of a new granular rheology that is able to (smoothly) capture all phases of the tumbling mill flow at transition points that match leading experimental findings reported in the literature. Limiting our models to athermal boundary conditions, we then derive the power density for better understanding of flow dissipation that ultimately drives the comminution purpose of tumbling mills. The rheology and power density models were then applied to the 5 mm and 8 mm glass bead data to reveal that shear power density is an order of magnitude larger than the normal component. Notwithstanding, the effective friction coefficient - which is akin to viscosity in typical fluids - remains relatively constant across most of the flowing layer with notable exponential growth across the interface from dense-to-inertial that continued into the inertial regime.
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Bradshaw, Sean D. (Sean Darien) 1978. "Physics-based, reduced-order combustor flow modeling." Thesis, Massachusetts Institute of Technology, 2002. http://hdl.handle.net/1721.1/82215.
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Qazi, S. Junaid S. "Flow of Colloidal Mesophases." Doctoral thesis, Uppsala universitet, Materialfysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-152872.
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This dissertation presents new work and results in the flow of complex fluids and experimental methodologies for their investigation. Plate-like colloidal particles of kaolinite and nickel hydroxide are studied. A study of lamellar fragments and their mixture with the nickel hydroxide particles is also presented. The lamellar fragments are self-assembled structures of surfactant molecules that approximate disks. Particles are seen to align with their large faces parallel to the flow direction under shear and elongational strains. Order parameters have been calculated to quantify the extent of preferential alignment and direction of orientation. The experimental data are supported by comparisons with finite-element fluid mechanics calculations that provide estimates of the flow patterns and the strain rates. Elongational strain rates in the range of 5 − 20 s−1 are required to induce a high degree of alignment with the various sizes of the particles whereas about two to three order of magnitude higher shear strain rates are required. The combination of both elongational and shear strain is an effective means to provide a uniform alignment. Comparison of the Peclet numbers calculated for both the shear and elongational flow are presented and this explains that alignment occurs when the energy per particle of the strain is larger than the thermal energy. Mixtures have shown complex behavior: significant changes in the structure are observed that are not seen to the same extent in samples at rest. X-ray diffraction and small-angle neutron scattering techniques are used to characterize the samples and determine the structure in flowing systems. Laboratory X-ray diffraction can be used to characterize dispersed samples. The combination of dynamic light scattering and X-ray diffraction was used to estimate the thickness of the stabilizing layers of the polymer on the colloidal particles. Scattering of synchrotron radiation and neutrons are powerful complementary techniques to provide information about flow and the potential to apply them to systems that are beyond the scope of simple simulations has been demonstrated.
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Sekhar, Susheel Kumar. "Viscous hypersonic flow physics predictions using unstructured Cartesian grid techniques." Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/45857.
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Aerothermodynamics is an integral component in the design and implementation of hypersonic transport systems. Accurate estimates of the aerodynamic forces and heat transfer rates are critical in trajectory analysis and for payload weight considerations. The present work seeks to investigate the ability of an unstructured Cartesian grid framework in modeling hypersonic viscous flows. The effectiveness of modeling viscous phenomena in hypersonic flows using the immersed boundary ghost cell methodology of this solver is analyzed.
The capacity of this framework to predict the surface physics in a hypersonic non-reacting environment is investigated. High velocity argon gas flows past a 2-D cylinder are simulated for a set of freestream conditions (Reynolds numbers), and impact of the grid cell sizes on the quality of the solution is evaluated. Additionally, the formulation is verified over a series of hypersonic Mach numbers for the flow past a hemisphere, and compared to experimental results and empirical estimates.
Next, a test case that involves flow separation and the interaction between a hypersonic shock wave and a boundary layer, and a separation bubble is investigated using various adaptive mesh refinement strategies. The immersed boundary ghost cell approach is tested with two temperature clipping strategies, and their impact on the overall solution accuracy and smoothness of the surface property predictions are compared.
Finally, species diffusion terms in the conservation equations, and collision cross-section based transport coefficients are installed, and hypersonic flows in thermochemical nonequilibrium environments are studied, and comparisons of the off-surface flow properties and the surface physics predictions are evaluated. First, a 2-D cylinder in a hypersonic reacting air flow is tested with an adiabatic wall boundary condition. Next, the same geometry is tested to evaluate the viscous chemistry prediction capability of the solver with an isothermal wall boundary condition, and to identify the strengths and weaknesses of the immersed boundary ghost cell methodology in computing convective heating rates in such an environment.
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Alver, Burak Han. "Measurement of non-flow correlations and elliptic flow fluctuations in Au+Au collisions at RHIC." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/62643.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 95-102).
Measurements of collective flow and two-particle correlations have proven to be effective tools for understanding the properties of the system produced in ultrarelativistic nucleus-nucleus collisions at the Relativistic Heavy Ion Collider (RHIC). Accurate modeling of the initial conditions of a heavy ion collision is crucial in the interpretation of these results. The anisotropic shape of the initial geometry of heavy ion collisions with finite impact parameter leads to an anisotropic particle production in the azimuthal direction through collective flow of the produced medium. In "head-on" collisions of Copper nuclei at ultrarelativistic energies, the magnitude of this "elliptic flow" has been observed to be significantly large. This is understood to be due to fluctuations in the initial geometry which leads to a significant anisotropy even for most central Cu+Cu collisions. This thesis presents a phenomenological study of the effect of initial geometry fluctuations on two-particle correlations and an experimental measurement of the magnitude of elliptic flow fluctuations which is predicted to be large if initial geometry fluctuations are present. Two-particle correlation measurements in Au+Au collisions at the top RHIC energies have shown that after correction for contributions from elliptic flow, strong azimuthal correlation signals are present at A0 = 0 and A0 ~ 120. These correlation structures may be understood in terms of event-by-event fluctuations which result in a triangular anisotropy in the initial collision geometry of heavy ion collisions, which in turn leads to a triangular anisotropy in particle production. It is observed that similar correlation structures are observed in A Multi-Phase Transport (AMPT) model and are, indeed, found to be driven by the triangular anisotropy in the initial collision geometry. Therefore "triangular flow" may be the appropriate description of these correlation structures in data. The measurement of elliptic flow fluctuations is complicated by the contributions of statistical fluctuations and other two-particle correlations (non-flow correlations) to the observed fluctuations in azimuthal particle anisotropy. New experimental techniques, which crucially rely on the uniquely large coverage of the PHOBOS detector at RHIC, are developed to quantify and correct for these contributions. Relative elliptic flow fluctuations of approximately 30-40% are observed in 6-45% most central Au+Au collisions at s NN= 200 GeV. These results are consistent with the predicted initial geometry fluctuations.
by Burak Han Alver.
Ph.D.
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Santhanakrishnan, Arvind. "CHARACTERIZATION AND FLOW PHYSICS OF PLASMA SYNTHETIC JET ACTUATORS." UKnowledge, 2007. http://uknowledge.uky.edu/gradschool_diss/545.
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Plasma synthetic jet actuators are investigated experimentally, in which the geometrical design of single dielectric barrier discharge (SDBD) plasma actuators is modified to produce zero-mass flux jets similar to those created by mechanical devices. The SDBD plasma actuator consists of two rectangular electrodes oriented asymmetrically and separated by a layer of dielectric material. Under an input of high voltage, high frequency AC or pulsed DC, a region of plasma is created in the interfacial air gap on account of electrical breakdown of the ambient air. A coupling between the electric field in the plasma and the neutral air near the actuator is introduced, such that the latter experiences a net force which results in a horizontal wall jet. This effect of the actuator has been demonstrated to be useful in mitigating boundary layer separation in aerodynamic flows. To increase the impact that a plasma actuator may have on the flow field, this research investigates the development and characterization of a novel flow control device, the plasma synthetic jet actuator, which tailors the residual air in the form of a vertical jet resembling conventional continuous and synthetic jets. This jet can be either three dimensional using annular electrode arrays, or nearly two dimensional using two rectangular strip exposed electrodes and one embedded electrode. Detailed measurements on the isolated plasma synthetic jet reveal that pulsed operation of the actuator results in the formation of multiple counterrotating vortical structures in the flow field. The output jet velocity and momentum are found to be higher for unsteady pulsing as compared to steady operation. In the case of flow over a flat plate, the actuator is observed to create a localized interaction region within which the baseline flow direction and boundary layer characteristics are modified. The efficiency of the actuator in coupling momentum to the neutral air is found to be related to the plasma morphology, pulsing frequency, actuator dimension, and input power. An analytical scaling model is proposed to describe the effects of varying the above variables on the output jet characteristics and actuator efficiency, and the experimental data is used for model validation.
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Freed, David M. (David Michael). "A digital physics method for two phase flow." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/43571.
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Chinn, John Joss. "The internal flow physics of swirl atomizer nozzles." Thesis, University of Manchester, 1996. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488302.
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The internal flow of pressure swirl atomizers is studied with the ultimate aim of developing a design tool for such atomizers, particularly to enable the production of the finest possible water sprays (for any given supply pressure) for fire suppression purposes. The primary direction of the research was in computationally modelling this flow with a two-dimensional assumption and a methodology is described which is capable of determining the velocity profiles and, for the first time in the literature, the air-core topology and discharge coefficient, for nozzles which approximate the axially-symmetric condition. The results are compared with the experimental results from both the literature and also from research which was carried out under the supervision of the author. The predictions show important flow features, which are found in the experiments and have not been recognised in the "classically" assumed internal flow. These include concentration of the axial flow near the air-core and toroidal vortices, similar to Taylor-Göertler vortices found in Taylor-Couette flow, which are visible in planes through the nozzle axis. The secondary direction of the research is in both reviewing and improving upon simplified analytical techniques which have been used to estimate the size of the air-core radius at the nozzle exit together with the values of the discharge coefficient and the cone angle of the resultant spray. A critical review is given of many of the existing analytical techniques and a new analytical theory is presented which is based upon a balance of the axial momentum across a control volume. The results of the new theory are compared with the experimental findings reported in the literature and show the need to include the swirl chamber/nozzle orifice ratio as an independent variable. Suggestions are given on how the computational methodology might be employed to determine the spray drop size for a given atomizer design and on the direction the computational work might take in order to predict a full two-phase internal flow using volume of fluid (VOF) techniques.
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Guy, Ben Michael. "The physics of the flow of concentrated suspensions." Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/29572.
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A particulate suspension under shear is a classic example of a system driven out of equilibrium. While it is possible to predict the equilibrium phase behaviour of a quiescent suspension, linking microscopic details to bulk properties under flow remains an open challenge. Our current understanding of sheared suspensions is restricted to two disparate regimes, the colloidal regime, for particle sizes d < 1 μm and the granular regime, for d > 50 μm. The physics of the industrially-relevant intermediate size regime, 1 μm ≲ d ≲ 50 μm, is unclear and has not been explored previously. In this thesis, we use conventional rheometry on a range of model spheres to develop the foundations of a predictive understanding of suspension flow across the entire size spectrum. In the first part of the thesis, we show that in repulsive particulate systems the rheology is characterised by two viscosity "branches" diverging at different volume fractions φRCP and φm, which represent states of flow with lubricated (frictionless) and frictional interactions between particles. In the intermediate size regime, there is a transition between these two branches above a critical onset stress σ* which manifests as shear thickening. This σ* is related to a barrier (invariably due to the charge or steric stabilisation) keeping particle surfaces apart. Our data are quantitatively fit by the Wyart and Cates theory for frictional thickening [1] if we assume that probability distribution of forces in the system is similar to in dry granular media. The onset stress for shear thickening is found to decrease with the inverse square of the particle size σ* / d ̄ 2 for diverse systems. We show that it is the competition between the scaling of σ*(d) and the size dependence of the entropic stress scale (~ d ̄ 3) that controls the crossover from colloidal to granular rheology with increasing size. Granular systems are "always shear thickened" under typical experimental conditions, while colloidal systems are always in a frictionless state. In the second part of the thesis, we explore the validity of the frictional framework for shear thickening. Although it quantitatively predicts our steady-state rheology, the frictional framework contradicts traditional fluid-mechanical thinking and has yet to be rigorously tested experimentally. In fact, there is a large body of literature that attributes thickening to purely hydrodynamic effects. Using dimensional analysis and simple physical arguments we examine possible physical origins for thickening and show that previously-proposed mechanisms can be subdivided into three types: two-particle hydrodynamic thickening, many-particle hydrodynamic thickening ("hydroclusters") and frictional-contact driven thickening. Many of these mechanisms can are inconsistent with the experimental two-branch phenomenology and can be disregarded. We further narrow down possible causes of thickening using the technique of flow reversal, which disentangles the relative contributions of contact and hydrodynamic forces to the viscosity. Consistent with recent simulations [2] and theory [1], we find that in each case thickening is dominated by the formation of frictional contacts and that hydrodynamic thickening, if present, is subdominant.
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Bronk, Lawrence Fernando. "Particle detector optimization via particle flow algorithms." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/44465.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.Includes bibliographical references (p. 51).
Using the the SLIC simulator software and the org.lcsim reconstruction framework package, the performance of Mat Charles' NonTrivialPfa.java PFA for several different detector variations was found by determining the mass resolution for a given detector geometry. The variations tested included the layering of the hadronic calorimeter, the radius of the calorimeter, the interaction material utilized in the hadronic calorimeter and the type of read-out used in the calorimeter. Based on the performance of the PFA for the different variations, the optimal detector specifications for use with the PFA were discovered. The optimal detector was found to use scintillator as the sensitive layer and steel as the interaction material in the hadronic calorimeter. A general trend in increased performance with more layering was also observed for the calorimeter. Also illuminated in the study was the discovery of unexpected performance for radius variations.
by Lawrence Fernando Bronk.
S.B.
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Bhandari, Sagar. "Imaging Electron Flow in Graphene." Thesis, Harvard University, 2015. http://nrs.harvard.edu/urn-3:HUL.InstRepos:17467347.
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Scanning probe techniques can be used to probe electronic properties at the nanoscale, to shed light on the physics of nanoscale devices: Graphene is of great interest for its promise in both applied (e.g. spintronics and valleytronics) and fundamental research (e.g. quantum Hall and Dirac fermions). We successfully used a cooled scanning gate microscope to image the motion of electrons along cyclotron orbits for magnetic focusing in graphene. Part of my time at Harvard was also spent incorporating a low temperature scanning capacitance setup into the existing microscope as well as building a low temperature coarse positioning system.
To image magnetic focusing of electrons in graphene, a conducting tip of a scanned probe microscope is held just above the sample surface, and an applied tip-to-sample voltage creates an image charge that is moved while the transresistance between two leads is measured. The sample is a high mobility hBN-graphene-hBN sandwich etched into hall bar geometry with two point contacts along each side. By tuning the transverse magnetic field B and electron density n in the graphene layer, we observe the first few magnetic focusing peaks. For values of B and n that correspond to the first magnetic focusing peak, we observe an image of the cyclotron orbit that extends from one point contact to the other. We also study the effects of B and n on the spatial distribution of electron trajectories as we move away from the magnetic focusing peak.
We also present the design and implementation of a cooled scanning capacitance probe that operates at liquid He temperatures to image electrons in nanodevices. In this setup, an applied sample-to-tip voltage creates an image charge that is measured by a cooled charge amplifier adjacent to the tip. The circuit is based on a low-capacitance, high-electron-mobility transistor(HEMT) (Fujitsu FHX35X). The input is a capacitance bridge formed by a low capacitance pinched-off HEMT transistor and the tip-sample capacitance. We have achieved a low noise level (0.13 e/ Hz^0.5) and high spatial resolution (100 nm) for this technique, which promises to be a useful tool to study electronic behavior in nanoscale devices.Engineering and Applied Sciences - Applied Physics
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Isa, Lucio. "Capillary flow of dense colloidal suspensions." Thesis, University of Edinburgh, 2008. http://hdl.handle.net/1842/2600.
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The purpose of this thesis is to study the flow of dense colloidal suspensions into micronsized capillaries at the particle level. Understanding the flow of complex fluids in terms of their constituents (colloids, polymers, or surfactants) poses deep fundamental challenges, and has wide applications in many industrial processes. Through the use of a novel experimental procedure we find results contrasting with the predicted bulk rheological behaviour of dense colloidal systems and propose an alternative approach based on the analogy with granular systems. Quantitative predictions which successfully explain the data are obtained. In order to obtain quantitative information on the dynamics of the system, we image the flow using a fast confocal microscope and identify the trajectories of each particle. Due to the nature of the flow, conventional techniques for locating and tracking the particles fail to yield satisfactory results. To overcome this limitation, we have developed a novel technique which allows us to successfully track the particles in strongly non-uniform flow fields (published, 2006). We focus our attention on three main aspects of the flow: concentration gradients, velocity profiles and time behaviour. We initially discuss the occurrence of concentration gradients along the flow direction and relate them to the local flow profiles. We observe high density regions where the velocity is uniform across the channel (complete plugs) and lower density regions where shear is present. The observed concentration profiles can be qualitatively explained by considering the relative ow between the solvent and the suspended particles. The flow profiles in the presence of shear consist of a plug in the centre while shear occurs localized adjacent to the channel walls, reminiscent of yield-stress fluid behaviour. However, the observed scaling of the velocity profiles with the flow rate strongly contrasts yield-stress fluid predictions. Instead, the velocity profiles can be captured by a theory of stress fluctuations originally developed for chute flow of dry granular media (published ,2007). We extend the model to our case and discuss it as a function of a series of parameters (boundary conditions, volume fraction, channel size, etc.) highlighting differences and similarities with granular media. Finally we discuss the time behaviour of complete plug flows relating it to the microscopic dynamics of the particles. At variance with dilute systems, dense systems exhibit velocity fluctuations when driven into channels by a constant pressure difference. We find that there exists a threshold value of the flow rate below which oscillations in the velocity are absent and above which their frequency scales as a power law of the flow rate. Despite quantitative predictions on this issue that are still missing, we present a microscopic description of the phenomenon highlighting the interplay between the particles and the solvent.
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Pathak, Mihir Gaurang. "Periodic flow physics in porous media of regenerative cryocoolers." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/49056.
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Pulse tube cryocoolers (PTC) are a class of rugged and high-endurance refrigeration systems that operate without moving parts at their low temperature ends, and are capable of reaching temperatures down to and below 123 K. PTCs are particularly suitable for applications in space, guiding systems, cryosurgery, medicine preservation, superconducting electronics, magnetic resonance imaging, weather observation, and liquefaction of gases. Applications of these cryocoolers span across many industries including defense, aerospace, biomedical, energy, and high tech. Among the challenges facing the PTC research community is the improvement of system efficiency, which is a direct function of the regenerator component performance. A PTC implements the theory of oscillatory compression and expansion of the gas within a closed volume to achieve desired refrigeration. An important deficiency with respect to the state of art models dealing with PTCs is the limited understanding of the hydrodynamic and thermal transport parameters associated with periodic flow of a cryogenic fluid in micro-porous structures. In view of the above, the goals of this investigation include: 1) experimentally measuring and correlating the steady and periodic flow Darcy permeability and Forchheimer’s inertial hydrodynamic parameters for available rare-Earth ErPr regenerator filler; 2) employing a CFD-assisted methodology for the unambiguous quantification of the Darcy permeability and Forchheimer’s inertial hydrodynamic parameters, based on experimentally measured steady and periodic flow pressure drops in porous structures representing recently developed regenerator fillers; and 3) performing a direct numerical pore-level investigation for steady and periodic flows in a generic porous medium in order to elucidate the flow and transport processes, and quantify the solid-fluid hydrodynamic and heat transfer parameters. These hydrodynamic resistances parameters were found to be significantly different for steady and oscillatory flows.
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Kroeger, Jens. "Diffusion and flow in growing pollen tubes." Thesis, McGill University, 2009. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=66841.
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The growth of walled cells is due to the cooperation of physical and chemical mechanisms leading to the controlled mechanical deformation of the cells. Plant cells, for example, need to expand the surface area of their cell wall in order to grow in size. This can be done in a uniform manner called diffusive growth or through tip-growth. Tip-growth is characterised by a mechanical deformation that is confined to a specific region of the cell wall, namely its tip. Tip-growth generally leads to tubular cell wall shapes and has been observed in plant cells such as pollen tubes and root hairs but also in fungal hyphae, algae and neuronal growth cones. The pollen tube is a protuberance growing on the pollen grain. As part of plant fertilisation, its purpose is to extend until it reaches the ovule of a host plant and allow for the passage of a sperm cell contained in the pollen grain. This thesis studies how physical principles direct and control the tip-growth of a pollen tube. The three particular phenomena that were addressed concern the effects of elastic forces and short-range electrical potentials on ion diffusion, actin protein aggregation and the viscous flow in growing pollen tubes. A very intriguing phenomenon, observed a decade ago, concerning growing pollen tubes is the oscillation in time of the growth rate, the concentration of signalling molecules such as calcium and the thickness of the cell wall. While the total interaction of the components of such a complex system is hard to asses, I show that a few key elements, when coupled adequately, lead to the steady oscillation similar to that observed experimentally. The growing cell was modelled as a fluid finger in the viscous regime with elastic properties depending on the material delivery to the growing region. The difference between the material delivered and the material required for growth led to the stable oscillation of the growth rate. A seLa croissance de cellules ayant une paroie cellulaire est due \'a la collaboration de processus physiques et chimiques menant \'a une déformation mécanique contrôlée de ces cellules. Les cellules végétales, par exemple, doivent augmenter la surface de leur paroie afin de croître. Ceci peut être accompli de façon uniforme appellée croissance diffuse ou par croissance apicale. La croissance apicale est caractérisée par une déformation mécanique qui est confinée à une région spécifique de la cellule, soit l'apex. La croissance apicale méne générallement à des paroies cellulaires de forme cylindrique. Elle est observée chez des cellules végétales telles que le tube pollinique ou des racines mais aussi chez des algues ou des hyphes fongiques. Le tube pollinique est une protubérance qui croît sur un grain de pollen. Lors de la fertilisation, son rôle est de s'allonger jusqu'à ce qu'il atteigne l'ovule de la plante hôte et de permettre le passage d'une cellule de sperme contenue dans le grain de pollen.Cette thése étudie les principes physiques qui contrôlent la croissance apicale d'une cellule végétale. Elle s'applique plus spécifiquement à la croissance d'un tube pollinique. Les trois systémes particuliers que nous étudierons concernent les effets élastiques et les potentiels électriques sur la diffusion d'ions, l'aggrégation de protéines et le flot visqueux dans les tubes de pollen en croissance. Un premier phénoméne, observé il y a une décennie, est l'oscillation dans le temps de la vitesse de croissance ainsi que de la concentration de calcium et de l'épaisseur de la paroie cellulaire d'un tube pollinique. Bien que les interactions entre toutes les composantes du tube pollinique sont complexes, nous avont démontré que quelles composantes, lorsque couplées de maniére adéquate, produisent l'oscillation stable observée en laboratoire.La c
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Sharma, Amil Yograj. "Arbitrary-wavelength, dynamic-strong-flow gyrokinetics." Thesis, University of Warwick, 2016. http://wrap.warwick.ac.uk/78144/.
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Gyrokinetics is a maximally optimal description of low-frequency magnetised plasma turbulence. We present a generalisation of gyrokinetic theory that allows dynamic strong flows and is valid for arbitrary-wavelength electrostatic potential perturbations in slab magnetic geometry. We obtain a substantially simpler gyrocentre Lagrangian than that of Dimits (2010a,b). We present a symplectic strong-flow generalisation of gyrokinetic field theory such that, unlike Dimits (2010a,b), our Vlasov-Poisson system is manifestly conservative as, ultimately, we obtain it as a whole, directly from our gyrocentre particle Lagrangian (Scott and Smirnov, 2010). Despite the symplectic representation of our strong-flow theory, our Poisson equation is consistent with that for weak flows (Hahm, 1988) at all wavelengths and the fluid equation obtained from our Vlasov-Poisson system is consistent with fluid theory. Again, despite our symplectic representation, we demonstrate numerical tractability by solving implicit dependences using an iterative scheme. Weak-and strong- flow code verification are performed in limits that admit analytic comparison. Simulations show strong-flow symmetry breaking of the Kelvin-Helmholtz in-stability of a shear layer and blobs that manifest as asymmetries in the growth rate and propagation, respectively, that depend on the sign of the parallel vorticity.
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Kolnaar, J. W. H. "A temperature window of reduced flow resistance in polyethylene with implications for melt flow rheology." Thesis, University of Bristol, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.357887.
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Bos, Wouter. "Passive scalar mixing in turbulent flow." Phd thesis, Ecole Centrale de Lyon, 2005. http://tel.archives-ouvertes.fr/tel-00199364.
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Le mélange d'un scalaire passif par un écoulement turbulent est étudié. D'abord, la simulation numérique directe (DNS), la simulation des grandes échelles (LES) et des arguments dimensionnels sont employés pour étudier le spectre du flux de scalaire dans une turbulence isotrope avec un gradient moyen uniforme de scalaire. Une loi d'échelle est dérivée. Cette loi conduit à des pentes du spectre variant entre -5/3 et -7/3 en zone inertielle. De premiers résultats de LES plaident en faveur d'un comportement en K^-2. Ensuite, en utilisant une fermeture en deux points (EDQNM), nous montrons qu'aux nombres de Reynolds très élevés, le spectre de flux de scalaire dans la zone intertielle se comporte en K^-7/3. Ce résultat est en accord avec l'analyse dimensionnelle classique de Lumley (1967). Aux nombres de Reynolds correspondant aux expériences de laboratoire, la fermeture conduit à des spectres plus près de K^-2. Nous montrons ensuite que le comportement en K^-2 trouvé en LES est induit par le forçage à grande échelle. La fermeture est alors appliquée au cas des écoulements homogènes cisaillés et les spectres du flux de scalaire longitudinal et transverse sont étudiés. Le spectre du flux longitudinal est trouvé proportionnelle à K^-23/9. Ce résultat est en accord avec l'expérience mais est en désaccord avec l'analyse dimensionnelle classique. Finalement, nous montrons que le lien entre la dispersion de particules et le mélange d'un scalaire permet de formuler une fermeture en deux points et un temps qui ne nécessite l'introduction d'aucune constante dans le modèle.
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Ilic, Ognjen. "Nanophotonics for tailoring the flow of thermal electromagnetic radiation." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/103227.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 117-129).
In this thesis, we explore the interaction of thermal radiation with nano-scale structures. First, we introduce the concept of radiative energy transfer between two objects of different temperatures in the near field, and theoretically argue that the radiation tunneling of evanescent surface modes can enable energy transfer that is orders of magnitude stronger than the energy transfer in the far field. Specifically, we develop a new computational approach-based on a finite-difference time-domain (FDTD) method that incorporates the Langevin approach to Brownian motion-which enables calculations of heat transfer for arbitrary geometries and materials. Second, we study the near-field heat transfer between two sheets of graphene and show that thermally excited plasmon-polariton modes can strongly mediate, enhance, and tune the energy exchange in this system. We predict maximum transfer at low doping and for plasmons in two graphene sheets in resonance, with orders-of-magnitude enhancement over the Stefan-Boltzmann law. Third, we develop the concept of a near-field thermophotovoltaic (NFTPV) system, and analyze several different implementations that use plasmonic materials as thermal emitters. In particular, we quantify the properties of an optimal near-field photovoltaic cell, argue that large plasmonic losses can-contrary to intuition-be helpful in enhancing the overall heat transfer, and propose and develop the concept of graphene as a tunable thermal emitter for a NFTPV system. Fourth, we tailor the far-field thermal emission from objects at high temperatures and experimentally demonstrate a method where the emission spectrum is controlled on the cold-side by implementing a nano-layer structure that surrounds the hot emitter and recycles unwanted emission. We find that this approach can enable lighting sources with luminous efficiencies close to the fundamental limit for lighting applications. Finally, we study opto-thermal effects in asymmetric nanoparticles. Specifically, we show that a type of metal-dielectric (Janus) particle in uniform light field exhibits a new class of stable rotational dynamics. We demonstrate (in a simulation) opto-thermal guiding of a composite asymmetric particle by switching the light beam frequency, without regard to the direction or the shape of the light beam.
by Ognjen Ilic.
Ph. D.
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Bremner, Sherry. "A granular flow model of an annular shear cell." Doctoral thesis, University of Cape Town, 2016. http://hdl.handle.net/11427/20304.
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Machinery such as an IsaMillTM used in communition to produce fine particle sizes that allow minerals to be extracted are best modelled using granular flows. A single rheological description that captures all the features of granular flows has not yet been realised, although considerable progress towards a complete theory has been made. Existing models of such horizontally stirred mills are empirical, tend to be extremely dependent on boundary conditions and do not allow for confident extrapolation beyond their window of design. As a first step to understanding the dynamics inside the IsaMillTM,a constitutive stress model of a horizontal annular shear cell is developed. This shear stress model was used in an athermal energy balance to develop a description of the power dissipation, which drives the communition purpose of the IsaMillTM. The key ingredients (velocity, shear rate and volume fraction distributions) to the granular ow model are extracted from experiments using Positron Emission Particle Tracking (PEPT), as well as Discrete Element Method (DEM) simulations. 5mm glass beads were used to fill an annulus 51mm wide. In the PEPT experiments, two different surfaces of the driving wall (the inner cylinder of the shear cell) were used, over two shearing velocities. The effect of two friction coefficients over a range of shearing wall velocities were examined in the DEM simulations. The data were examined over 3 selected radial lines and utilised to calculate the shear stress distribution and the power dissipation from the developed models. It was found that even the usually simple relations describing the dynamics within a vertical shear cell are greatly modified by changing the orientation of the rotation axis.
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Courts, Samuel Scott. "Superfluid turbulence in two-fluid flow of helium II /." The Ohio State University, 1988. http://rave.ohiolink.edu/etdc/view?acc_num=osu148759165817599.
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Hegseth, John. "Spatiotemporal patterns in flow between two independently rotating cylinders /." The Ohio State University, 1990. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487676847116532.
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Morris, Robert Christian. "Neoclassical poloidal flow damping in a tokamak." Thesis, Imperial College London, 1996. http://hdl.handle.net/10044/1/59970.
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The damping rate of the poloidal flow in a tokamak is determined in the banana regime as an initial value problem. The bounce averaged drift kinetic equation is solved analytically for early times, and on longer time scales (of the order To) a numerical solution is performed. Initial conditions are chosen for the ion distribution function t = 0) describing states with similar flows ug(t = 0), but differing structure in the pitch angle variable A = 2/x/t;^. At early times an analytical treatment shows that the damping characteristics of ug{t) depend sensitively on whether the ions responsible for the flow be close to the trapped-passing boundary. Initial decay is shown to be of the form ^ ~ A numerical treatment then confirms this early time result and extends the solution to the long term aymptotic decay, which is found to be independent of the initial preparation of the system. A spread of characteristic times are estimated for differing initial conditions and in terms of the ion-ion collision frequency tu can be written Tp % 0.2 — O.Grf,-. The previous attempts at such a calculation are criticised and, as far as possible, explained in terms of the present work. The paper by Shaing and Hirshman (1989) claims an inverse aspect ratio dependence of Tp DC and this result is found to be most compatible with the asymptotic result of the present work.
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Bartlett, Bruce. "Flow equations for hamiltonians from continuous unitary transformations." Thesis, Stellenbosch : Stellenbosch University, 2003. http://hdl.handle.net/10019.1/53428.
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Thesis (MSc)--Stellenbosch University, 2003.ENGLISH ABSTRACT: This thesis presents an overview of the flow equations recently introduced by Wegner. The little known mathematical framework is established in the initial chapter and used as a background for the entire presentation. The application of flow equations to the Foldy-Wouthuysen transformation and to the elimination of the electron-phonon coupling in a solid is reviewed. Recent flow equations approaches to the Lipkin model are examined thoroughly, paying special attention to their utility near the phase change boundary. We present more robust schemes by requiring that expectation values be flow dependent; either through a variational or self-consistent calculation. The similarity renormalization group equations recently developed by Glazek and Wilson are also reviewed. Their relationship to Wegner's flow equations is investigated through the aid of an instructive model.
AFRIKAANSE OPSOMMING: Hierdie tesis bied 'n oorsig van die vloeivergelykings soos dit onlangs deur Wegner voorgestel is. Die betreklik onbekende wiskundige raamwerk word in die eerste hoofstuk geskets en deurgans as agtergrond gebruik. 'n Oorsig word gegee van die aanwending van die vloeivergelyking vir die Foldy-Wouthuysen transformasie en die eliminering van die elektron-fonon wisselwerking in 'n vastestof. Onlangse benaderings tot die Lipkin model, deur middel van vloeivergelykings, word ook deeglik ondersoek. Besondere aandag word gegee aan hul aanwending naby fasegrense. 'n Meer stewige skema word voorgestel deur te vereis dat verwagtingswaardes vloei-afhanklik is; óf deur gevarieerde óf self-konsistente berekenings. 'n Inleiding tot die gelyksoortigheids renormerings groep vergelykings, soos onlangs ontwikkel deur Glazek en Wilson, word ook aangebied. Hulle verwantskap met die Wegner vloeivergelykings word bespreek aan die hand van 'n instruktiewe voorbeeld.
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Crawford, Alasdair James. "A Chemistry Neutral Flow Battery Performance Model Development, Validation, and Application." Thesis, Portland State University, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10096820.
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A physical model for redox flow batteries is developed to estimate performance for any chemistry using parameters such as electrolyte conductivity and kinetic rate constants. The model returns the performance as a function of flow rate, current density, and state of charge. Two different models are developed to estimate the current density distribution throughout the electrode in order to evaluate physical performance of the battery. This is done using electrochemical parameters such as conductivity and kinetic rate constant. The models are analytical in order to produce a computationally cheap algorithm that can be used in optimization routines. This allows for evaluating the economic performance of redox flow batteries, and optimization of cost. The models are validated vs data and found to accurately predict performance in a V-V system for a wide variety of operating conditions.
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Ducharme, Réjean 1970. "Capillary flow of non-Newtonian fluids." Thesis, McGill University, 1995. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=23392.
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The flow of a two-dimensional incompressible non-Newtonian fluid, showing a viscoelastic behavior, has been studied using the White-Metzner model with a phenomenological law for the viscosity, the Spriggs' truncated power-law model. Our goal was to determine if these models could generate the oscillating instabilities appearing in such fluids at very high driving force. We studied the effect of various quantities on the time-dependent numerical simulations and noticed that the mesh length was not very important for the accuracy of the results. However, the time constant modulus appearing in the White-Metzner model and the applied pressure were of paramount importance for the relaxation time of a disruptive flow.We thus showed that this model was effective only at low pressure and that without adding new aspects to the study of the flow, such as compressibility, we could not obtain any oscillating flow at high pressure. Despite this fact, exact steady-state solutions, as well as a time-dependant solution in the case of very small Reynolds number ($R to$ 0), have been given.
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Kriel, Johannes Nicolaas. "Non-perturbative flow equations from continuous unitary transformations." Thesis, Link to the online version, 2005. http://hdl.handle.net/10019/1076.
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Quinn, Brenda E. "Rossby wave, drift wave and zonal flow turbulence." Thesis, University of Warwick, 2011. http://wrap.warwick.ac.uk/51779/.
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An extensive qualitative and quantitative study of Rossby wave, drift wave and zonal flow turbulence in the Charney-Hasegawa-Mima model is presented. This includes details of two generation mechanisms of the zonal flows, evidence of the nonlocal nature of this turbulence and of the energy exchange between the small and large scales. The modulational instability study shows that for strong primary waves the most unstable modes are perpendicular to the primary wave, which corresponds to the generation of a zonal flow if the primary wave is purely meridional. For weak waves, the maximum growth occurs for off-zonal modulations that are close to being in three-wave resonance with the primary wave. Nonlinear jet pinching is observed for all nonlinearity levels but the subsequent dynamics differ between strong and weak primary waves. The jets of the former further roll up into Kármán-like vortex streets and saturate, while for the latter, the growth of the unstable mode reverses and the system oscillates between a dominant jet and a dominant primary wave. A critical level of nonlinearity is defined which separates the two regimes. Some of these characteristics are captured by truncated models. Numerical proof of the extra invariant in Rossby and drift wave turbulence is presented. While the theoretical derivations of this invariant stem from the wave kinetic equation which assumes weak wave amplitudes, it is shown to be relatively-well conserved for higher nonlinearities also. Together with the energy and enstrophy, these three invariants cascade into anisotropic sectors in the k-space as predicted by the Fjørtoft argument. The cascades are characterised by the zonostrophy pushing the energy to the zonal scales. A small scale instability forcing applied to the model has demonstrated the wellknown drift wave - zonal flow feedback loop. The drift wave turbulence is generated from this primary instability. The zonal flows are then excited by either one of the generation mechanisms, extracting energy from the drift waves as they grow. Eventually the turbulence is completely suppressed and the zonal flows saturate. The turbulence spectrum is shown to diffuse in a manner which has been mathematically predicted. The insights gained from this simple model could provide a basis for equivalent studies in more sophisticated plasma and geophysical fluid dynamics models in an effort to fully understand the zonal flow generation, the turbulent transport suppression and the zonal flow saturation processes in both the plasma and geophysical contexts.
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Klees, Ilja. "AGN outflows and the cluster cooling flow conundrum." Thesis, University of Southampton, 2011. https://eprints.soton.ac.uk/200125/.
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Temperature distributions within cores of galaxy clusters indicate gas in the core should cool down rapidly and contract. The cooling flow problem involves the hypothesis that the rapidly cooling gas should eventually condensate into unobserved molecular clouds. Active galactic nuclei (AGNs) are thought to be able to counter catastrophic cooling. This thesis provides an overview of the theory and design philosophy of hydrodynamical simulations and a hydrodynamical code called FLASH, which was created by Fryxell et al. (2000). This code is expanded with gravity and AGN outflow units to support the simulation of a pair of outflows in the intracluster medium whilst using classical fluid dynamics. The three simulation chapters involve a single parameter being varied and its impact on jet morphology and the heating process studied. The jet opening angle was found not to impact the heating process, while a study on wave heating demonstrated the presence of sound waves. The third project involved varying the jet heating power as a trade-off of thrust, while keeping the energy output constant. That proved to greatly influence the nmorphology of the backflow and the bubble inflation process. The simulations show that despite the superficial simplicity of the model, the results provide valuable insight into the physics behind an AGN outflow and the available heating mechanisms. Future challenges include the extension of the model, which would require more reliable data and hypotheses on cluster cores
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Collier, James D. "Investigating strong flow-turbulence dynamics via numerical simulations." Thesis, University of Warwick, 2016. http://wrap.warwick.ac.uk/77692/.
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In a magnetically confined fusion plasma, small scale instabilities drive the anomalous transport which determines the confinement. It is possible for the plasma in a tokamak to have a toroidal rotation, either formed spontaneously or by being given some external drive. This rotation may help improve confinement. Therefore, this thesis aims to investigate the effects of strongly rotating plasmas, with toroidal Mach number approaching unity, on the turbulence dynamics in numerical simulations. For this purpose, the global gyrokinetic PIC code ORB5 has been extended to include these strong- flow terms; retaining the background E x B drift terms typically neglected. Investigations into GAMs appearing as eigenfunctions with forms similar to the Airy function found that although the behaviour of GAMs with increasing radial wavenumber were poorly predicted, eigenfunctions of the predicted form were still found. As radial wavenumber increased the eigenfunctions became less well defined. Linear simulations with a solid-body rotation found that the frequency of the GAMs and toroidal modes with n n = 0 exhibited an increase for larger magnitude of toroidal velocity and were largely independent of direction of rotation. Further studies found that an increasing toroidal rotation initially showed a destabilization effect on the linear modes, acting against the stabilizing effects of already present gradient profile flows, before beginning to reduce the mode growth at large toroidal velocities. This behaviour was found to be largely reflected in the tilting of the mode structures caused by rotation. A stabilizing effect was also observed in collisionless non-linear simulations. The presence of a positively rotating plasma gave reductions to turbulence, but a much stronger turbulence suppression was found when the plasma was rotated in the opposite direction. It is suspected that the large ows caused by the equilibrium profile gradients give rise to some of this observed asymmetry.
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Braun, Robert, Fred Feudel, and Parvez Guzdar. "The route to chaos for a two-dimensional externally driven flow." Universität Potsdam, 1998. http://opus.kobv.de/ubp/volltexte/2007/1471/.
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We have numerically studied the bifurcations and transition to chaos in a two-dimensional fluid for varying values of the Reynolds number. These investigations have been motivated by experiments in fluids, where an array of vortices was driven by an electromotive force. In these experiments, successive changes leading to a complex motion of the vortices, due to increased forcing, have been explored [Tabeling, Perrin, and Fauve, J. Fluid Mech. 213, 511 (1990)]. We model this experiment by means of two-dimensional Navier-Stokes equations with a special external forcing, driving a linear chain of eight counter-rotating vortices, imposing stress-free boundary conditions in the vertical direction and periodic boundary conditions in the horizontal direction. As the strength of the forcing or the Reynolds number is raised, the original stationary vortex array becomes unstable and a complex sequence of bifurcations is observed. Several steady states and periodic branches and a period doubling cascade appear on the route to chaos. For increasing values of the Reynolds number, shear flow develops, for which the spatial scale is large compared to the scale of the forcing. Furthermore, we have investigated the influence of the aspect ratio of the container as well as the effect of no-slip boundary conditions at the top and bottom, on the bifurcation scenario.
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Andrew, Philip L. "Experimental and numerical investigations of the off-design flow physics in a supersonic through-flow fan cascade." Diss., This resource online, 1992. http://scholar.lib.vt.edu/theses/available/etd-07282008-134047/.
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Gingell, Peter W. "Hybrid simulations of flow bursts in magnetically confined plasmas." Thesis, University of Warwick, 2013. http://wrap.warwick.ac.uk/58230/.
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Strongly localised concentrations or depressions of plasma density and magnetic field strength (\blobs") are ubiquitous in the edge region of tokamak fusion experiments. They contribute significantly to heating and transport in that region, and therefore to overall energy confinement. The existing fusion plasma literature in this area focuses primarily on blobs sufficiently large that a uid description is appropriate. However, the blob population may include some - not necessarily easily detectable - whose characteristic lengthscales are on the order of the ion gyro-scales. This implies that a description at the uid level is unlikely to capture the full dynamics. In this Thesis, therefore, we report hybrid (particle ions, uid electrons) particle-in-cell simulations of ion gyro-scale blobs, which enable us to examine the effects of finite Larmor radius on their dynamics, evolution, and their ability to heat the near-edge plasma. We find that ion gyro-scale blobs are advected with the background flow, and develop a twin-celled vortex structure. Asymmetry then arises from finite ion Larmor radius kinetics, manifesting in the size of the internal vortices, the shape of tails forming from blob ejecta, and the growth of a Kelvin-Helmholtz instability. Small scale blobs are also found to increase ion energies more than larger blobs as a result of ion pick-up at the upstream blob-background boundary, which may result in a significant increase in plasma energy caused by a blob population that is not yet directly observable. Finally, we examine the creation of ion gyro-scale blobs using hybrid simulations of kinetic interchange and Kelvin-Helmholtz instabilities, and present statistics of the sizes of blobs created by these instabilities, and power-laws for the resulting particle displacements.
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Adams, Matthew Michael. "Magnetic and acoustic investigations of turbulent spherical Couette flow." Thesis, University of Maryland, College Park, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10160544.
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This dissertation describes experiments in spherical Couette devices, using both gas and liquid sodium. The experimental geometry is motivated by the Earth's outer core, the seat of the geodynamo, and consists of an outer spherical shell and an inner sphere, both of which can be rotated independently to drive a shear flow in the fluid lying between them. In the case of experiments with liquid sodium, we apply DC axial magnetic fields, with a dominant dipole or quadrupole component, to the system. We measure the magnetic field induced by the flow of liquid sodium using an external array of Hall effect magnetic field probes, as well as two probes inserted into the fluid volume. This gives information about possible velocity patterns present, and we extend previous work categorizing flow states, noting further information that can be extracted from the induced field measurements. The limitations due to a lack of direct velocity measurements prompted us to work on developing the technique of using acoustic modes to measure zonal flows. Using gas as the working fluid in our 60 cm diameter spherical Couette experiment, we identified acoustic modes of the container, and obtained excellent agreement with theoretical predictions. For the case of uniform rotation of the system, we compared the acoustic mode frequency splittings with theoretical predictions for solid body flow, and obtained excellent agreement. This gave us confidence in extending this work to the case of differential rotation, with a turbulent flow state. Using the measured splittings for this case, our colleagues performed an inversion to infer the pattern of zonal velocities within the flow, the first such inversion in a rotating laboratory experiment. This technique holds promise for use in liquid sodium experiments, for which zonal flow measurements have historically been challenging.
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Vale, Carla Manuel 1973. "Elliptic flow in Au+Au collisions at 200 GeV per nucleon pair." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/17843.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2004.Includes bibliographical references (p. 135-142).
The Relativistic Heavy Ion Collider (RHIC) has provided its experiments with the most energetic nucleus-nucleus collisions ever achieved in a laboratory. These collisions allow for the study of the properties of nuclear matter at very high temperature and energy density, and may uncover new forms of matter created under such conditions. This thesis presents measurements of the elliptic flow amplitude, v₂, in Au+Au collisions at RHIC's top center of mass energy of 200 GeV per nucleon pair. Elliptic flow is interesting as a probe of the dynamical evolution of the system formed in the collision. The elliptic flow dependences on transverse momentum, centrality, and pseudorapidity were measured using data collected by the PHOBOS detector during the 2001 RHIC run. The reaction plane of the collision was determined using the multiplicity detector, and the azimuthal angles of tracks reconstructed in the spectrometer were then correlated with the found reaction plane. The v₂ values grow almost linearly with transverse momentum, up to P[sub]T of approximately 1.5 GeV, saturating at about 14%. As a function of centrality, v₂ is minimum for central events, as expected from geometry, and increases up to near 7% (for 0 < [eta] < 1) at (N[sub]part) = 83. The v2 dependence on pseudorapidity was measured over the range 0 < [eta] < 1.8 for three centrality rangess: 3-15%, 15-25% and 25-50%. For all but the most central of the three centrality ranges, v₂ is seen to decrease with increasing starting already near mid-rapidity. The results, their comparison to models and interpretation are discussed.
by Carla Manuel Vale.
Ph.D.
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Kraenzel, Carl Joseph. "A superconducting memory switch based on the principles of flux-flow resistivity." Thesis, Massachusetts Institute of Technology, 1989. http://hdl.handle.net/1721.1/14530.
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Povall, Timothy Mark. "Dense granular flow in rotating drums: a computational investigation of constitutive equations." Doctoral thesis, University of Cape Town, 2018. http://hdl.handle.net/11427/29694.
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The constitutive laws of dense granular flow are investigated. Simulations of a drum, with periodic boundary conditions, rotating at varying speeds are performed. From the resulting data, kinematic and kinetic fields are extracted and used to investigate the validity of constitutive relations proposed in the literature. Two key constitutive assumptions are (a) isotropy and (b) incompressibility. The rotating drum system is found to be largely isotropic for high rotational speeds. For low rotational speeds, anisotropy is observed in the bottom part of the system, where the particles are flowing upwards. A small degree of compressibility is observed in the downward-flowing layer. The friction coefficient for the granular constitutive relations is also investigated. An empirically-derived friction law has a better fit to the data when compared to other friction laws proposed in the literature. Lastly, two scaling laws are investigated: the scaling between the scaled flow-rate (flux) and the thickness of the downward- flowing layer and the scaling between the dynamic angle of repose of the bed and the flux through the downward- flowing layer. The thickness-flux scaling is measured by interpolating the flux over a number of slices through the flowing layer, this is done in a number of different ways. The size of the measured section through the flowing layer is varied. The orientation of the slices is also varied. Also investigated is whether the total velocity or the tangential velocity produce the same scaling. The size of the section of the flowing layer significantly changes the scaling, this shows that the scaling is not constant throughout the flowing layer. The dynamic angle of repose is determined using two methods, one which is determined unambiguously as the repose angle of the ellipse fitted to the equilibrium surface and the other which is the changing angle of the tangent to the equilibrium surface or free surface. The first repose angle is found to be highly dependent on the flux even in the limit of infinite drum length, which is modelled using axial periodic boundary conditions. The second definition results in two sets of repose angles with complex behaviour that may be due to inertial effects. An instability in the system is observed, this is conjectured to be due to a frictional threshold that is breached as the rotational speed of the drum increases. Algorithms for calculating field variables and features of the charge are presented.
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Aversano, Gianmarco. "Development of physics-based reduced-order models for reacting flow applications." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLC095/document.
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L’objectif final étant de développer des modèles d’ordre réduit pour les applications de combustion, des techniques d’apprentissage automatique non supervisées et supervisées ont été testées et combinées dans les travaux de la présente thèse pour l’extraction de caractéristiques et la construction de modèles d’ordre réduit. Ainsi, l’application de techniques pilotées par les données pour la détection des caractéristiques d’ensembles de données de combustion turbulente (simulation numérique directe) a été étudiée sur deux flammes H2 / CO: une évolution spatiale (DNS1) et une jet à évolution temporelle (DNS2). Des méthodes telles que l’analyse en composantes principales (ACP), l’analyse en composantes principales locales (LPCA), la factorisation matricielle non négative (NMF) et les autoencodeurs ont été explorées à cette fin. Il a été démontré que divers facteurs pouvaient affecter les performances de ces méthodes, tels que les critères utilisés pour le centrage et la mise à l’échelle des données d’origine ou le choix du nombre de dimensions dans les approximations de rang inférieur. Un ensemble de lignes directrices a été présenté qui peut aider le processus d’identification de caractéristiques physiques significatives à partir de données de flux réactifs turbulents. Des méthodes de compression de données telles que l’analyse en composantes principales (ACP) et les variations ont été combinées à des méthodes d’interpolation telles que le krigeage, pour la construction de modèles ordonnées à prix réduits et calculables pour la prédiction de l’état d’un système de combustion dans des conditions de fonctionnement inconnues ou des combinaisons de modèles valeurs de paramètre d’entrée. La méthodologie a d’abord été testée pour la prévision des flammes 1D avec un nombre croissant de paramètres d’entrée (rapport d’équivalence, composition du carburant et température d’entrée), avec des variantes de l’approche PCA classique, à savoir PCA contrainte et PCA locale, appliquée aux cas de combustion la première fois en combinaison avec une technique d’interpolation. Les résultats positifs de l’étude ont conduit à l’application de la méthodologie proposée aux flammes 2D avec deux paramètres d’entrée, à savoir la composition du combustible et la vitesse d’entrée, qui ont donné des résultats satisfaisants. Des alternatives aux méthodes non supervisées et supervisées choisies ont également été testées sur les mêmes données 2D. L’utilisation de la factorisation matricielle non négative (FNM) pour l’approximation de bas rang a été étudiée en raison de la capacité de la méthode à représenter des données à valeur positive, ce qui permet de ne pas enfreindre des lois physiques importantes telles que la positivité des fractions de masse d’espèces chimiques et comparée à la PCA. Comme méthodes supervisées alternatives, la combinaison de l’expansion du chaos polynomial (PCE) et du Kriging et l’utilisation de réseaux de neurones artificiels (RNA) ont été testées. Les résultats des travaux susmentionnés ont ouvert la voie au développement d’un jumeau numérique d’un four à combustion à partir d’un ensemble de simulations 3D. La combinaison de PCA et de Kriging a également été utilisée dans le contexte de la quantification de l’incertitude (UQ), en particulier dans le cadre de collaboration de données lié (B2B-DC), qui a conduit à l’introduction de la procédure B2B-DC à commande réduite. Comme pour la première fois, le centre de distribution B2B a été développé en termes de variables latentes et non en termes de variables physiques originalesWith the final objective being to developreduced-order models for combustion applications,unsupervised and supervised machine learningtechniques were tested and combined in the workof the present Thesis for feature extraction and theconstruction of reduced-order models. Thus, the applicationof data-driven techniques for the detection offeatures from turbulent combustion data sets (directnumerical simulation) was investigated on two H2/COflames: a spatially-evolving (DNS1) and a temporallyevolvingjet (DNS2). Methods such as Principal ComponentAnalysis (PCA), Local Principal ComponentAnalysis (LPCA), Non-negative Matrix Factorization(NMF) and Autoencoders were explored for this purpose.It was shown that various factors could affectthe performance of these methods, such as the criteriaemployed for the centering and the scaling of theoriginal data or the choice of the number of dimensionsin the low-rank approximations. A set of guidelineswas presented that can aid the process ofidentifying meaningful physical features from turbulentreactive flows data. Data compression methods suchas Principal Component Analysis (PCA) and variationswere combined with interpolation methods suchas Kriging, for the construction of computationally affordablereduced-order models for the prediction ofthe state of a combustion system for unseen operatingconditions or combinations of model input parametervalues. The methodology was first tested forthe prediction of 1D flames with an increasing numberof input parameters (equivalence ratio, fuel compositionand inlet temperature), with variations of the classicPCA approach, namely constrained PCA and localPCA, being applied to combustion cases for the firsttime in combination with an interpolation technique.The positive outcome of the study led to the applicationof the proposed methodology to 2D flames withtwo input parameters, namely fuel composition andinlet velocity, which produced satisfactory results. Alternativesto the chosen unsupervised and supervisedmethods were also tested on the same 2D data.The use of non-negative matrix factorization (NMF) forlow-rank approximation was investigated because ofthe ability of the method to represent positive-valueddata, which helps the non-violation of important physicallaws such as positivity of chemical species massfractions, and compared to PCA. As alternative supervisedmethods, the combination of polynomial chaosexpansion (PCE) and Kriging and the use of artificialneural networks (ANNs) were tested. Results from thementioned work paved the way for the developmentof a digital twin of a combustion furnace from a setof 3D simulations. The combination of PCA and Krigingwas also employed in the context of uncertaintyquantification (UQ), specifically in the bound-to-bounddata collaboration framework (B2B-DC), which led tothe introduction of the reduced-order B2B-DC procedureas for the first time the B2B-DC was developedin terms of latent variables and not in terms of originalphysical variables
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Paraschiv, Ioana. "Shear flow stabilization of Z-pinches." abstract and full text PDF (free order & download UNR users only), 2007. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3264527.
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Dai, Yi 1964. "Study of directed flow in Au+Au collisions at 11.5 A.GeVc." Thesis, McGill University, 1998. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=35689.
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This thesis presents the systematic analysis of azimuthal distributions of identified charged particles produced in Au+Au collisions at 11.5 A·GeV/c at the AGS measured with respect to the reaction plane. A Fourier expansion is used to describe the azimuthal anisotropy in particle production. Directed flow of protons, deuterons, pi+/-, K+/- and antiprotons is presented as a function of transverse momentum for different particle rapidities and collision centralities. Directed flow of protons and that of deuterons are well described by a simple sideward moving and radially expanding thermal source model; the observed behavior of the directed flow of pi+/- indicates the interplay between shadowing and expansion; direct flow of K + and K- is observed to be anti-correlated to that of protons at low pt, implying that the role of absorption and rescattering is significant for kaons in the dense nuclear medium; the first observation of a large "antiflow" signal for antiprotons suggests that a strong annihilation process occurs in heavy-ion collisions at the AGS. The experimental results are compared to the predictions of the event generator RQMD (version 2.3) in both the mean-field and cascade modes.
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Hausner, Alejo. "Non-linear effects in pulsating pipe flow." Thesis, McGill University, 1992. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=61228.
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The present thesis considers the phenomenon of flow-rate enhancement of polymer solutions in a pipe due to pulsating pressure gradients. It presents an historical review of the problem. The unexplained experimental dependence of enhancement on pulsation frequency reported by Barnes et al is examined, as are later theoretical attempts to reproduce their results. We find that the results can be reproduced only by omitting the important inertial term. The Modified Moment Method is applied to the problem. The results confirm the predictions of other models. The enhancement is of second order in the pulsation amplitude, exhibits a maximum when the pressure gradient is varied, and declines with increasing pulsation frequency. An expansion in powers of the pulsation amplitude gives a satisfactory approximation. Less power is consumed at the same rate of flow if the pressure gradient is constant and not pulsated.
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Akbari, Mohammad Hadi. "Bluff-body flow simulations using vortex methods." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/nq55294.pdf.
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Golingo, Raymond Peter. "Formation of a sheared flow Z-pinch /." Thesis, Connect to this title online; UW restricted, 2003. http://hdl.handle.net/1773/9960.
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Matthews, Jason E. "Thermoelectric and Heat Flow Phenomena in Mesoscopic Systems." Thesis, University of Oregon, 2011. http://hdl.handle.net/1794/12108.
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xvii, 189 p. ： ill. (some col.)Low-dimensional electronic systems, systems that are restricted to single energy levels in at least one of the three spatial dimensions, have attracted considerable interest in the field of thermoelectric materials. At these scales, the ability to manipulate electronic energy levels offers a great deal of control over a device's thermopower, that is, its ability to generate a voltage due to a thermal gradient. In addition, low-dimensional devices offer increased control over phononic heat flow. Mesoscale geometry can also have a large impact on both electron and phonon dynamics. Effects such as ballistic transport in a two-dimensional electron gas structure can lead to the enhancement or attenuation of electron transmission probabilities in multi-terminal junctions. The first half of this dissertation investigates the transverse thermoelectric properties of a four-terminal ballistic junction containing a central symmetry-breaking scatterer. It is believed that the combined symmetry of the scatterer and junction is the key component to understanding non-linear and thermoelectric transport in these junctions. To this end, experimental investigations on this type of junction were carried out to demonstrate its ability to generate a transverse thermovoltage. To aid in interpreting the results, a multi-terminal scattering-matrix theory was developed that relates the junction's non-linear electronic properties to its thermoelectric properties. The possibility of a transverse thermoelectric device also motivated the first derivation of the transverse thermoelectric efficiency. This second half of this dissertation focuses on heat flow phenomena in InAs/InP heterostructure nanowires. In thermoelectric research, a phononic heat flow between thermal reservoirs is considered parasitic due to its minimal contribution to the electrical output. Recent experiments involving heterostructure nanowires have shown an unexpectedly large heat flow, which is attributed in this dissertation to an interplay between electron-phonon interaction and phononic heat flow. Using finite element modeling, the recent experimental findings have provided a means to probe the electron-phonon interaction in InAs nanowires. In the end, it is found that electron-phonon interaction is an important component in understanding heat flow at the nanoscale. This dissertation includes previously unpublished co-authored material.
Committee in charge: Dr. Richard Taylor， Chair； Dr. Heiner Linke， Advisor； Dr. David Cohen， Member； Dr. John Toner， Member； Dr. David Johnson， Outside Member
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Dauphinais, Guillaume. "Fabrication, structural relaxation, and flow in solid-state nanopores." Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=107651.
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Single solid-stat nanopores have been fabricated in free-standing layers of amorphous silicon nitride. Using a focused electron beam and a focused ion beam, diameters ranging from about 0.7 nm to a few hundred nanometerswere achieved. Structural relaxation of nanopores fabricated by focused electron beam was observed. Depending on the initial diameter and on the length of the nanopores, two distinct phases for the relaxation process were found. An experimental setup for themeasurement of pressure-driven mass flow of helium through a single nanopore was built. The conductance of nanopores with diameters ranging from 25 nm to 315 nm was measured. A semi-phenomenological model was developed and it was shown to quantitatively describe the conductance of fluid through a short cylindrical nanopore.Des nanopores ont été fabriqués dans de minces couches suspendues de nitrure de silicium amorphe. En utilisant un faisceau d'électrons focalisé et un faisceau d'ions focalisé, des diamètres entre 0.7 nm et 315 nm ont été obtenus. La relaxation struturelle de nanopores fabriqués par faisceau focalisé d'électrons a été observée. Dépendamment du diamètre inital du nanopore et de sa longueur, deux phases distinctes ont été identifiées. Un montage expérimental permettant la mesure de l'écoulement de masse d'hélium causé par l'application d'une différence de pression a été réalisé. La mesure de conductance de nanopores ayant un diamètre compris entre 25 nm et 315 nm a été effectuée. Un simple modèle phénoménologique permet de décrire quantitativement l'écoulement de gaz dans un court nanopore cylindrique.
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Datta, Sujit Sankar. "Getting Out Of A Tight Spot: Physics Of Flow Through Porous Materials." Thesis, Harvard University, 2013. http://dissertations.umi.com/gsas.harvard:11113.
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We study the physics of flow through porous materials in two different ways: by directly visualizing flow through a model three-dimensional (3D) porous medium, and by investigating the deformability of fluid-filled microcapsules having porous shells.Physics
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Franke, Arthur James. "Simulation of measuring bottom quark flow in heavy ion collisions using the CMS detector." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/40900.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.Includes bibliographical references (leaves 55-59).
In this thesis, I carried out a simulation study to characterize the measurement of bottom quark flow in relativistic Pb+Pb collisions using the Compact Muon Solenoid experiment. The Hydjet event generator is used to produce sample collision events at four centrality values between 0 and 12 fm. These events are processed using the simulation, digitization, and reconstruction modules of the CMSSW software framework. Studies of these data produce information necessary to create a flow signal simulator, and attempts are made at suggesting values for experimental cuts in PT and DCA to reduce backgrounds. Using the signal simulator, it is determined that the muon elliptic flow coefficient, v [mu]/2, varies linearly with that of open beauty, v B/2, with the same constant of proportionality applicable in all background conditions. The expected statistical uncertainty of the flow measurement, RMSv2, is shown to vary linearly with background level. Finally, RMSv2 is shown to vary with event sample size as predicted by background-to-signal ratio counting statistics.
by Arthur James Franke.
S.B.
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Varma, Mukund Madhav. "Flow of [phi] mesons in pPb collisions at the LHC with the CMS detector." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/99277.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Physics, 2015.In title on title-page, "[phi]" is the Greek letter. Cataloged from PDF version of thesis.
Includes bibliographical references (pages 71-75).
Measurements of two-particle angular correlations between unidentified charged particles and [phi]-mesons are shown over a wide pseudorapidity range ([delta] [eta] < 2) in pPb collisions. The data, collected during 2013 pPb run at a nucleon-nucleon center-ofmass energy of 5.02 TeV with the CMS detector, corresponds to a total integrated luminosity of aproximately 35nb-1. [Phi]-mesons are reconstructed via their hadronic decay channel, [phi] --> K+K-, for high multiplicity events. The observed two-particle long range correlations are quantified by the second-order Fourier coefficients (v2) as a function of transverse momentum. The v2 of [phi]-mesons is compared to that of other identified particles, to study the mass ordering and quark number scaling effects in measurements of elliptic flow.
by Mukund Madhav Varma.
S.M.
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Castiglione, James A. "Superfluid turbulence for converging and diverging heat flow in a weakly nonuniform circular channel /." The Ohio State University, 1996. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487935573772597.
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