Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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Now showing 1 - 6 of 6
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    The role of consistent turbulence energetics in the representation of dry and shallow convection
    (2019) New, David Andrew; Liang, Xin-Zhong; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this doctoral dissertation, the role of consistent turbulence energetics is examined in the context of sub-grid turbulence, convection, and cloud condensation parameterizations for numerical weather and climate models. The property of energetic consistency is formally defined and divided into two categories, local and non-local, and various common parameterization approaches are classified according this framework. I show theoretically that the basis of local energetic consistency is the inclusion of mean-gradient transport and buoyancy acceleration terms in the diagnostic and prognostic budget equations of all second-order statistical moments, including fluxes. Effectively, these terms account for the conversion between turbulent kinetic energy (TKE) and turbulent potential energy (TPE) under stably stratified conditions. With simple numerical experiments, I show that if local energetic consistency is not satisfied, then thermodynamic profiles cannot be correctly predicted under stably conditions, such as in the boundary layer capping inversion. I then extend the concept of energetic consistency from local turbulent mixing to non-local convective transport. I show that the popular eddy diffusivity-mass flux (EDMF) approach for unified parameterization of turbulence and convection treats the turbulent transport of turbulent energy in two parallel but inconsistent ways: advectively and diffusively. I introduce a novel parameterization approach, inspired by EDMF, that consistently partitions all second-order moments, including TKE, between convective and non-convective parts of a grid cell and show that this approach predicts significantly more realistic depths of convective boundary layers than conventional EDMF schemes. Finally, I introduce a novel method for validating this parameterization approach, based on Langragian particle tracking in large-eddy simulations.
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    Modeling buoyancy-driven instability and transport in porous media with application to geological carbon dioxide storage
    (2017) Ghorbani, Zohreh; Riaz, Amir; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Buoyancy-driven convection in porous media plays a central role in a wide range of industrial and environmental settings and has received renewed attention recently because of the role of convection in geologic carbon dioxide (CO2) sequestration-- a viable solution to mitigate climate change by reducing the concentration of atmospheric CO2. The idea is to inject CO2 into brine saturated formations, whereby a gradual dissolution of CO2 into the underlying brine forms a mixture that is denser than either fluids. The unstable stratified flow eventually results in a Rayleigh--Benard type flow, in which the formation of sinking plumes acts to mix the CO2 more thoroughly into the aquifer and increases the security of storage. Consequently, accurate prediction and characterization of the mixing process is crucial in estimating and managing storage security against leakage risks. Computational modeling of CO2 storage in subsurface formations, however, is a complex multiscale transport process because of several competing flow paths, regimes, and displacement patterns accompanied by a series of geochemical reactions, across a hierarchy of length and time scales associated with multiphase flow in porous formations. This has motivated studies of simplified system where although various features of real formations are neglected, it provides a valuable framework to investigate the underlying key processes in detail. To this end, the present work aims to improve the understanding of buoyancy-driven convection in an idealized 2D porous layer by addressing two fundamental issues that have not been investigated in the past using multiple theoretical and high resolution numerical simulation: (i) convective mixing in a vertically-layered porous media; and (ii) convective mixing in a continuously perturbed porous media. We uncover new physics, both in the dynamics of convective flow in a layered porous media as well as natural convection in a system subjected to continuous forcing. These contributions can be used as a stepping stone for modeling geological scale systems. Among the main contributions of this study is the finding that, when the porous medium is vertically-layered, thick permeability layers enhance instability compared to thin layers when heterogeneity is increased. In contrast, for thin layers the instability is weakened progressively with increasing heterogeneity to the extent that the corresponding homogeneous case, with the same density contrast, is more unstable. A resonant amplification of instability is observed within the linear regime when the dominant perturbation mode is equal to half the wavenumber of permeability variation. A weaker resonance also occurs when the dominant perturbation mode of the heterogeneous system coincides with the corresponding homogeneous system. On the other hand, substantial damping occurs when the perturbation mode is equal to the harmonic and sub-harmonic components of the permeability wavenumber. The phenomenon of such harmonic interactions influences both the onset of instability as well as the onset of convection. Of particular physical importance is a multimodal horizontal perturbation structures, in contrast to the situation for vertical permeability variation. As a consequence, the standard eigenvalue analysis can not be used. In the case of a continuously perturbed porous system, perturbations that are required to induce convection are introduced in the form a spatial variation of porosity in the system, a feature reflecting realistic geological settings. This form of perturbation results in an unconditionally unstable system for which the prescription of initial perturbation time and shape function are not needed. This is in contrast to a system which is perturbed in the conventional manner by introducing disturbances in the initial concentration. Using a reduced nonlinear method, the effect of harmonic variations of porosity in the transverse and streamwise direction on the onset time of convection and late time behavior is examined. It was found that the choice of perturbation method has a noticeable effect on the onset of convection and the subsequent nonlinear regime, in that the onset time of convection is reached more quickly in an impulsively perturbed system. Subsequently, an optimization procedure based on a Lagrange multiplier technique are utilized to find the optimal porosity structure that leads to the earliest onset time of convection. Scaling relationships for the optimal onset of convection and wavenumber are developed in terms of aquifer properties and initial perturbation magnitude.
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    Convection and Flow Boiling in Microgaps and Porous Foam Coolers
    (2007-10-05) Kim, Dae Whan; Bar-Cohen, Avram; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    An open and foam-filled microgap cooler, providing direct liquid cooling for a simulated electronic/photonic component and which eliminates the problematic thermal resistance of the commonly-used thermal interface material (TIM), is examined. The single phase heat transfer and pressure drop results of water are used to validate a detailed numerical model and, together with the convective FC-72 data, establish a baseline for microgap cooler performance. The two-phase heat transfer characteristics of FC-72 are examined at various microgap dimensions, heat fluxes, and mass fluxes and the results are projected onto a flow regime map. Infrared (IR) thermography is used to explore the two-phase characteristic of FC-72 inside the channel instantaneously. Also the single and two-phase heat transfer and pressure drop of porous metal foam which can enhance the cooling capability of low conductive fluid are studied and compared with the performance of the open channel microgap cooler in terms of volumetric heat transfer rate and required pumping power. The single-phase experimental results were in good agreement (within 10% error) with classical correlation of single-phase heat transfer coefficient and pressure drop in micro single gap channel with heat transfer coefficients as high as 23 kW/m2-K at 260 µm gap with water and 5 kW/m2-K at 110 µm gap with FC-72. Annular flow was found to dominate the two-phase behavior in the open channel yielding FC-72 heat transfer coefficients as high as 10 kW/m2-K at 110 µm gap channel. The single-phase pressure drop and heat transfer coefficient experimental results are compared with existing correlations and achieved 10 kW/m2-K of heat transfer coefficient at 95% porosity and 20PPI with water and 2.85 kW/m2-K with FC-72 at the same configuration. For the two-phase flow boiling, it is found that large pore size provides better cooling capability.
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    An analysis of convective transport, Lightning NO.sub.x production, and chemistry in midlatitude and subtropical thunderstorms
    (2006-10-18) Ott, Lesley Elaine; Dickerson, Russell R.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The impact of lightning NO.sub.x production and convective transport on tropospheric chemistry was studied in four thunderstorms observed during field projects using a 3-dimensional (3-D) cloud-scale chemical transport model (CSCTM). The dynamical evolution of each storm was simulated using a cloud-resolving model, and the output used to drive the off-line CSCTM which includes a parameterized source of lightning NO.sub.x based on observed cloud-to-ground (CG) and intracloud (IC) flash rates. Simulated mixing ratios of tracer species were compared to anvil aircraft observations to evaluate convective transport in the model. The production of NO per CG flash (P.sub.CG) was estimated based on mean observed peak current, and production per IC flash (P.sub.IC) was scaled to P.sub.CG. Different values of P.sub.IC/P.sub.CG were assumed and the results compared with in-cloud aircraft measurements to estimate the ratio most appropriate for each storm. The impact of lightning NO.sub.x on ozone and other species was examined during the storm in the CSCTM and following each storm in the convective plume using a chemistry-only version of the model which includes diffusion but without advection, and assumes clear-sky photolysis rates. New lightning parameterizations were implemented in the CSCTM. One parameterization uses flash length data, rather than flash rates, as input, and production per meter of flash channel length is estimated. A second parameterization simulates indivdual lightning flashes rather than distributing lightning NOx uniformly among a large number of gridcells to better reproduce the variability of observations. The results suggest that PIC is likely on the order of PCG and not significantly less as has been assumed in many global modeling studies. Mean values of PCG=500 moles NO and PIC=425 moles NO have been estimated from these simulations of midlatitude and subtropical continental thunderstorms. Based on the estimates of production per flash, and an assumed ratio of the number of IC to CG flashes and global flash rate, a global annual lightning NO source of 8.6 Tg N yr-1 is estimated. Based on these simulations, vertical profiles of lightning NOx mass for subtropical and midlatitude continental regimes have been computed for use in global and regional chemical transport models.
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    The Effect of Deep Convection on Temperatures in the Tropical Tropopause Layer and Its Implications to the Regulation of Tropical Lower Stratospheric Humidity
    (2005-04-19) Kim, Hyun Cheol; Dessler, Andrew E; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation focuses on the impact of deep convection on the thermal structure in the Tropical Tropopause Layer (TTL). Temperatures in this region play an important role in the regulation of water vapor, which in turn affects radiation, chemistry, and dynamics in the lower stratosphere. This dissertation includes two important conclusions concerning the regulation of temperature in the TTL. First, significant cooling near the tropical tropopause is observed during the time when active convection is occurring. A composite technique is used to relate the local temperature anomalies to the evolution of local convection. Temperature profiles are measured by the Atmospheric Infrared Sounder (AIRS) onboard the Aqua satellite, and the time evolution of local convections are determined by the National Centers for Environmental Protection / Aviation Weather Center (NCEP/AWS) half-hourly infrared global geostationary composite. The observations demonstrate that the TTL is cooled by convection, in agreement with previous observations and model simulations. By using a global data set, the variations in this convective cooling are investigated by season and region. The estimated cooling rate during active convection is - 7 K/day. This exceeds the likely contribution from cloud-top radiative cooling, suggesting turbulent mixing of deep convection plays a role in cooling the TTL. Second, height and thermal structure of the overshooting deep convection in the TTL are investigated using visible and infrared observations from the Visible and Infrared Scanner (VIRS) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. The heights of overshooting clouds are estimated from the sizes of the visible shadows that these clouds cast. The temperature information is obtained from the mid-infrared channel. From these, the lapse rate in the cloud is estimated. The result shows that the measured lapse rate of these clouds is significantly below adiabatic. Mixing between these clouds and the near-tropopause environment is the most likely explanation. As a result, these clouds will likely settle at a final altitude above the convections' initial level of neutral buoyancy.
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    Liquid sodium model of Earth's outer core
    (2004-08-27) Shew, Woodrow Lee; Lathrop, Daniel P.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Convective motions in Earth's outer core are responsible for the generation of the geomagnetic field. We present liquid sodium convection experiments in a spherical vessel, designed to model the convective state of Earth's outer core. Heat transfer, zonal fluid velocities, and properties of temperature fluctuations were measured for different rotation rates and temperature drops across the convecting sodium. The small scale fluid motion was highly turbulent, despite the fact that less than half of the total heat transfer was due to convection. Retrograde zonal velocities were measured at speeds up to 0.02 times the tangential speed of the outer wall of the vessel. Power spectra of temperature fluctuations indicate a well defined knee characterizing the convective energy input. This frequency is proportional to the ballistic velocity estimate. In the context of Earth's outer core, our observations imply a thermal Rayleigh number Ra=10^22, a convective velocity near 10^-5 m/s, and length and time scales of convective motions of 100 m and 2 days.