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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Item Experimental Investigations and Scaling Analyses of Whirling Flames(2020) Hariharan, Sriram Bharath; Gollner, Michael J; Oran, Elaine S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Swirling flows are ubiquitous in nature, occurring over a large range of length scales -- on the order of many tens-of-thousands of kilometers in the case of Saturn's hexagonal polar vortex, to just a few centimeters in dandelion flight. Most instances of swirling flow involve momenta competing in two different directions, axial and azimuthal. Whirling flames (also known as fire whirls) occur at the intersection of vortical flow fields and buoyant, reactive plumes, and they represent a general class of flows that may be considered slender vortices involving axial momentum from heat-release and tangential momentum from air entrainment. In this work, two previously unexplored characteristics of whirling flames are considered over a wide range of scales, spanning three orders of magnitude in length and four orders in heat-release rate. First, emissions of particulate matter (PM) from fire whirls (FW) were measured and compared to those from free-buoyant pool fires (PF). For different pool diameters and fuels, FWs showed higher burning rate and fuel-consumption efficiency, but lower PM-emission rate, leading to lower PM-emission factors. The lower PM emissions from FWs is attributed to a feedback cycle between higher oxygen consumption from improved entrainment, higher average temperatures, increased heat feedback to the fuel pool, which in turn increases burning rate and entrainment. A scaling analysis showed that the PM emission factor decreased linearly with the ratio of inverse Rossby number to nondimensional heat-release rate. Second, the structure of the blue whirl (BW), a soot-free regime, was investigated using dimensional analysis and non-intrusive optical diagnostics. Experimental data of heat-release rates and circulation for BWs and FWs from the literature were used to define the nondimensional equivalents of buoyant and azimuthal momenta. The combinations of these parameters showed that FWs primarily formed in a buoyancy-dominated regime, and that a circulation-dominated regime was required for BW formation, corroborating hypotheses that the transition was caused by the bubble mode of vortex breakdown, resulting in the formation of a recirculation zone. Finally, OH- and PAH-PLIF, OH* and CH* chemiluminescence suggest a triple-flame structure anchored at the blue ring region of the BW, with the rich branch formed by the lower blue cone, and the lean branch by the upper purple haze. These results show that the mixing process occurs upstream of the conical region and that the recirculation zone is comprised of combustion products.Item Investigation into the Effects of Aeolian Scaling Parameters on Sediment Mobilization below a Hovering Rotor(2011) Baharani, Ajay Kumar; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flow visualization and particle image velocimetry (PIV) experiments were conducted on a small-scale rotor hovering over a ground plane covered with a mobile sediment bed to help understand the effects of certain selected scaling parameters on the processes of sediment mobilization, entrainment, and uplift as induced by the rotor. Flow visualization using high-speed videography was used to study the rotor flow structures, their evolution in the rotor wake, and their interaction with the ground plane. Time-resolved PIV measurements of the rotor wake flow at the sediment bed quantified the flow velocities where most of the sediment mobilization was observed to occur. Dual-phase PIV experiments were conducted using ten different sediment samples of known characteristics to vary the values of five of the similarity parameters: 1. Particle diameter-to-rotor radius ratio, 2. Particle-to-fluid density ratio, 3. Ratio of characteristic flow (or wind) speed to particle terminal speed, 4. Densimetric Froude number, and 5. Threshold friction velocity ratio. The particle-to-fluid density ratio was shown to have the greatest effect on the resulting two-phase flow, followed by the threshold friction velocity ratio. The flow was also sensitive to changes in the particle diameter-to-rotor radius ratio. Changes in the densimetric Froude number and ratio of the characteristic flow speed to particle terminal speed also showed good correlations to observations of the quantity of uplifted particles. The effects of the passage of the tip vortex near the bed was shown to increase the shear stresses on the bed, which was observed to be closely correlated to an increase in the quantity of entrained sediment particles if the threshold conditions for particle mobility was exceeded. The observations and results were used to make recommendations regarding scaling on dual-phase experiments to better simulate the problem of rotorcraft brownout in the laboratory environment.Item An Empirical Investigation of Unscalable Components in Scaling Models(2009) Braaten, Kristine Norene; Dayton, C. M.; Measurement, Statistics and Evaluation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Guttman (1947) developed a scaling method in which the items measuring an attribute can be ordered according to the strength of the attribute. The Guttman scaling model assumes that every member of the population belongs to a scale type and does not allow for response errors. The Proctor (1970) and the intrusion-omission (Dayton and Macready, 1976) models introduced the notion that observed response patterns deviate from Guttman scale types because of response error. The Goodman (1975) model posited that part of the population is intrinsically unscalable. The extended Proctor and intrusion-omission (Dayton and Macready, 1980) models, commonly called extended Goodman models, include both response error and an intrinsically unscalable class (IUC). An alternative approach to the Goodman and extended Goodman models is the two-point mixture index of fit developed by Rudas, Clogg, and Lindsay (1994). The index, pi-star, is a descriptive measure used to assess fit when the data can be summarized in a contingency table for a hypothesized model. It is defined as the smallest proportion of cases that must be deleted from the observed frequency table to result in a perfect fit for the postulated model. In addition to contingency tables, pi-star can be applied to latent class models, including scaling models for dichotomous data. This study investigates the unscalable components in the extended Goodman models and the two-point mixture where the hypothesized model is the Proctor or intrusion-omission model. The question of interest is whether the index of fit associated with the Proctor or intrusion-omission model provides a potential alternative to the IUC proportion for the extended Proctor or intrusion-omission model, or in other words, whether or not pi-star and the IUC proportion are comparable. Simulation results in general did not support the notion that pi-star and the IUC proportion are comparable. Six-variable extended models outperformed their respective two-point mixture models with regard to the IUC proportion across almost every combination of condition levels. This is also true for the four-variable case except the pi-star models showed overall better performance when the true IUC proportion is small. A real data application illustrates the use of the models studied.