UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
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Item A STUDY ON CRITICAL HEAT FLUX MECHANISMS AND THE TRANSITION TO FILM BOILING(2015) Thompson, Jason Christopher; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An infrared (IR) thermometry technique was used to study the critical heat flux (CHF) and the transition to film boiling during pool boiling and submerged jet impingement. Highly resolved temporal and spatial heat transfer measurements were obtained by measuring temperature distributions on the surfaces of an IR transparent test heater with a mid-range IR camera. Measurements were obtained for the nucleate boiling regime, CHF, early transition boiling regime, and through the transition to film boiling. The local heat flux, temperature, and dryout characteristics were used to compare the submerged jet and pool boiling conditions. It was found that similar mechanisms govern CHF and the transition to film boiling. This finding supports that the hydrodynamic models are incorrect, and CHF is governed by the surface characteristics and the dynamics of the microlayer.Item Near Threshold Sediment Transport by a Forced Jet Impinging on a Mobile Sediment Bed(2015) Corfman, Kyle; Kige, Kenneth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Although sediment transport has been extensively studied in the past, flows such as rotorcraft brownout with large-scale coherent structures call many of the simplifying assumptions into question. The objective of this study is to develop a model for the prediction of sediment removal, referred to as erosion, based on independent measurements of the single-phase flow and the evolution of bedforms on the surface of a mobile sediment bed. A series of phase-resolved particle image velocimetry (PIV) flow measurements have been conducted to quantify the stress induced by an acoustically forced impinging jet, analagous to tip-vortices within the rotor wake. The threshold conditions for incipient particle motion are quantified through a series of PIV measurements of the single-phase flow at conditions found to produce quantifiable erosion of the surface. A force balance approach is used to develop a model, following the theory presented by Bagnold (1966), to predict the transport of sediment due to the stress above the theshold. A series of surface elevation measurements are analyzed to quantify the removal of sediment, for the evaluation of the predicted model. An additional series of PIV measurements are performed on a prototype bedform, modeled after the observed bedforms, to quantify the changes in the flow field caused by their developement. The proposed model is shown to provide a better prediction of the observed erosion than classical sediment transport models, especially for cases close to the threshold conditions. For higher speed cases however, the model dramatically over predicts the observed erosion. Several physcially-based explanations are provided for this kink in the trend.Item EFFECTS OF WALL PLANE TOPOLOGY ON VORTEX-WALL INTERACTIONS IN A FORCED IMPINGING JET(2011) Geiser, Jayson Spencer; Kiger, Ken; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The phenomenon of a three-dimensionally unstable vortex-ground interaction is studied, motivated by the problem of sediment suspension by vortex-wall interactions from landing rotorcraft. In the current work, the downwash of a rotorcraft is simplified using a prototype flow consisting of an acoustically forced impinging jet. The goal of the current investigation is to quantify the effects of disturbances to the ground-plane boundary layer on the three-dimensional development of the vortex ring as it interacts with the ground plane. A small radial fence is employed to perturb the natural evolution of the secondary vortex, which typically exhibits azimuthal instabilities as it is wrapped around the primary vortex. The fence is observed to localize and intensify the azimuthal development, dramatically altering the mean flow in this region and generating corresponding azimuthal variations in the turbulent near-wall stresses. Multi-plane ensemble-averaged stereo PIV is employed to obtain volumetric, phase-averaged data that are subjected to a triple decomposition to quantify the unsteady behavior resulting from the coherent and stochastic fluctuations of the impinging structures. The effects of the radial fence are examined at both a high and low Reynolds number flows (Re = 50,000 and 10,000, respectively (Γ/ν)), and the data is analyzed in the context of structures leading to significant sediment mobilization.