UMD Theses and Dissertations
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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 given thesis/dissertation in DRUM.
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Item DEVELOPMENT OF TWO-POINT FOCUSED LASER DIFFERENTIAL INTERFEROMETRY FOR APPLICATIONS IN HIGH-SPEED WIND TUNNELS(2022) Ceruzzi, Andrew; Cadou, Christopher; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Focused laser differential interferometry (FLDI) and its relative two-point focused laser differential interferometry (2pFLDI) are completely non-intrusive (i.e. seedless) optical techniques for measuring density fluctuations and velocity respectively that offer high frequency response (>10MHz). Developed in the 1970s, FLDI is receiving renewed attention today for its potential usefulness in measuring turbulent fluctuations and velocity in hypersonic flows. In the technique, two focused, closely-spaced (~100microns), orthogonally-polarized beams pass through a region of interest and are subsequently combined and focused onto a photodetector. Differences in refractive index between the two focal volumes cause a phase shift, thus interference, between the beams which is measured by the detector. In this way the instrument is sensitive to the gradient in refractive index along a line between the two focal volumes perpendicular to the beams (dn/dx). Since gradients in index of refraction arise from gradients in density (in homogeneous flows), fluctuations in the FLDI signal are proportional to local fluctuations in density. If the fluctuations are due to localized eddies convecting through the FLDI measurement volume, then the cross-correlation of the FLDI signal with a that from a second FLDI instrument located a known distance downstream of the first provides a measure of convection velocity (2pFLDI). The ability to measure density fluctuations and velocity simultaneously and at the same point in the flow is critical because it enables one to relate the temporal scales measured by the instrument to the spatial scales present in the flow. In spite of the technique's age, a unified theory for the FLDI operation and sensitivity limits which is simple and easy to use does not exist so the first objective of this thesis is to develop such a theory. It does so using transfer functions that enable one to isolate the effects of focusing, beam separation, and disturbance frequency on the performance (i.e. sensitivity and spatial resolution) of the instrument. While the transfer functions have been previously proposed by others, an application of these functions which accounts for velocity variation in space (u_c(z)) and frequency (u_c(f)) is unique to this work. The theory is validated via comparison to experimental measurements in a canonical turbulent jet where the distributions of velocity and density fluctuations are well known. Measurements made using different FLDI instruments collapse when the differences between them are accounted for, indicating that the unified theory is correctly capturing the effects of instrument parameters like beam separation and beam diameter. FLDI response to the jet is also modeled by substituting the velocity distribution for a dispersion relation, u_c(f), measured by 2pFLDI. The advantage of the latter procedure is it allows for signal interpretation in flows where historical measurements are unavailable. This is demonstrated by comparing modeled FLDI response to experimental measurements in the flow downstream of a ramp in a small (6.4cm square) Mach 3 wind tunnel. The second objective of this thesis is to demonstrate 2pFLDI in other industrially-relevant flows. To this end, density fluctuations and convection velocities are measured in the near-wall flow in a 61cm square Mach 4 wind tunnel (Ludwieg tube) and in the free-stream flow of a 1.5m diameter Mach 18 tunnel. In each case, a method for estimating the spatiotemporal resolution using transfer functions is demonstrated. The spatiotemporal resolution of the instrument was not well understood prior to this work so quantifying it is an important contribution. Achieving acceptable signal/noise at Mach 18 was difficult because densities were so low. However, convection velocities of ~75-80 of the freestream velocity are measured above 200kHz in two runs. Spatiotemporal analysis suggests these measurements are the result of freestream disturbances; the first measurement of its kind in a Mach 18 flow.Item Connecting Molecular Clouds to Clustered Star Formation using Interferometry(2018) Dhabal, Arnab; Mundy, Lee G.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Stars are commonly formed in clusters in dense regions of interstellar medium called molecular clouds. In this thesis, we improve our understanding of the physics of star formation through multiple experiments involving interferometry. We use CARMA observations of filaments in Serpens and Perseus molecular clouds to study their morphology and kinematics using dense gas tracers. The observations are compared against predictions from simulations to explain how filaments form and evolve to form stars. Ammonia inversion transitions data is obtained from VLA observations of the NGC 1333 molecular cloud. From this data, we derive temperature, structural and kinematic information about the gas participating in star formation on scales from 2 parsec to 0.01 parsec, thereby connecting the large scale gas and dust structure to individual protostellar envelopes. These observations from ground-based arrays are complemented by the development of the Balloon Experimental Twin Telescope for Infra-red Interferometry (BETTII). This pioneering instrument performs Michelson interferometry along with Fourier Transform Spectroscopy, thereby providing sub-arcsecond angular resolution and spectroscopic capabilities at far-infrared wavelengths 30-100 microns. Using this capability, BETTII will study the dusty envelopes around protostars in clustered star forming regions. The instrument development is a component of the thesis with focus on the optics designing, evaluation and alignment for the completed and upcoming flights. We discuss how the optical system mitigates the challenges of phase control for such a balloon borne interferometer. Further, interferometric simulations of BETTII observations are carried out to investigate how well these observations can constrain the defining parameters of protostars.Item BETTII: A pathfinder for high angular resolution observations of star-forming regions in the far-infrared(2016) Rizzo, Maxime Jean; Mundy, Lee G; Rinehart, Stephen A; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we study clustered star formation in nearby star clusters and discuss how high angular resolution observations in the far-infrared regime could help us understand these important regions of stellar birth. We use the increased angular resolution from the FORCAST instrument on the SOFIA airborne observatory to study 10 nearby star-forming regions, and discuss the physical properties of sources in these regions that we can infer from radiative transfer modeling using these new observations. We discuss the design of BETTII, a pathfinder balloon-borne interferometer which will provide significantly better angular resolution in the far-infrared regime, and pave the way for future space-borne observatories. We elaborate on the details of BETTII's core technique, called Double-Fourier interferometry, and how to accurately compute the sensitivity of instruments which use this technique. Finally, we show our design and implementation results of the control and attitude estimation system for the BETTII payload, which poses unique challenges as an interferometer on a balloon platform.Item STUDY OF THE FEMTOSECOND DYNAMICS AND SPECTROSCOPY OF LASER IONIZED PLASMAS.(2015) Elle, Jennifer; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultra-short laser pulses are used to ionize gas in different configurations and study the plasma and ionization dynamics. The variation in non-linear index of refraction as a function of time is used to diagnose laser-plasma interactions. First, a proposed novel method to stimulate lasing in the atmosphere is studied. A few mJ pulse is used to ionize nitrogen gas in a long column without dissociating the molecular nitrogen. A 140ps laser is used to heat the resulting electron population in an attempt to generate a population inversion between the C3u and B3g states of molecular nitrogen. No evidence of lasing from this transition is observed. Next, a few mJ pulse is used to ionize xenon gas, creating Xe+ plasma. Ionization in Xe+ is observed far below the threshold predicted by multiphoton ionization theory due to resonant multiphoton ionization of collisionally excited states. To my knowledge, this is the first observation of resonant ionization involving multiple resonances. Finally, construction of an experiment to detect predicted birefringence in a relativistic laser-plasma interaction is described, with preliminary testing of diagnostics included.