A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Response of hypersonic boundary-layer disturbances to compression and expansion corners(2021) Butler, Cameron Scott; Laurence, Stuart; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An experimental campaign was conducted at the University of Maryland - College Park to examine the impact of abrupt changes in surface geometry on hypersonic boundary-layer instability waves. A model consisting of a 5-degree conical forebody was selected to encourage the dominance of second-mode wavepackets upstream of the interaction region. Interchangeable afterbody attachments corresponding to flow deflections of -5-degree to +15-degree in 5-degree increments were considered. The adverse pressure gradient imposed by the +10-degree and +15-degree configurations caused the boundary layer to separate upstream, creating a region of recirculating flow. High-speed schlieren (440-822 kHz) was employed as the primary means of flow interrogation, with supplemental surface measurements provided by PCB132B38 pressure transducers. A lens calibration was applied to the images to provide quantitative fluctuations in density gradient. The high frame rate made possible the use of spectral analysis techniques throughout the entire field of view. This analysis reveals complex growth and decay trends for incoming second-mode disturbances. Additional, low-frequency content is generated by the deflected configurations. This is most pronounced for the separated cases where distinct, shear-generated disturbances are observed. Spectral proper orthogonal decomposition (SPOD) is demonstrated as a powerful tool for resolving the flow structures tied to amplifying frequencies. Nonlinear interactions are probed through bispectral analysis. Resonance of low-frequency structures is found to play a large role in nonlinear energy transfer downstream of the compression corners, particularly for the separated cases. Concave streamline curvature appears to result in concentrated regions of increased nonlinearity. These nonlinear interactions are shown to be spatially correlated with coherent flow structures resolved through SPOD. Finally, a limited computational study is carried out to demonstrate the ability of linear stability theory and the parabolized stability equations to reproduce experimental results obtained for the +10-degree extension. The development of the second-mode and shear-generated disturbances resolved by the computational analysis shows excellent agreement with the experimental results.Item Parallel Algorithms for Burrows-Wheeler Compression and Decompression(2012-11-12) Edwards, James A.; Vishkin, UziWe present work-optimal PRAM algorithms for Burrows-Wheeler compression and decompression of strings over a constant alphabet. For a string of length n, the depth of the compression algorithm is O(log2 n), and the depth of the the corresponding decompression algorithm is O(log n). These appear to be the first polylogarithmic-time work-optimal parallel algorithms for any standard lossless compression scheme. The algorithms for the individual stages of compression and decompression may also be of independent interest: 1. a novel O(log n)-time, O(n)-work PRAM algorithm for Huffman decoding; 2. original insights into the stages of the BW compression and decompression problems, bringing out parallelism that was not readily apparent, allowing them to be mapped to elementary parallel routines that have O(log n)-time, O(n)-work solutions, such as: (i) prefix-sums problems with an appropriately-defined associative binary operator for several stages, and (ii) list ranking for the final stage of decompression.Item A Viscoelastoplastic Continuum Damage Model for the Compressive Behavior of Asphalt Concrete(2006-10-23) Gibson, Nelson Harold; Schwartz, Charles W.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mechanistic performance prediction of asphalt concrete pavements has been a goal for the pavement industry for some time. A comprehensive material model is essential for such predictions. This dissertation illustrates the development, calibration and validation of a comprehensive constitutive material model for asphalt concrete in unconfined and confined compression. A continuum damage-based viscoelastic model is extended with viscoplasticity. Thermodynamic principles, an elastic-viscoelastic correspondence principle and internal state variables quantify degradation by accounting for linear viscoelasticity and any nonlinear viscoelasticity with cumulative damage. Viscoplastic effects are addressed separately. Two distinctly different strain-hardening viscoplastic models were investigated. A more capable multiaxial model with primary-secondary hardening improved upon the original uniaxial. These characteristics enable the whole model to decompose total strain into individual response components of viscoelasticity, viscoplasticity and damage. Separate laboratory tests were required to measure and calibrate the individual response components. The calibration tests include small strain dynamic modulus tests for undamaged viscoelastic properties, cyclic creep and recovery tests for viscoplastic properties, and constant rate of strain tests for damage properties. All tests were performed at appropriate temperatures and loading rates. An extensive set of validation tests was used to confirm each model, which were very different from the calibration conditions to evaluate the models' capabilities. The predictions at these different conditions indicate that the comprehensive model can realistically simulate a wide range of asphalt concrete behavior. Recommendations are given based on lessons learned in the laboratory experiments and analyses of the data generated.