Browsing by Subject "frequency"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item A Comparison of Vegetation in Artificially Isolated Wetlands on West Galveston Island(2012-07-16) Wilson, AshleyThe purpose of this study was to compare vegetation systems among three artificially isolated wetlands on the west end of Galveston Island. Sample sites were identified as isolated wetlands and anthropogenic impact was observed. Wetland plant communities were identified through representative field studies using a modified quadrat method. Species composition, species diversity, evenness, cover and frequency were compared among the three sample sites. Salinity at all three sample sites remained at 0 ppt through June, July and August. Salinity increased to 10 ppt in both Lafitte?s Cove Nature Preserve and Dos Vacas Muertas Bird Sanctuary in September. No change in salinity was recorded at Isla Del Sol. At Lafitte?s Cove Nature Preserve the majority of the soil composition included Mustang-Nass. Dos Vacas Muertas Bird Sanctuary consisted of a Mustang fine sand complex while Isla Del Sol consisted of Mustang fine sand and Nass very fine sandy loam. Sampling at Lafitte?s Cove Nature Preserve produced 15 species. Dos Vacas Muertas Bird Sanctuary added 7 new species while Isla Del sol added 6 new species for a total of 28 species within the three sites. The overall plant species? richness of Lafitte?s Cove Nature Preserve and Dos Vacas Muertas Bird Sanctuary remained low. A high frequency was observed in Sesbania drummondii and Cyperus odoratus at Lafitte?s Cove Nature Preserve. Sesbania drummondii retained the highest percent cover for the site. At Dos Vacas Muertas Bird Sanctuary, Phragmites australis and Spartina patens demonstrated a high frequency as well as percent cover. Frequency was highest in Juncus roemerianus and Eleocharis geniculata at Isla Del Sol. Several plant species exhibited a high frequency while overall frequency was more evenly distributed in Isla Del Sol than the other sample sites. Percent cover was highest in Juncus roemerianus and Borrichia frutescens. Isla Del Sol had the highest species diversity and evenness of all three sample sites. Similarity in species composition was high with the coefficient for pair-wise comparisons in Isla Del Sol and Lafitte?s Cove Nature Preserve being the highest. The three sample sites shared 53% to 73% of their species. Isla Del Sol possessed 6 species that were absent from the other sample sites. Dos Vacas Muertas Bird Sanctuary contained 4 unique species while Lafitte?s Cove Nature Preserve had only 3 unique species. Dos Vacas Muertas Bird Sanctuary had the lowest index score at 10. Lafitte?s Cove Nature Preserve had the highest index score at 24 while Isla Del Sol followed close behind at 22. For the Anthropogenic Activity Index, Lafitte?s Cove Nature Preserve still retained the highest score at 14. Results for Isla Del Sol showed an index score of 13 while Dos Vacas Muertas Bird Sanctuary scored only 7. The results of this study show that although Dos Vacas Muertas Bird Sanctuary had lower levels of disturbance when compared to the other sample sites, it still experienced a lower species diversity. Isla Del Sol had the highest species diversity and evenness of the sites. Lafitte?s Cove Nature Preserve had the highest level of disturbance and maintained a low level of diversity as well. When comparing the results to historical data, a reduction in salt marsh plant species was observed. Species that are often associated with freshwater to brackish marsh wetlands have become more dominant in the sample sites.Item Experimental evaluation of wire mesh for design as a bearing damper(Texas A&M University, 2004-11-15) Choudhry, Vivek VaibhavWire mesh vibration dampers have been the subject of some very encouraging experiments at the Texas A&M Turbomachinery laboratories for the past several years and have emerged as an excellent replacement for squeeze film dampers. Their capability to provide damping for a wide range of temperatures (even cryogenic), fluid free operation and ability to perform even when soaked with lubricants makes them a suitable option as a bearing damper. Experiments were conducted to investigate the effect of design parameters like axial thickness and axial compression that influence the characteristics of wire mesh as a bearing damper. Two groups of wire mesh were tested to show that the stiffness and damping are directly proportional to the axial thickness, if all the other parameters are kept constant. Tests on four wire mesh donuts of different radial thickness showed that stiffness and damping vary inversely with radial thickness. Rigorous tests were also conducted to quantify the effects of axial compression, radial interference and displacement amplitude on stiffness and damping of the wire mesh. Another novel kind of mesh damper tested was comprised of two small segments instead of a whole donut. The results showed that wire mesh exhibited good damping characteristics even when used in small segments. Empirical expressions were developed using MathCADTM worksheets, and an existing ExcelTM design worksheet was modified to include these factors. The effect of frequency variation was also included to give a comprehensive design tool for wire mesh. A new design worksheet was developed that can predict rotordynamic coefficients for a wire mesh bearing damper having a different size as well as different installation and operational conditions.Item Fluidic, Solid-State, and Hybrid Reconfiguration Techniques in a Frequency and Polarization Reconfigurable Antenna(2014-12-16) Barrera, Joel DanielThis work presents the development of a hybrid reconfiguration technique used to achieve both frequency and polarization diversity in a 2.4 ? 2.5 GHz microstrip antenna. This hybrid solution for the first time combines current state-of-the-art fluidic and solid-state reconfiguration mechanisms in a collaborative effort. Two orthogonally-crossed and co-located narrow microstrip patches with gap discontinuities separating a central probe-fed section from the radiating slots provides the base antenna structure. The fluidic mechanisms use high strength dielectric fluids or liquid metal loaded across the gap discontinuities and the solid-state mechanisms uses readily available RF PIN and varactor diodes integrated across the gaps to enable reconfiguration. Accurate and robust circuit modeling concepts are presented to provide insight on antenna performance and loss mechanisms from each reconfiguration technique. A polarization-only reconfigurable version of this antenna utilizing dielectric fluids, RF PIN didoes, and liquid metal in separate design iterations were examined to introduce design and circuit modeling concepts and provide a first comparison between the reconfiguration techniques. While all iterations achieved good linear polarization switching, dielectric fluids and the RF PIN didoes are found to have large negative impacts on radiation performance due to ohmic losses (radiation efficiencies between 8 ? 35%). In the liquid metal iteration, ohmic losses are significantly reduced to boost radiation efficiencies near that of a tradition patch antenna (near 80%). The hybrid reconfiguration solution utilizes liquid metal and solid-state varactors for polarization and frequency diversity, respectively. Non-hybrid design iterations using only dielectric fluids and solid-state RF PIN diodes with varactors provide a comparison between all reconfiguration techniques and demonstrate the advantages of the hybrid solution. It was found that broadly variable dielectric strength fluids used as a sole reconfiguration mechanism can achieve a wide frequency tuning range of 700 MHz, maintain linear polarization switching, and have radiation efficiencies near 60%. However, the fluids must have loss tangents less than 0.02 and are currently not readily available. The RF PIN and varactor diode combination provides a realizable solution, however, suffers from excessive DC control power requirements, a limited tuning range of 100 MHz, and low radiation efficiency around 16%. The hybrid solution combines the best aspects of all subsequent design iterations to achieve a realizable frequency and polarization reconfigurable antenna with a tuning range of 263 MHz and 41.7% radiation efficiency average across reconfiguration states.Item Mathematical Modeling of Stress Fiber Reorganization Induced by Cyclic Stretch(2010-01-14) Hsu, Hui-JuArterial endothelial cells (ECs) are subjected to pulsatile strain due to pressure changes in the cardiac cycle and this may play a significant role in vascular function in health and disease. Further, ECs differentially respond to different patterns of strain. There is much evidence that cyclic uniaxial strain results in a perpendicular orientation of ECs and their stress fibers, while no such alignment occurs in response to cyclic equaibiaxial stretch. It is unclear how cells and their stress fibers determine their specific response to particular spatiotemporal changes in the matrix, however. Given that ECs located at regions in the arterial tree prone to atherogenesis are non-aglined, while ECs in relatively healthy regions are oriented perpendicular to the principal direction of cyclic stretch, it is important to understand the mechanisms which regulate stretch-induced stress fiber alignment. The focus of this thesis was to develop realistic models to describe the dynamic changes in the organization of stress fibers in response to diverse spatiotemporal patterns of stretch. The model is based on the premise that stress fibers are pre-stressed at a ?homeostatic? level so that stress fibers are extended beyond their unloaded lengths, and that perturbation in stress fiber length from the homeostatic level destabilizes the stress fibers. A deterministic model described experimentally measured time courses of stress fiber reorientation perpendicular to the direction of cyclic uniaxial stretch, as well as the lack of alignment in response to equibiaxial stretch. In the case of cyclic simple elongation with transverse matrix contraction, stress fibers oriented in the direction of least perturbation in stretch. Model analysis indicated the need for a time-dependent stress fiber mechanical property, however. Thus, a stochastic model was developed that incorporated the concept that stress fibers tend to self-adjust to an equilibrium level of extension when they are perturbed from their unload lengths with the turnover of stress fibers. The stochastic model successfully described experimentally measured time courses of stress fiber reorganization over a range of frequencies. At a frequency of 1 Hz, stress fibers predominantly oriented perpendicular to stretch, while at 0.1 Hz the extent of stress fiber alignment was markedly reduced and at 0.01 Hz there was no alignment at all. Both the deterministic and stochastic models accurately described the relationship between stretch magnitude and the extent of stress fiber alignment in endothelial cells subjected to cyclic uniaxial stretch. Parameter sensitivity analyses for each model were used to demonstrate the effects of each parameter on the characteristics of the system response. In summary, the mathematical models were capable of describing stress fiber reorganization in response to diverse temporal and spatial patterns of stretch. These models provide a theoretical framework to elucidate the mechanisms by which adherent cells sense the characteristics of matrix deformation and describe a mechanism by which the cells can then adapt to such deformations to maintain mechanical homeostasis.