28,385 research outputs found
Extended Defects in the Potts-Percolation Model of a Solid: Renormalization Group and Monte Carlo Analysis
We extend the model of a 2 solid to include a line of defects. Neighboring
atoms on the defect line are connected by ?springs? of different strength and
different cohesive energy with respect to the rest of the system. Using the
Migdal-Kadanoff renormalization group we show that the elastic energy is an
irrelevant field at the bulk critical point. For zero elastic energy this model
reduces to the Potts model. By using Monte Carlo simulations of the 3- and
4-state Potts model on a square lattice with a line of defects, we confirm the
renormalization-group prediction that for a defect interaction larger than the
bulk interaction the order parameter of the defect line changes discontinuously
while the defect energy varies continuously as a function of temperature at the
bulk critical temperature.Comment: 13 figures, 17 page
Ground-water resources data of Charlotte, DeSoto, and Hardee Counties, Florida
Charlotte, De Soto, and Hardee counties are east-southeast of
Tampa in west-central peninsular Florida, figure 1. In order to
plan the future water-resource development of the area, information
about the water resources is needed. To meet this need, the Water
Resources Division of the U.S. Geological Survey, in cooperation
with the Peace River Basin Board of the Southwest Florida Water
Management District as part of the statewide cooperative program
with the Division of Geology, Florida Board of Conservation, began
a continuing hydrologic data collection program in July, 1963, as
an initial step in the investigation and evaluation of the groundwater
resources of Hardee and De Soto counties. A similar hydrologic
data program commenced in Charlotte County in July, 1964.
Previous work in Hardee and De Soto counties included a
one year reconnaissance by the Division of Water Resources and
Conservation, Florida Board of Conservation, which concluded in
June, 1963, and resulted in a hydrologic report (Woodard, 1964).
As an outgrowth of the hydrologic data program, a Map Series
report portraying the chemical character of water in the Floridan
aquifer in the southern Peace River basin was prepared in 1967
(Kaufman and Dion).
The data contained herein constitute the basis for the Map
Series report. Additional selected data, including records of wells
and chemical analyses,, on the ground-water resources of the three
county area are also included and are published to make the data
available.
(Document has 28 pages.
Stable Direct Adaptive Control of Linear Infinite-dimensional Systems Using a Command Generator Tracker Approach
A command generator tracker approach to model following contol of linear distributed parameter systems (DPS) whose dynamics are described on infinite dimensional Hilbert spaces is presented. This method generates finite dimensional controllers capable of exponentially stable tracking of the reference trajectories when certain ideal trajectories are known to exist for the open loop DPS; we present conditions for the existence of these ideal trajectories. An adaptive version of this type of controller is also presented and shown to achieve (in some cases, asymptotically) stable finite dimensional control of the infinite dimensional DPS
Cyclic structural analyses of anisotropic turbine blades for reusable space propulsion systems
Turbine blades for reusable space propulsion systems are subject to severe thermomechanical loading cycles that result in large inelastic strains and very short lives. These components require the use of anisotropic high-temperature alloys to meet the safety and durability requirements of such systems. To assess the effects on blade life of material anisotropy, cyclic structural analyses are being performed for the first stage high-pressure fuel turbopump blade of the space shuttle main engine. The blade alloy is directionally solidified MAR-M 246 alloy. The analyses are based on a typical test stand engine cycle. Stress-strain histories at the airfoil critical location are computed using the MARC nonlinear finite-element computer code. The MARC solutions are compared to cyclic response predictions from a simplified structural analysis procedure developed at the NASA Lewis Research Center
Simplified cyclic structural analyses of SSME turbine blades
Anisotropic high-temperature alloys are used to meet the safety and durability requirements of turbine blades for high-pressure turbopumps in reusable space propulsion systems. The applicability to anisotropic components of a simplified inelastic structural analysis procedure developed at the NASA Lewis Research Center is assessed. The procedure uses as input the history of the total strain at the critical crack initiation location computed from elastic finite-element analyses. Cyclic heat transfer and structural analyses are performed for the first stage high-pressure fuel turbopump blade of the space shuttle main engine. The blade alloy is directionally solidified MAR-M 246 (nickel base). The analyses are based on a typical test stand engine cycle. Stress-strain histories for the airfoil critical location are computed using both the MARC nonlinear finite-element computer code and the simplified procedure. Additional cases are analyzed in which the material yield strength is arbitrarily reduced to increase the plastic strains and, therefore, the severity of the problem. Good agreement is shown between the predicted stress-strain solutions from the two methods. The simplified analysis uses about 0.02 percent (5 percent with the required elastic finite-element analyses) of the CPU time used by the nonlinear finite element analysis
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