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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Modelling including testing of cohesionless soils using disturbed state concept.

Armaleh, Sonia Hanna January 1990 (has links)
A powerful and general concept, the DSC concept, that can capture a wide spectrum of behavior of geologic materials is developed in this dissertation. Factors such as nonassociativeness, induced anisotropy and damage are included in the concept as disturbances with respect to two reference states. One reference state is the intact state where the material is assumed to be associative initially isotropic and hardening isotropically. It is modelled by the basic hierarchical model δₒ. Its hardening parameters are modified to include the influence of relative density Dᵣ and confinements, σₒ. The other reference state is the fully disturbed state which is assumed to be the critical state. In this state the material is assumed to experience no change in void ratio under further shearing. The average response is expressed in terms of the responses corresponding to the reference states through a disturbance function D. The disturbance function parameters have been found to be dependent on Dᵣ and σₒ. Comprehensive laboratory tests have been performed on Leighton Buzzard sand covering a wide range of relative densities and confinements. Some of these tests were used in the formulation of the DSC concept to incorporate the effect of Dᵣ and σₒ into the model parameters. Verification of the new model is performed with respect to the observed behavior of loose and dense sand. Verification was done with respect to laboratory tests that were used and the ones that were not used in the development of the model. The proposed model provides overall highly satisfactory predictions of the observed behavior for the entire range of loose to dense sand. It is concluded that the DSC concept along with the new contributions of this research which allow incorporation of the entire range of loose to dense behavior can provide a powerful and general approach for constitutive modelling of geomaterials.
22

Analysis of parameters for evaluation of canopy and aerodynamic resistances over turfgrass.

Guerra, Antonio Fernando. January 1990 (has links)
The estimation of the surface roughness parameters and the choice of the non-dimensional gradient functions for stability correction are important steps in characterizing the transfer of momentum, heat, and water vapor over vegetated surfaces. The analysis of hypothetical and experimental wind profiles indicated that the zero-plane displacement (d(m)) is an unnecessary parameter in the log-wind profile model. The inclusion of d(m) in the model causes a drastic underestimation of the roughness length for momentum (z(0m)). The analysis of data collected over bermudagrass indicated that z(0m) is virtually constant at wind speeds larger than about 2.5 m/s but increases at lower wind speeds. The roughness length for heat (z(0h)) was found to be about 1/7.6 of z(0m). The calculation of the roughness length for water vapor (z(0v)) was not possible because there is no practical method to measure the specific humidity at the surface (q(s)). The calculated canopy resistances from the humidity profiles (assuming z(0h) = z(0v)) were variable over time. Minimum canopy resistance was about 78 s/m which produced a minimum stomatal resistance of about 360 s/m. The fitting of the non-dimensional gradient functions for momentum (φ(m)) and for water vapor (φᵥ) for unstable conditions, and φ(m) for stable conditions agreed very well with regression lines from the literature. A large scatter was apparent in the measured values of non-dimensional gradient function for heat (φ(h)) under unstable conditions. Under stable conditions large scatter was found in the measured values of φ(h) and φᵥ. The results seem to indicate that φ(h) is equal to φᵥ but different from φ(m). Further research is needed to determine the roughness lengths for various crops without the inclusion of d(m) in the log-wind profile model. The effect of wind speeds on z(0m) should also be studied more carefully. Also, more research is needed in order to better characterize the fluxes of heat and water vapor under stable atmospheric conditions.
23

Optimization schemes for queueing networks with applications to flexible manufacturing systems.

Krisht, Ali Hussein January 1990 (has links)
Product-form queueing networks have been useful for modeling complex systems such as flexible manufacturing systems and computer systems. While the literature is rich with queueing models, little attention has been paid to the use of these models in optimization schemes. This dissertation addresses the optimal design of complex systems in conjunction with closed queueing network theory. The overall plan is as follows: Product-form queueing network models are used to evaluate system "performance measures" for a given setting of system "decision parameters". The performance measures are useful in the computation of system cost functions and/or their sensitivities with respect to decision parameters. Optimization algorithms are applied in order to find the set of decision parameter values which optimize performance measures and/or minimize the cost of the system. Typical performance measures are the throughput (production rate) and average queue lengths at individual nodes of the system. Sensitivities of performance measures with respect to the decision parameters are derived in closed-form. These sensitivities are used to study the concavity (convexity) properties of performance measures. Both the concavity properties and the sensitivities of performance measures are then utilized in the formulation and solution procedures of the optimization models. Decision variables for the design and operation of queueing systems include service rates, routing of jobs, number of servers, and level of work-in-process. Models with a single decision variable type, such as service rates, are considered first. Hybrid models which include several types of decision variables such as service rates and work-in-process levels are then addressed. Constraints include meeting production goals, capital budgeting, and bounds on decision variables. The optimization models are discussed and solved to optimality. Numerical examples are provided and results are analysed.
24

Scan parameter optimization and a temperature controller for scanned focussed ultrasound hyperthermia: A theoretical and experimental study.

Lin, Win-Li. January 1990 (has links)
Maintenance of the treatment temperatures at their target levels in the face of disturbances, a uniform temperature distribution within the treatment region, an acceptable temperature rise outside that volume, a fast temperature rise, and stability are desirable characteristics of an optimal hyperthermia treatment control system. Since scanned, focussed ultrasound systems (SFUS) have a great deal of flexibility it is necessary to use such a feedback system in shaping the power field during hyperthermia treatments. For best use of such a system many complicated, interacting decisions must be made to obtain an optimal hyperthermia treatment. This dissertation studies this optimization problem using a simulation program which searches for the optimal scan parameters, and presents a PID plus bang-bang feedback control system which gives a suitable power distribution to meet the above requirements for this hyperthermia system. Several objective functions were studied and compared based on temperature criteria. An extensive objective function study has been done in order to determine; the best formulation for that objection function, the characteristics of that objective function near the optimal operating point, the effects of the scan parameters on that objective function, and the domain of acceptable initial guess points for obtaining a globally optimal solution. A further comprehensive study of the optimal temperature distributions attainable with single and multiple circular scans of a tumor was done. The results show that the optimal scan parameter configuration will allow this SFUS to produce a close to ideal treatment temperature distribution for a wide variety of clinically relevant conditions. To further study the variation of the temporal and spatial blood perfusion, a controller was used to obtain a more suitable power to meet the treatment needs. Both the simulation results and experimental animal results show that the controlled region can be rapidly heated to the target temperature with a small overshoot and maintained at that level in the face of disturbances. In vitro dog kidney model and in vivo dog thigh experiments show that the controller works well in practice, and verify that it can compensate for spatial and temporal blood perfusion variations. As shown in both these experiments and in simulations, the controller can be used for controlling a single temperature or multiple temperature points simultaneously, thus allowing relatively uniform temperature fields to be created.
25

Integration of operating room monitors for development of a smart alarm system.

Navabi-Shirazi, Mohammad Jafar. January 1990 (has links)
A computer based system was designed and used to collect physiologic and respiratory data (13 variables and 3 waveforms) from six routinely used operating room monitors. 23 hours of data were collected during 20 general surgery cases (ASA III patients). Part of the data were used to design and implement an integrated monitor with intelligent alarm capability. The system used a rule based approach to reduce false alarms and artificial neural networks (ANN) for classification of physiological waveforms. The integrated monitor was able to correctly identify 13 of 17 intubations which resulted in a 42% reduction in low end-tidal-CO₂ false alarms. False heart rate alarms were reduced to 2.6% of total alarms using multi-variable analysis and rate of change limits. A combination of ANN's and an edge detection filter was used to classify CO₂ waveforms into spontaneous, mechanical, and mechanical with spontaneous breathing attempts. The edge detection algorithm was able to detect 171 of 182 breaths. The ANN's properly classified 65 of 67 mechanical, 47 of 71 spontaneous, and 37 of 44 mechanical breaths with spontaneous breathing attempts. Another ANN was used for detection of elevated and depressed ST segments in the ECG signal. All ST segment elevations and depressions of 0.1 mV were correctly identified. An attempt was made to use ANN's to classify ECG waveforms according to anesthetic levels. However, the back-propagation algorithm used to train the network did not converge perhaps due to the variety of drugs used in the different cases. The system met our goals of providing an integrated operating room monitor with intelligent alarm capability. The system significantly reduced false heart rate alarms, detected intubation and classified ECG and CO₂ waveforms.
26

High-temperature removal of metal vapors by solid sorbents.

Uberoi, Mohit. January 1990 (has links)
Emissions of metal vapor compounds during incineration and combustion is becoming an increasingly important problem. The kinetics and mechanism of high temperature removal of various metal vapors by solid sorbents has been investigated in this study. The kinetics experiments were performed in a high temperature microbalance reactor system under simulated flue gas atmosphere. Scanning electron microscopy, X-ray diffraction analysis, atomic absorption/emission spectrophotometry, Energy dispersive X-ray analysis, mercury porosimetry, and BET surface area analysis were used for characterization of the fresh and reacted sorbents. The results show that the process of metal vapor capture is not just physical condensation, but rather a complex combination of various chemical and physical processes. There are some similarities in the sorption process. For all the sorbents the rate of metal vapor sorption decreases with time and there is a final limit beyond which no more metal vapor gets captured. However, there are differences in the rate and reaction mechanism of metal vapor removal. Kaolinite and bauxite are suitable sorbents for lead and cadmium capture. The melting point of the lead aluminosilicate product formed after reaction of lead chloride with kaolinite and bauxite has a low melting point. Therefore, these sorbents are more suitable for downstream fixed bed removal of lead compounds. Removal of cadmium by bauxite occurs due to chemical reaction to form a cadmium aluminum silicate and a cadmium aluminate. Removal of cadmium by kaolinite occurs due to the formation of only the cadmium aluminosilicate. The final products of cadmium sorption have a higher water solubility as compared to that of the corresponding products for lead. Chlorine is not retained by the sorbents during the sorption process. Kaolinite, bauxite and emathlite are suitable sorbents for removal of alkali compounds. In adsorbing alkali chloride vapors, kaolinite and emathlite release all the chlorine back to the gas phase while bauxite retains some of the chlorine. Moreover, the products of reaction with emathlite have a melting point significantly lower than those for kaolinite and bauxite. At lower alkali concentrations, NaCl reacts irreversibly with kaolinite to form a sodium aluminosilicate product. When the local metal vapor concentration in the sorbent pores becomes higher than the saturation concentration for condensation, the metal vapor physically condenses in the sorbent pores and may subsequently react with the solid. The theoretical models developed were used to extract kinetic parameters from experimental data and for parametric studies. The kinetic data obtained can be used in design of practical metal removal systems.
27

In-situ tests of the hydraulic performance of grout borehole seals.

Greer, William Bryan. January 1990 (has links)
Three tests are proposed for determining the hydraulic properties of in-situ borehole seals. Two consist of monitoring the rate of injection of water at constant pressure into an injection zone at one end of a seal and monitoring the collection rate or rate of flow out of a free-draining collection zone at the other end. The third test is performed by shutting in the collection zone and monitoring the buildup in hydraulic head. One-dimensional and axisymmetric three-dimensional flow models are presented for analyzing test results. In the one-dimensional models, the seal is assumed to be a homogeneous and isotropic porous medium. In the axisymmetric models, the seal and surrounding rock mass are taken as homogeneous and isotropic porous media. The equation for saturated, confined ground-water flow is assumed to apply. The hydraulic properties of the seal are expressed by its hydraulic conductivity and specific storage. In the axisymmetric models, the conductivity and specific storage of the rock mass are included in the formulation. Closed-form solutions are presented for the analysis of tests using the one-dimensional models. Analysis with the axisymmetric models is numerical using an available computer code for ground-water flow. The code is used to examine the effects of variations in hydraulic parameters on the measured quantities in the tests (i.e. flow rates or head) and to compare the one-dimensional and axisymmetric models. Methods are presented for obtaining the hydraulic properties of the seal and/or rock mass by analysis of test results. A fourth test, a tracer travel-time test, is presented as a means for detecting the existence of a high-velocity flow path through or around the seal. The test methods are applied to cement grout borehole seals from 10 to 36 cm in length and 10 cm in diameter in two rock types, a recrystallized limestone and a dense basalt.
28

A fully coupled Monte Carlo/discrete ordinates solution to the neutron transport equation.

Baker, Randal Scott. January 1990 (has links)
The neutron transport equation is solved by a hybrid method that iteratively couples regions where deterministic (S(N)) and stochastic (Monte Carlo) methods are applied. Unlike previous hybrid methods, the Monte Carlo and S(N) regions are fully coupled in the sense that no assumption is made about geometrical separation or decoupling. The hybrid method provides a new means of solving problems involving both optically thick and optically thin regions that neither Monte Carlo nor S(N) is well suited for by themselves. The fully coupled Monte Carlo/S(N) technique consists of defining spatial and/or energy regions of a problem in which either a Monte Carlo calculation or an S(N) calculation is to be performed. The Monte Carlo region may comprise the entire spatial region (with vacuum boundary conditions) for selected energy groups, or may consist of a rectangular areas that is either completely or partially embedded in an arbitrary S(N) region. The Monte Carlo and S(N) regions are then connected through the common angular boundary fluxes, which are determined iteratively using the response matrix technique, and volumetric sources. The hybrid method has been implemented in the S(N) code TWODANT by adding special-purpose Monte Carlo subroutines to calculate the response matrices and volumetric sources, and linkage subroutines to carry out the interface flux iterations. The common angular boundary fluxes are included in the S(N) code as interior boundary sources, leaving the logic for the solution of the transport flux unchanged, while, with minor modifications, the diffusion synthetic accelerator remains effective in accelerating the S(N) calculations. The special-purpose Monte Carlo routines used are essentially analog, with few variance reduction techniques employed. However, the routines have been successfully vectorized, with approximately a factor of five increase in speed over the non-vectorized version. The hybrid method is capable of solving forward, inhomogeneous source problems in X - Y and R - Z geometries. This capability includes multigroup problems involving upscatter and fission in non-highly multiplying (k(eff) ≤ .8) systems. The hybrid method has been applied to several simple test problems with good results.
29

Optical, chemical and protective properties of thin films produced by ion-assisted deposition.

Cornett, Kenneth D. January 1990 (has links)
Magnetooptical data storage materials, such as terbium-iron-cobalt (TbFeCo) alloys are susceptible to pinhole formation, as well as oxidation, particularly of the TB component. Previous studies have shown that oxide overlayers or substrates can be directly reduced by the terbium component. Ultra-thin, partially oxidized samarium films (nominally 5-10 nm) were found to serve as a suitable barrier layer between the TbFeCO film and Al₂O₃ overlayers. Furthermore, the interfacial Sm layer was found to increase the coercivity of the TbFeCo film by nearly a factor of two without degrading the magnetooptic figure of merit or significantly changing its curie point. Ion-assisted deposition (IAD) has been used to alter and improve the protective barrier properties of dielectric materials such as alumina (Al₂O₃) and zirconia (ZrO₂). In particular, pinhole formation in iron films (in lieu of TbFeCo) protected by alumina or zirconia films was greatly affected by the use of ion assisted deposition. In the case of the alumina overlayers, any bombardment was found to improve the protection afforded to underlying iron when the samples were exposed to warm, humid environments. Increasing the intensity of the ion bombardment further improved the quality of the alumina overlayers. The protective properties of zirconia films were excellent when deposited without IAD, but were much more sensitive to the choice of ion bombardment and oxide deposition parameters. Reactive ion-assisted deposition of zirconium oxyfluoride was studied as a potential method for fabricating laterally-graded index profiles in planar waveguides for use in integrated optical devices. For some applications, index changes in excess of 0.2 are required, such as a proposed technique for chromatic aberation compensation of focusing grating couplers. We found waveguide losses in evaporated ZrF₄ films of 5.5 dB/cm at 632.8 nm, and refractive index changes in O₂⁺ bombarded films of 0.23.
30

Sealing performance assessments of bentonite and bentonite/crushed rock plugs.

Ouyang, Shoung. January 1990 (has links)
Bentonite and mixtures of bentonite and crushed rock are potential sealing materials for high level nuclear waste repositories. The materials have been used to form cap layers to reduce infiltration for mined waste tailings and can also be used to construct clay liners for municipal as well as industrial waste managements. This study includes a systematic investigation of the sealing performance of bentonite and bentonite/crushed rock plugs under diverse conditions. American Colloid C/S granular bentonite and Apache Leap tuff have been mixed to prepare samples for laboratory flow testing. Bentonite weight percent and crushed tuff gradation are the major variables studied. The sealing performance assessments include high injection pressure flow tests, polyaxial flow tests, high temperature flow tests, and piping tests. The results indicate that an appropriate composition would have at least 25% bentonite by weight mixed with well-graded crushed rock. Hydraulic properties of the mixture plugs may be highly anisotropic if significant particle segregation occurs during sample installation and compaction. Temperature has no negative effects on the sealing performance within the test range from room temperature to 60°C. The piping damage to the sealing performance is small if the maximum hydraulic gradient does not exceed 120 and 280 for 25 and 35% bentonite content, respectively. The hydraulic gradients above which flow of bentonite may take place are deemed critical. Analytical work includes the introduction of bentonite occupancy percentage and water content at saturation as two major parameters for the plug design. A permeability model developed is useful for the prediction of permeability in clays, especially in view of the difficulties in obtaining such a property experimentally. A piping model is derived based on the plastic flow theory. This piping model permits the estimation of critical hydraulic gradient allowed before the flow of bentonite takes place. It can also be used to define the maximum allowable pore diameter of a protective filter layer.

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