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1,675 Matching Results

Start Over Year 1999 Month April

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(open access)

Advanced Concepts for High-Power VCSELS and 2-Dimensional VCSEL Arrays

We have developed high power vertical cavity surface emitting lasers (VCSELS) for multimode or single mode operation. We have characterized new cavity designs for individual lasers and 2-dimensional VCSEL arrays to maximize output power. Using broad area high power VCSELS under pulsed excitation, we have demonstrated the triggering of a photoconductive semiconductor switch (PCSS) with a VCSEL. We also have developed designs for high output power in a single mode. The first approach is to engineer the oxide aperture profile to influence the optical confinement and thus modal properties. A second approach focuses on "leaky-mode" concepts using lateral modification of the cavity resonance to provide the lateral refractive index difference. To this end, we have developed a regrowth process to fabricate single-mode VCSELS. The overall objective of this work was to develop high-power single-mode or multimode sources appropriate for many applications leveraging the many inherent advantages of VCSELS.

Date: April 1, 1999

Creator: Allerman, A. A.; Choquette, Kent D.; Chow, W. W.; Geib, K. M.; Hadley, R.; Hou, H. Q. et al.

Object Type: Report

System: The UNT Digital Library

(open access)

Context-Dependent Prognostics and Health Assessment: A Condition-Based Maintenance Approach That Supports Mission Compliance

In today's manufacturing environment, plants, systems, and equipment are being asked to perform at levels not thought possible a decade ago. The intent is to improve process operations and equipment reliability, availability, and maintainability without costly upgrades. Of course these gains must be achieved without impacting operational performance. Downsizing is also taking its toll on operations. Loss of personnel, particularly those who represent the corporate history, is depleting US industries of their valuable experiential base which has been relied on so heavily in the past. These realizations are causing companies to rethink their condition-based maintenance policies by moving away from reacting to equipment problems to taking a proactive approach by anticipating needs based on market and customer requirements. This paper describes a different approach to condition-based maintenance-context-dependent prognostics and health assessment. This diagnostic capability is developed around a context-dependent model that provides a capability to anticipate impending failures and determine machine performance over a protracted period of time. This prognostic capability links operational requirements to an economic performance model. In this context, a system may provide 100% operability with less than 100% functionality. This paradigm is used to facilitate optimal logistic supply and support.

Date: April 19, 1999

Creator: Allgood, G. O. & Kercel, S. W.

Object Type: Article

System: The UNT Digital Library

(open access)

An Anticipatory Model of Cavitation

The Anticipatory System (AS) formalism developed by Robert Rosen provides some insight into the problem of embedding intelligent behavior in machines. AS emulates the anticipatory behavior of biological systems. AS bases its behavior on its expectations about the near future and those expectations are modified as the system gains experience. The expectation is based on an internal model that is drawn from an appeal to physical reality. To be adaptive, the model must be able to update itself. To be practical, the model must run faster than real-time. The need for a physical model and the requirement that the model execute at extreme speeds, has held back the application of AS to practical problems. Two recent advances make it possible to consider the use of AS for practical intelligent sensors. First, advances in transducer technology make it possible to obtain previously unavailable data from which a model can be derived. For example, acoustic emissions (AE) can be fed into a Bayesian system identifier that enables the separation of a weak characterizing signal, such as the signature of pump cavitation precursors, from a strong masking signal, such as a pump vibration feature. The second advance is the development of extremely fast, …

Date: April 5, 1999

Creator: Allgood, G. O.; Dress Jr., W. B.; Hylton, J. O. & Kercel, S. W.

Object Type: Article

System: The UNT Digital Library

(open access)

Can Cavitation Be Anticipated?

The major problem with cavitation in pumps and hydraulic systems is that there is no effective (conventional) method for detecting or predicting its inception. The traditional method of recognizing cavitation in a pump is to declare the event occurring when the total head drops by some arbitrary value (typically 3%) in response to a pressure reduction at the pump inlet. However, the device is already seriously cavitating when this happens. What is actually needed is a practical method to detect impending rather than incipient cavitation. Whereas the detection of incipient cavitation requires the detection of features just after cavitation starts, the anticipation of cavitation requires the detection and identification of precursor features just before it begins. Two recent advances that make this detection possible. The first is acoustic sensors with a bandwidth of 1 MHz and a dynamic range of 80 dB that preserve the fine details of the features when subjected to coarse vibrations. The second is the application of Bayesian parameter estimation which makes it possible to separate weak signals, such as those present in cavitation precursors, from strong signals, such as pump vibration. Bayesian parameter estimation derives a model based on cavitation hydrodynamics and produces a figure …

Date: April 25, 1999

Creator: Allgood, G. O.; Dress, W. B.; Hylton, J. O. & Kercel, S. W.

Object Type: Article

System: The UNT Digital Library

Ensemble: 1999-04-05-African Cultural Festival

An African music festival performed at the UNT College of Music Winspear Hall.

Date: April 5, 1999

Creator: Alorwoyie, Gideon Foli

Object Type: Sound

System: The UNT Digital Library

(open access)

Auditory Brainstem Responses in Golden Syrian Hamsters (Mesocricetus auratus) Affected with the Wh Gene

Article on auditory brainstem responses in golden Syrian hamsters (Mesocricetus auratus) affected with the Wh gene.

Date: April 1999

Creator: Amedofu, Geoffrey K. P.; Gopal, Kamakshi V.; Asher, James H.; Ahmadizadeh, Massum & Moore, Ernest J.

Object Type: Article

System: The UNT Digital Library

(open access)

Secret Sessions of Congress: A Brief Historical Overview

“Secret” or “closed door” sessions of the House of Representatives and Senate are held periodically to discuss business, including impeachment deliberations, deemed to require confidentiality and secrecy. Authority for the two chambers to hold these sessions is implied by Article I, section 5, of the Constitution. Both the House and the Senate have supplemented this clause through rules and precedents.

Date: April 6, 1999

Creator: Amer, Mildred L.

Object Type: Report

System: The UNT Digital Library

(open access)

Southwest Retort, Volume 51, Number [7], April 1999

This publication of the Dallas-Fort Worth Section of the American Chemical Society includes information about research, prominent scientist, organizational business, and various other stories of interest to the community.

Date: April 1999

Creator: American Chemical Society. Dallas/Fort Worth Section.

Object Type: Journal/Magazine/Newsletter

System: The UNT Digital Library

(open access)

Appropriations for FY1999: An Overview

None

Date: April 28, 1999

Creator: Anderson, J. Michael

Object Type: Report

System: The UNT Digital Library

(open access)

Appropriations for FY1999: An Overview

Appropriations are one part of a complex federal budget process that includes budget resolutions, appropriations (regular, supplemental, and continuing) bills, rescissions, and budget reconciliation bills. This report is a guide to CRS reports that provide analytical perspectives on the 13 annual appropriations bills, and other related appropriation measures. It does not include a detailed explanation or description of the budget or appropriations processes.

Date: April 28, 1999

Creator: Anderson, J. Michael

Object Type: Report

System: The UNT Digital Library

(open access)

(129)Xe NMR of Mesoporous Silicas

The porosities of three mesoporous silica materials were characterized with {sup 129}Xe NMR spectroscopy. The materials were synthesized by a sol-gel process with r = 0, 25, and 70% methanol by weight in an aqueous cetyltrimethylammonium bromide solution. Temperature dependent chemical shifts and spin lattice relaxation times reveal that xenon does not penetrate the pores of the largely disordered (r= 70%) silica. For both r = 0 and 25%, temperature dependent resonances corresponding to physisorbed xenon were observed. An additional resonance for the r = 25% sample was attributed to xenon between the disordered cylindrical pores. 2D NMR exchange experiments corroborate the spin lattice relaxation data which show that xenon is in rapid exchange between the adsorbed and the gas phase.

Date: April 23, 1999

Creator: Anderson, M.T.; Asink, R.A.; Kneller, J.M. & Pietrass, T.

Object Type: Article

System: The UNT Digital Library

(open access)

Mobile Robotic Teams Applied to Precision Agriculture

The Idaho National Engineering and Environmental Laboratory (INEEL) and Utah State University’s Center for Self-Organizing and Intelligent Systems (CSOIS) have developed a team of autonomous robotic vehicles applicable to precision agriculture. A unique technique has been developed to plan, coordinate, and optimize missions in large structured environments for these autonomous vehicles in realtime. Two generic tasks are supported: 1) Driving to a precise location, and 2) Sweeping an area while activating on-board equipment. Sensor data and task achievement data is shared among the vehicles enabling them to cooperatively adapt to changing environmental, vehicle, and task conditions. This paper discusses the development of the autonomous robotic team, details of the mission-planning algorithm, and successful field demonstrations at the INEEL.

Date: April 1, 1999

Creator: Anderson, Matthew Oley; Kinoshita, Robert Arthur; Mckay, Mark D; Willis, Walter David; Gunderson, R.W. & Flann, N.S.

Object Type: Article

System: The UNT Digital Library

(open access)

Extruded plastic scintillation detectors

As a way to lower the cost of plastic scintillation detectors, commercially available polystyrene pellets have been used in the production of scintillating materials that can be extruded into different profiles. The selection of the raw materials is discussed. Two techniques to add wavelength shifting dopants to polystyrene pellets and to extrude plastic scintillating strips are described. Data on light yield and transmittance measurements are presented.

Date: April 16, 1999

Creator: Anna Pla-Dalmau, Alan D. Bross and Kerry L. Mellott

Object Type: Article

System: The UNT Digital Library

(open access)

The mean evolution and variability of the Asian summer monsoon: comparison of ECMWF and NCEP/NCAR reanalyses

The behavior of the Asian Summer Monsoon is compared using the European Centre for Medium Range Weather Forecasts Reanalysis (ERA) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis (Kalnay et al. 1996). The goals of this paper are to identify common features between the reanalyses, to assess their robustness for model validation, and especially to use reanalyses to develop their understanding of the mean evolution of the Asian Summer Monsoon and the characteristics of its interannual and intraseasonal variability (Annamalai et al. 1999).

Date: April 21, 1999

Creator: Annamalai, H.; Hodges, K.; Slingo, J. M. & Sperber, K. R.

Object Type: Article

System: The UNT Digital Library

(open access)

The relationship between intraseasonal and interannual variability during the asian summer monsoon

The purpose of this paper is to investigate intraseasonal (30-70 days) and higher frequency (5-30 days) variability and its relationship to interannual variability. Various modelling studies have suggested a link between intraseasonal and interannual variability of the Asian summer monsoon. This relationship has been mainly based upon the similar spatial structures of the dominant EOF patterns of the monsoon circulation on intraseasonal and interannual time scales from simulations with simple models and atmospheric general circulation models. Here we investigate these relationships using 40 years of NCEP/NCAR Reanalysis. Motivation for this study is embodied in the suggestions of Charney and Shukla (1981) that boundary forcing (e.g., sea surface temperature) may predispose the monsoon system towards a dry or wet state, and the result of Palmer (1994), using the Lorenz (1963) model, that the probability of being in one regime of phase space or another is no longer equally probable in the presence of external forcing. To investigate the influence of the boundary forcing, the probability distribution functions (PDF�s) of the principal components are given.

Date: April 21, 1999

Creator: Annamalai, H; Slingo, J M & Sperber, K R

Object Type: Article

System: The UNT Digital Library