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Browsing by Author "Mahan, J. Robert"

Now showing 1 - 8 of 8
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    Comparison Of The Measured And Predicted Response Of The Earth Radiation Budget Experiment Active Cavity Radiometer During Solar Observations
    Mahan, J. Robert; Tira, N. E.; Lee, R. B.; Keynton, R. J. (Optical Society of America, 1989-05-01)
    The Earth Radiation Budget Experiment consists of an array of radiometric instruments placed in earth orbit by the National Aeronautics and Space Administration to monitor the longwave and visible components of the earth's radiation budget. Presented is a dynamic electrothermal model of the active cavity radiometer used to measure the earth's total radiative exitance. Radiative exchange is modeled using the Monte Carlo method and transient conduction is treated using the finite element method. Also included is the feedback circuit which controls electrical substitution heating of the cavity. The model is shown to accurately predict the dynamic response of the instrument during solar calibration.
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    The constitutive modeling of shape memory alloys
    Liang, Chen (Virginia Tech, 1990-08-15)
    This dissertation presents a one-dimensional thermomechanical constitutive model for shape memory alloys based on basic concepts of thermodynamics and phase transformation kinetics. Compared with other developed constitutive relations, this thermomechanical constitutive relation not only reflects the physical essence of shape memory alloys, i.e., the martensitic phase transformation involved, but also provides an easy-to-use design tool for engineers. It can predict and describe the behavior of SMA quantitatively. A multi-dimensional constitutive relation for shape memory alloys is further developed based on the one-dimensional model. It can be used to study the mechanical behavior including shape memory effect of complex SMA structures that have never been analytically studied, and provide quantitative analysis for many diverse applications of shape memory alloys. A general design method for shape memory alloy actuators has also been developed based on the developed constitutive relation and transient thermal considerations. The design methodology provides a quantitative approach to determine the design parameters of shape memory alloy force actuators, including both bias spring SMA force actuators and differential SMA force actuators.
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    Determination Of The Thermal Offset Of The Eppley Precision Spectral Pyranometer
    Haeffelin, M.; Kato, S.; Smith, A. M.; Rutledge, C. K.; Charlock, T. P.; Mahan, J. Robert (Optical Society of America, 2001-10-01)
    Eppley's precision spectral pyranometer (PSP) is used in networks around the world to measure down-welling diffuse and global solar irradiance at the surface of the Earth. In recent years several studies have shown significant discrepancy between irradiances measured by pyranometers and those computed by atmospheric radiative transfer models. Pyranometer measurements have been questioned because observed diffuse irradiances sometimes are below theoretical minimum values for a pure molecular atmosphere, and at night the instruments of ten produce nonzero signals ranging between +5 and -10 W m(-2). We install thermistor sondes in the body of a PSP as well as on its inner dome to monitor the temperature gradients within the instrument, and we operate a pyrgeometer (PIR) instrument side by side with the PSP. We derive a relationship between the PSP output and thermal radiative exchange by the dome and the detector and a relationship between the PSP output and the PIR thermopile output (net-IR). We determine the true PSP offset by quickly capping the instrument at set time intervals. For a ventilated and shaded PSP, the thermal offset can reach -15 W m(-2) under clear skies, whereas it remains close to zero for low overcast clouds. We estimate the PSP thermal offset by two methods: (1) using the PSP temperatures and (2) using the PIR net-IR signal. The offset computed from the PSP temperatures yields a reliable estimate of the true offset (+/-1 W m(-2)). The offset computed from net-IR is consistent with the true offset at night and under overcast skies but predicts only part of the true range under clear skies. (C) 2001 Optical Society of America.
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    Linear-Array Apertures For In-Flight Dynamic Solar Calibration Of Radiometric Channels For Earth Radiation-Budget Applications
    Tira, N. E.; Mahan, J. Robert; Lee, R. B.; Keynton, R. J. (Optical Society of America, 1994-07-01)
    The zero-frequency gain of nonimaging radiometers used in Earth radiation-budget applications is usually verified by a procedure that allows the instrument to view the Sun through an appropriate attenuating aperture and then equates its response to the known attenuated solar constant. However, channel intercomparison often requires that data from a low-resolution, relatively slow instrument such as an active-cavity radiometer be compared with data from a high-resolution, fast instrument such as a scanning thermistor-bolometer radiometer. In such a case, consideration of the difference in the dynamic responses of the two channels may be important. A novel technique for in-flight measurement of the radiometric transfer function of such instruments is described and then demonstrated through the use of a high-order dynamic model of the total, wide-field-of-view, nonscanning channel of NASA's Earth Radiation Budget Experiment (ERBE).
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    Numerical Focusing of a Wide Field-of-View Instrument for Monitoring the Planetary Energy Budget
    Ashraf, Anum Barki; Mahan, J. Robert; Priestley, Kory J.; Shankar, Mohan (MDPI, 2019-09-21)
    Wide field-of-view optical instruments based on Ritchey-Crétien telescopes have been proposed to replace narrow field-of-view scanning instruments for Earth radiation budget monitoring applications. A disadvantage of such instruments is that they are subject to significant focal plane distortion. A novel numerical focusing scheme is proposed and demonstrated using a Monte Carlo ray-trace-based simulation of the performance of a candidate instrument. Results are presented which indicate that image recovery error can be significantly reduced using the proposed algorithm.
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    Numerical Focusing of a Wide-Field-Angle Earth Radiation Budget Imager Using an Artificial Neural Network
    Yarahmadi, Mehran; Mahan, J. Robert; McFall, Kevin; Ashraf, Anum Barki (MDPI, 2020-01-03)
    Narrow field-of-view scanning thermistor bolometer radiometers have traditionally been used to monitor the earth’s radiant energy budget from low earth orbit (LEO). Such instruments use a combination of cross-path scanning and along-path spacecraft motion to obtain a patchwork of punctual observations which are ultimately assembled into a mosaic. Monitoring has also been achieved using non-scanning instruments operating in a push-broom mode in LOE and imagers operating in geostationary orbit. The current contribution considers a fourth possibility, that of an imager operating in LEO. The system under consideration consists of a Ritchey-Chrétien telescope illuminating a plane two-dimensional microbolometer array. At large field angles, the focal length of the candidate instrument is field-angle dependent, resulting in a blurred image in the readout plane. Presented is a full-field focusing algorithm based on an artificial neural network (ANN). Absorbed power distributions on the microbolometer array produced by discretized scenes are obtained using a high-fidelity Monte Carlo ray-trace (MCRT) model of the imager. The resulting readout array/scene pairs are then used to train an ANN. We demonstrate that a properly trained ANN can be used to convert the readout power distribution into an accurate image of the corresponding discretized scene. This opens the possibility of using an ANN based on a high-fidelity imager model for numerical focusing of an actual imager.
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    Predicted Dynamic Electrothermal Performance Of Thermistor Bolometer Radiometers For Earth Radiation Budget Applications
    Haeffelin, M. P. A.; Mahan, J. Robert; Priestley, Kory J. (Optical Society of America, 1997-05-01)
    The Earth Radiation Budget Experiment (ERBE) and the Clouds and the Earth's Radiant Energy System (CERES) rely on scanning thermistor bolometer radiometers of a similar design for accomplishing their mission. High-level dynamic electrothermal models of these instruments have been developed on the basis of the Monte Carlo ray-trace, finite-difference, and finite-element methods. The models are capable of simulating the end-to-end response of the ERBE and the CERES instruments to simulated sequences of Earth scenes. Such models will prove useful in the design of future generations of similar instruments, in defining ground-based and in-flight calibration and data-reduction strategies, in the interpretation of flight data, and in understanding data anomalies that might arise after the instruments have been placed in orbit. Two modules that make up the end-to-end model are presented: the optical-thermal radiative module and the thermistor bolometer dynamic electrothermal module. The optics module is used to determine the point-spread function of the optics, which establishes that the instrument has sharply defined footprints on the Earth. Results obtained with the thermistor bolometer dynamic electrothermal module provide valuable insights into the details of channel operation and establish its high level of equivalence. The combination of the two modules allows the point-spread function of the instrument to be determined and reveals the potential of this tool for scanning realistic Earth scenes. (C) 1997 Optical Society of America.
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    A temperature correlation for the radiation resistance of a thick-walled circular duct exhausting a hot gas
    Mahan, J. Robert; Cline, J. G.; Jones, J. D. (Acoustical Society of America, 1984-01-01)
    It is often useful to know the radiation impedance of an unflanged but thick-walled circular duct exhausting a hot gas into relatively cold surroundings. The reactive component is shown to be insensitive to temperature, but the resistive component is shown to be temperature dependent. A temperature correlation is developed permitting prediction of the radiation resistance from a knowledge of the temperature difference between the ambient air and the gas flowing from the duct, and a physical basis for this correlation is presented.