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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Low-energy gamma ray attenuation characteristics of aviation fuels

Am241 (59.5 keV) gamma ray attenuation characteristics were investigated in 270 aviation fuel (Jet A and Jet A-1) samples from 76 airports around the world as a part of world wide study to measure the variability of aviation fuel properties as a function of season and geographical origin. All measurements were made at room temperature which varied from 20 to 27 C. Fuel densities (rho) were measured concurrently with their linear attenuation coefficients (mu), thus providing a measure of mass attenuation coefficient (mu/rho) for the test samples. In 43 fuel samples, rho and mu values were measured at more than one room temperature, thus providing mu/rho values for them at several temperatures. The results were found to be independent of the temperature at which mu and rho values were measured. It is noted that whereas the individual mu and rho values vary considerably from airport to airport as well as season to season, the mu/rho values for all samples are constant at 0.1843 + or - 0.0013 cu cm/gm. This constancy of mu/rho value for aviation fuels is significant since a nuclear fuel quantity gauging system based on low energy gamma ray attenuation will be viable throughout the world.

Singh, Jag J.↗
NASA NTRS1992

On the estimation of risk associated with an attenuation prediction

Viewgraphs from a presentation on the estimation of risk associated with an attenuation prediction is presented. Topics covered include: link failure - attenuation exceeding a specified threshold for a specified time interval or intervals; risk - the probability of one or more failures during the lifetime of the link or during a specified accounting interval; the problem - modeling the probability of attenuation by rainfall to provide a prediction of the attenuation threshold for a specified risk; and an accounting for the inadequacy of a model or models.

Crane, R. K.↗
NASA NTRS1992

Robust stabilization, robust performance, and disturbance attenuation for uncertain linear systems

This paper presents a linear quadratic regulator approach to the robust stabilization, robust performance, and disturbance attenuation of uncertain linear systems. The state-feedback designed systems provide both the robust stability with optimal performance and the disturbance attenuation with H-infinity-norm bounds. The proposed approach can be applied to matched and/or mismatched uncertain linear systems. For a matched uncertain linear system, it is shown that the disturbance attenuation robust-stabilizing controllers with or without optimal performance always exist and can be easily determined without searching; whereas, for a mismatched uncertain linear system, the introduced tuning parameters greatly enhance the flexibility of finding the disturbance-attenuation robust-stabilizing controllers.

Wang, Yeih J.↗
NASA NTRS1995

Effect of attenuation models on communication system design

The atmosphere has a significant impact on the design of a global communication system operating at 20 GHz. The system under consideration has a total atmospheric link attenuation budget that is less than 6 dB. For this relatively small link margin, rain, cloud, and molecular attenuation have to be taken into account. For an assessment of system performance on a global basis, attenuation models are utilized. There is concern whether current models can adequately describe the atmospheric effects such that a system planner can properly allocate his resources for superior overall system performance. The atmospheric attenuation as predicted by models will be examined.

Shimabukuro, Fred I.↗
NASA NTRS2000

Methods of Attenuation Correction for the TRMM Precipitation Radar Data

The surface reference technique (SRT) has been studied extensively both theoretically and experimentally over the last decade. It is only with the launch of the Tropical Rain Measuring Mission (TRMM) Precipitation Radar, however, that we can begin to test directly whether the technique provides a reliable means of attenuation correction for spaceborne weather radars. Preliminary results indicate that the method yields results that are qualitatively reasonable when the rain rate is moderate or high and when the surface provides a stable reference value. The structure of the normalized radar cross section of the surface (NRCS), however, is highly complex in the sense that the statistics change with background type, incidence angle, location, and season. As a consequence of this, the reliability of the path attenuation estimate is not proportional simply to the amount of rain attenuation but to the rain attenuation relative to the inherent variability of the reference target. To illustrate the behavior of the SRT, several overpasses of the Hurricane Bonnie are shown. We also show that the proper choice of reference data set (spatial, temporal and global) can be critical to the success of the method.

Meneghini, Robert↗
NASA NTRS2002

The Velocity and Attenuation of Acoustic Emission Waves in SiC/SiC Composites Loaded in Tension

The behavior of acoustic waves produced by microfracture events and from pencil lead breaks was studied for two different silicon carbide fiber-reinforced silicon carbide matrix composites. The two composite systems both consisted of Hi-Nicalon (trademark) fibers and carbon interfaces but had different matrix compositions that led to considerable differences in damage accumulation and acoustic response. This behavior was primarily due to an order of magnitude difference in the interfacial shear stress for the two composite systems. Load/unload/reload tensile tests were performed and measurements were made over the entire stress range in order to determine the stress-dependence of acoustic activity for increasing damage states. It was found that using the extensional wave velocities from acoustic emission (AE) events produced from pencil lead breaks performed outside of the transducers enabled accurate measurements of the stiffness of the composite. The extensional wave velocities changed as a function of the damage state and the stress where the measurement was taken. Attenuation for AE waveforms from the pencil lead breaks occurred only for the composite possessing the lower interfacial shear stress and only at significantly high stresses. At zero stress after unloading from a peak stress, no attenuation occurred for this composite because of crack closure. For the high interfacial stress composite no attenuation was discernable at peak or zero stress over the entire stress-range of the composite. From these observations, it is believed that attenuation of AE waveforms is dependent on the magnitude of matrix crack opening.

Morscher, Gregory N.↗
NASA NTRS2006

Radio Frequency (RF) Attenuation Measurements of the Space Shuttle Vehicle

Following the loss of Columbia, the Columbia Accident Investigation Board (CAIB) provided recommendations to be addressed prior to Return To Flight (RTF). As a part of CAIB Recommendation 3.4.1 - Ground Based Imagery, new C-band and X-band radars were added to the array of ground-based radars and cameras already in-situ at Kennedy Space Center. Because of higher power density considerations and new operating frequencies, the team of Subject Matter Experts (SMEs) assembled to investigate the technical details of introducing the new radars recommended a series of radio frequency (RF) attenuation tests be performed on the Space Shuttle vehicle to establish the attenuation of the vehicle outer mold line structure with respect to its external RF environment. Because of time and complex logistical constraints, it was decided to split the test into two separate efforts. The first of these would be accomplished with the assistance of the Air Force Research Laboratory (AFRL), performing RF attenuation measurements on the aft section of OV-103 (Discovery) while in-situ in Orbiter Processing Facility (OPF) 3, located at Kennedy Space Center. The second would be accomplished with the assistance of the National Institute of Standards and Technology (NIST) and the electromagnetic interference (EMI) laboratory out of the Naval Air Warfare Center, Patuxent River, Maryland (PAX River), performing RF attenuation measurements on OV-105 (Endeavour) in-situ inside the Space Shuttle Landing Facility (SLF) hangar, also located at Kennedy Space Center. This paper provides a summary description of these efforts and their results.

Scully, R. C.↗
NASA NTRS2001

Atmospheric attenuation calibrations of surface weather observations

A correlation between near-IR atmospheric attenuation measurements made by the Atmospheric Visibility Monitor (AVM) at the Table Mountain Facility and airport surface weather observations at Edwards Air Force Base has been performed. High correlations (over 0.93) exist between the simultaneous Edwards observed sky cover and the average AVM measured attenuations over the course of the 10 months analyzed. The statistical relationship between the data-sets allows the determination of coarse attenuation statistics from the surface observations, suggesting that such statistics may be extrapolated from any surface weather observation site, Furthermore, a superior technique for converting AVM images to attenuation values by way of MODTRAN predictions has been demonstrated.

laser propogation↗
NASA NTRS2008

Evaluation of a Seat Attenuation System for the Orion Crew Module

The function of the crew seat attenuation system for the Orion Crew Module (CM) is to provide the crew with a low injury-risk landing environment under a range of crew configurations and landing conditions. The current design for the seat attenuation system provides the crew with a low risk of injury environment based on the Brinkley criteria for most of the landing conditions considered. Furthermore, the stroking of the seat attenuation system is within limits, and the clearance between the seat support platform and vehicle is not exceeded. For the limited number of landing conditions where a low injury risk is exceeded, the risk is never beyond a moderate level. The results presented in this study are based on a CM structural model that is rigid except for the pallet struts, which attenuate landing loads and reduce the accelerations transferred to the astronauts. The CM simulations include a soft soil landing. Several different crew configurations are evaluated in this study. It is expected that situations where the risk is above low can be eliminated in future design iterations.

Lawrence, Charles↗
NASA NTRS2013

Modified Hitschfeld-Bordan Equations for Attenuation-Corrected Radar Rain Reflectivity: Application to Nonuniform Beamfilling at Off-Nadir Incidence

As shown by Takahashi et al., multiple path attenuation estimates over the field of view of an airborne or spaceborne weather radar are feasible for off-nadir incidence angles. This follows from the fact that the surface reference technique, which provides path attenuation estimates, can be applied to each radar range gate that intersects the surface. This study builds on this result by showing that three of the modified Hitschfeld-Bordan estimates for the attenuation-corrected radar reflectivity factor can be generalized to the case where multiple path attenuation estimates are available, thereby providing a correction to the effects of nonuniform beamfilling. A simple simulation is presented showing some strengths and weaknesses of the approach.

nonuniform beamfilling↗
DOE OSTI2020

Seismic attenuation system for a nuclear reactor

A system for attenuating seismic forces includes a reactor pressure vessel containing nuclear fuel and a containment vessel that houses the reactor pressure vessel. Both the reactor pressure vessel and the containment vessel include a bottom head. Additionally, the system includes a base support to contact a support surface on which the containment vessel is positioned in a substantially vertical orientation. An attenuation device is located between the bottom head of the reactor pressure vessel and the bottom head of the containment vessel. Seismic forces that travel from the base support to the reactor pressure vessel via the containment vessel are attenuated by the attenuation device in a direction that is substantially lateral to the vertical orientation of the containment vessel.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2025

Origins of radiation-induced optical attenuation in neutron-irradiated single-crystal sapphire at elevated temperatures

Sapphire (α-Al 2 O 3 ) is a candidate fiber-optic sensor material for extreme temperature environments, potentially including those of nuclear reactors. However, its optical transmission under high-dose neutron irradiation is not well understood compared with that of conventional fused silica. This study examined dimensional changes, optical transmission, and irradiation-induced defects in neutron-irradiated α-Al 2 O 3 at temperatures of 298 °C to 688 °C and doses of 3.2 to 12 dpa. Although previous studies attributed radiation-induced attenuation (RIA) at the highest irradiation temperatures to increased optical scattering from radiation-induced voids, our findings indicate that scattering from neither voids nor dislocation loops can explain the measured attenuation. Instead, absorption due to aluminum vacancy centers appears more likely based on a comparison of the spectral features of the measured optical attenuation with previous literature. Significant c-axis swelling (5.51 % ± 0.83 %) was observed in the 12 dpa, 592 °C irradiated sample, much higher than earlier measurements, suggesting temperature sensor drift of 543 °C to 1,140 °C. Void patterning was predominantly observed along the a-axis, differing from previous studies on polycrystalline samples, which showed c-axis patterning. Dislocation loops evolved into network dislocations with increasing temperature and dose; voids formed within these structures, showing no size or density changes, indicating an atypical growth mechanism.

Neutron irradiation↗
DOE OSTI2023

Generalized Theory and Realization of Continuously Loss-Programmable Bandpass Filtering Attenuators

With the increased demand for modern wireless systems in various applications, the need for adjustable radio frequency (RF) systems has dramatically increased. These modern systems rely on operating in the microwave frequency spectrum (1 GHz to 1 THz) without interference from other devices while also retaining the ability to detect very low and very high power signals simultaneously. There is also an ever-increasing demand for reducing the cost, size, weight, and power (C-SWaP) of these modern RF systems, driving the demand for more agile filters in the microwave frequency spectrum. In this paper, a generalized theory of bandpass filtering attenuators (filtenuators) is proposed. A filtenuator is a device that combines the frequency- selective characteristics of a filter and the loss-programmable characteristics of an attenuator into a single component. The loss- programmable aspect of the filtenuator is based on the tuning of a π-network of resistances, which are implemented using PIN diodes to control the individual resistance values electronically. A loss-programmable, third-order Chebyshev bandpass filtenuator is designed, fabricated, and measured to verify the generalized theory. The filtenuator is designed to operate at 1 GHz and have a tunable attenuation range of 2-10 dB. This proposed filtenuator demonstrates the feasibility of a tunable, low C-SWaP solution to increase RF system dynamic range and a design process that allows for future development of filtenuators.

42 ENGINEERING↗
DOE OSTI2020

Measured Energy-Dependent Neutron Attenuation Through the Stacked Printed Circuit Boards

Engineers often stack printed circuit boards (PCBs), populated with semiconductor devices, one behind the other in neutron beams to improve the error statistics in radiation-effects tests. As the neutron beam traverses through the boards, the neutron flux is both attenuated and the energy spectrum changes, which may have a significant impact on the results of such tests. In this work, we have developed a technique to measure the energy-dependent neutron attenuation due to the PCBs and to determine the energy-dependent effective cross section of neutron interactions with a single PCB. With this effective cross section, engineers can calculate the change in the neutron energy spectrum and the total neutron attenuation after any number of identical PCBs in the stack. This information will allow test engineers to partially correct inaccuracies in soft error tests.

42 ENGINEERING↗
NASA NTRS1965

Attenuation of Sinusoidal Perturbations Superimposed on Laminar Flow of a Liquid in a Long Line

The attenuation constant for sinusoidal pressure and flow perturbations superimposed on the laminar flow of a viscous liquid was measured in a system consisting of a long, straight, cylindrical hydraulic line. The upstream and downstream ends of the line were securely fastened t o the ground. A sinusoidal perturbation was imposed on the mean flow at the upstream end by means of a s m a l l oscillation of a throttle valve abmt a partly open mean position. The downstream end was terminated in a restricting orifice. Pressure perturbations were measured at three locations along the line for frequencies from 15 t o 100 cps. These pressure measurements were reduced by use of a pair of complex damped acoustic one-dimensional wave equations to obtain the attenuation constant along with the phase constant and the dimensionless downstream admittance. For the range of frequencies investigated, the experimental values of the attenuation constant are in good agreement with classical theory.

PERTURBATION↗
NASA NTRS2020

Path Attenuation Estimates for the GPM Dual-Frequency Precipitation Radar (DPR)

Estimation of path attenuation is a critical part of retrieving precipitation parameters using measurements from the Dual-Frequency Precipitation Radar (DPR) on board the Global Precipitation Measurement Mission (GPM) satellite. In this paper, we describe the latest implementation of the Surface Reference Technique that uses surface scattering properties to infer path attenuation through the precipitation. Both single- and dual-frequency versions of this method are available and while the dual-frequency version appears to be more accurate at moderate rain rates, the single-frequency approach at Ku-band is needed when the Ka-band data are not available. Despite improvements afforded by the dual-frequency version of the method, other methods such as the Hitschfeld-Bordan and standard dual-frequency approaches offer advantages particularly at lighter rain rates and at near-nadir incidence angles over land. Weighted averages of the results from these methods appear to offer the best estimate of path attenuation presently available.

attenuation↗
NASA NTRSDate not supplied

Preliminary Design of a Distributed Facesheet Acoustic Liner for Broadband Acoustic Attenuation

The purpose of this study is to investigate the acoustic performance of a liner with a distributed facesheet and a uniform depth core for broadband attenuation. The distributed facesheet is comprised of a cluster of three distinct cell resonators of varying hole diameter and porosity replicated over the active liner treatment area. A target frequency range of 1000 to 2000 Hz and an attenuation metric of 10 dB are chosen. An optimizer is used to determine the optimal facesheet designs for flow conditions of Mach 0.0 and Mach 0.3. The samples are tested in the Grazing Flow Impedance Tube at NASA Langley Research Center. The two liners are shown to achieve at least 10 dB attenuation over frequency ranges of 700 and 400 Hz, respectively, when tested at their respective designed flow speeds. This study demonstrate that a distributed facesheet with a uniform depth core can be successfully used to achieve broadband sound absorption.

acoustic liners↗
DOE OSTI2021

AGTR1 blocker attenuates activation of Tenon's capsule fibroblasts after glaucoma filtration surgery via the NF-κB signaling pathway

Activation of Tenon's capsule fibroblasts limits the success rate of glaucoma filtration surgery (GFS), the most efficacious therapy for patients with glaucoma. Angiotensin type 1 receptor (AGTR1) is involved in tissues remodeling and fibrogenesis. However, whether AGTR1 is involved in the progress of fibrogenesis after GFS is not fully elucidated. The aim of this study was to investigate the role of an AGTR1 in scar formation after GFS and the potential anti-fibrosis effect of AGTR1 blocker. AGTR1 expression level was increased in subconjunctival tissues in a rat model of GFS and transforming growth factor-beta 2 (TGF-β2)-induced human Tenon's capsule fibroblasts (HTFs). AGTR1 blocker treatment suppressed TGF-β2-induced HTF migration and α-smooth muscle actin (α-SMA) and fibronectin (FN) expression. AGTR1 blocker treatment also attenuated collagen deposition and α-SMA and FN expression in subconjunctival tissues of the rat model after GFS. Moreover, AGTR1 blocker decreased TGF-β2-induced P65 phosphorylation, P65 nuclear translocation, and nuclear factor kappa B (NF-κB) luciferase activity. Additionally, BAY 11–7082 (an NF-κB inhibitor) significantly suppressed HTF fibrosis. In conclusion, our results indicate that AGTR1 is involved in scar formation after GFS. The AGTR1 blocker attenuates subconjunctival fibrosis after GFS by inhibiting the NF-κB signaling pathway. These findings indicate that targeting AGTR1 is a potential approach to attenuate fibrosis after GFS.

60 APPLIED LIFE SCIENCES↗