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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Low-latency digital frequency synthesizer using the residue number system

A low-latency frequency synthesizer using the Direct Digital Synthesis (DDS) technique has been designed. Called the Residue Assisted Frequency Synthesizer (RAFS), it exhibits frequency switching times which are reduced by more than 50 percent below previously published designs. The switching speed advantage is made possible by the use of the Residue Number System, which allows the pipeline lengths in the Phase Accumulator and other circuitry to be reduced significantly.

Chren, William A., Jr.↗
NASA NTRS1994

Stochastic model of the residual acceleration environment in microgravity

We describe a theoretical investigation of the effects that stochastic residual accelerations (g-jitter) onboard spacecraft can have on experiments conducted in a microgravity environment. We first introduce a stochastic model of the residual acceleration field, and develop a numerical algorithm to solve the equations governing fluid flow that allow for a stochastic body force. We next summarize our studies of two generic situations: stochastic parametric resonance and the onset of convective flow induced by a fluctuating acceleration field.

Vinals, Jorge↗
NASA NTRS1995

Estimates of effects of residual acceleration on USML-1 experiments

The purpose of this study effort was to develop analytical models to describe the effects of residual accelerations on the experiments to be carried on the first U.S. Microgravity Lab mission (USML-1) and to test the accuracy of these models by comparing the pre-flight predicted effects with the post-flight measured effects. After surveying the experiments to be performed on USML-1, it became evident that the anticipated residual accelerations during the USML-1 mission were well below the threshold for most of the primary experiments and all of the secondary (Glovebox) experiments and that the only set of experiments that could provide quantifiable effects, and thus provide a definitive test of the analytical models, were the three melt growth experiments using the Bridgman-Stockbarger type Crystal Growth Furnace (CGF). This class of experiments is by far the most sensitive to low level quasi-steady accelerations that are unavoidable on space craft operating in low earth orbit. Because of this, they have been the drivers for the acceleration requirements imposed on the Space Station. Therefore, it is appropriate that the models on which these requirements are based are tested experimentally. Also, since solidification proceeds directionally over a long period of time, the solidified ingot provides a more or less continuous record of the effects from acceleration disturbances.

Naumann, Robert J.↗
NASA NTRS1995

Seasonal cycle of the residual mean meridional circulation in the stratosphere

The transformed Eurlerian-mean (TEM) residual circulation is used to study the zonally averaged transport of mass in the stratosphere. The residual circulation is estimated from heating rates computed with a radiative transfer model using data from the Upper Atmosphere Research Satellite (UARS) as inputs. An annual cycle exists in the resulting circulation in the lower stratosphere, with a larger net upward mass flux across a pressure surface in the tropics during northern hemisphere winter than during northern hemisphere summer. The annual cycle in upward tropical mass flux follows the annual cycle in downward mass flux across a pressure surface in the northern hemisphere extratropics. It is argued that the annual cycle in zonal momentum forcing in the northern hemisphere stratosphere is controlling mass flux across a pressure surface in the lower stratosphere both in the tropics and in the northern hemisphere extratropics.

Rosenlof, Karen H.↗
NASA NTRS1992

Scanning electron acoustic microscopy of residual stresses in ceramics: Theory and experiment

Several reviews have highlighted a number of applications of scanning electron acoustic microscopy (SEAM) to metals and semiconductors which show that SEAM can provide new information on surface and near-surface features of such materials, but there have been few studies attempting to determine the capabilities of SEAM for characterizing ceramic materials. We have recently observed image contrast in SEAM from residual stress fields induced in brittle materials by Vickers indentations that is strongly dependent on the electron beam chopping frequency. We have also recently developed a three-dimensional mathematical model of signal generation and contrast in SEAM, appropriate to the brittle materials studied, that we use as a starting point in this paper for modeling the effect of residual stress fields on the generated electron acoustic signal. The influence of the electron beam chopping frequency is also considered under restrictive assumptions.

Cantrell, John H.↗
NASA NTRS1996

Neutron diffraction measurements and modeling of residual strains in metal matrix composites

Neutron diffraction measurements at room temperature are used to characterize the residual strains in tungsten fiber-reinforced copper matrix, tungsten fiber-reinforced Kanthal matrix, and diamond particulate-reinforced copper matrix composites. Results of finite element modeling are compared with the neutron diffraction data. In tungsten/Kanthal composites, the fibers are in compression, the matrix is in tension, and the thermal residual strains are a strong function of the volume fraction of fibers. In copper matrix composites, the matrix is in tension and the stresses are independent of the volume fraction of tungsten fibers or diamond particles and the assumed stress free temperature because of the low yield strength of the matrix phase.

Saigal, A.↗
NASA NTRS1996

Analysis of Residual Acceleration Effects on Transport and Segregation During Directional Solidification of Tin-Bismuth in the MEPHISTO Furnace Facility

The object of this work, started in March of 1995, is to approach the problem of determining the transport conditions (and effects of residual acceleration) during the plane-front directional solidification of a tin-bismuth alloy under low gravity conditions. The work involves using a combination of 2- and 3-D numerical models, scaling analyses, 1-D models and the results of ground-based and low-gravity experiments. The experiments conducted in the MEPHISTO furnace facility during the USMP-3 spaceflight which took place earlier this year (22 Feb. - 6 Mar. 1996). This experiment represents an unprecedented opportunity to make a quantitative correlation between residual accelerations and the response of an actual experimental solidification system

Alexander, J. Iwan D.↗
NASA NTRS1994

Development of an Intermediate-Scale Aerobic Bioreactor to Regenerate Nutrients from Inedible Crop Residues

Three Intermediate-Scale Aerobic Bioreactors were designed, fabricated, and operated. They utilized mixed microbial communities to bio-degrade plant residues. The continuously stirred tank reactors operated at a working volume of 8 L, and the average oxygen mass transfer coefficient, k(sub L)a, was 0.01 s(exp -1). Mixing time was 35 s. An experiment using inedible wheat residues, a replenishment rate of 0.125/day, and a solids loading rate of 20 gdw/day yielded a 48% reduction in biomass. Bioreactor effluent was successfully used to regenerate a wheat hydroponic nutrient solution. Over 80% of available potassium, calcium, and other minerals were recovered and recycled in the 76-day wheat growth experiment.

Finger, Barry W.↗
NASA NTRS1996

Performance Enhancement of a High Speed Jet Impingement System for Nonvolatile Residue Removal

A high speed jet impingement cleaning facility has been developed to study the effectiveness of the nonvolatile residue removal. The facility includes a high pressure air compressor which charges the k-bottles to supply high pressure air, an air heating section to vary the temperature of the high pressure air, an air-water mixing chamber to meter the water flow and generate small size droplets, and a converging- diverging nozzle to deliver the supersonic air-droplet mixture flow to the cleaning surface. To reliably quantify the cleanliness of the surface, a simple procedure for measurement and calibration is developed to relate the amount of the residue on the surface to the relative change in the reflectivity between a clean surface and the greased surface. This calibration procedure is economical, simple, reliable, and robust. a theoretical framework is developed to provide qualitative guidance for the design of the test and interpretation of the experimental results. The result documented in this report support the theoretical considerations.

Klausner, James F.↗
NASA NTRS1998

Damage Characteristics and Residual Strength of Composite Sandwich Panels Impacted with and Without Compression Loading

The results of an experimental study of the impact damage characteristics and residual strength of composite sandwich panels impacted with and without a compression loading are presented. Results of impact damage screening tests conducted to identify the impact-energy levels at which damage initiates and at which barely visible impact damage occurs in the impacted facesheet are discussed. Parametric effects studied in these tests include the impactor diameter, dropped-weight versus airgun-launched impactors, and the effect of the location of the impact site with respect to the panel boundaries. Residual strength results of panels tested in compression after impact are presented and compared with results of panels that are subjected to a compressive preload prior to being impacted.

McGowan, David M.↗
NASA NTRS1998

Tropical Tropospheric Ozone (TTO) Maps from Nimbus 7 and Earth-Probe TOMS by the Modified-Residual Method: Validation, Evaluation and Trends based on Atlantic Regional Time Series - 1

The well-known wave-one pattern seen in tropical total ozone [Shiotani, 1992; Ziemke et al., 1996, 1998] has been used to develop a modified-residual (MR) method for retrieving time-averaged stratospheric ozone and tropospheric ozone column amount from TOMS (Total Ozone Mapping Spectrometer) over the 14 complete calendar years of Nimbus 7 observations (1979-1992) and from TOMS on the Earth-Probe (1996-present) and ADEOS platforms (1996- 1997). Nine- to sixteen-day averaged tropical tropospheric ozone (TTO) maps, validated with ozonesondes, show a seasonality expected from dynamical and chemical influences. The maps may be viewed on a homepage: http://metosrv2.umd.edu/~tropo. Stratospheric column ozone, which is also derived by the modified-residual method, compares well with sondes (to within 6-7 DU) and with stratospheric ozone column derived from other satellites (within 8-10 DU). Validation of the TTO time-series is presently limited to ozonesonde comparisons with Atlantic stations and sites on the adjacent continents (Ascension Island, Natal, Brazil; Brazzaville); for the sounding periods, TTO at all locations agrees with the sonde record to +/-7 DU. TTO time-series and the magnitude of the wave-one pattern show ENSO signals in the strongest El Nifio periods from 1979-1998. From 12degN and 12degS, zonally averaged tropospheric ozone shows no significant trend from 1980-1990. Trends are also not significant during this period in localized regions, e.g. from just west of South America across to southern Africa. This is consistent with the ozonesonde record at Natal, Brazil (the only tropical ozone data publicly available for the 1980's), which shows a not statistically significant increase. The lack of trend in tropospheric ozone agrees with a statistical analysis based on another method for deriving TTO from TOMS, the so-called Convective-Cloud-Differential approach of Ziemke et al. [1998].

Thompson, Anne M.↗
NASA NTRS1998

Analysis and Design of Fuselage Structures Including Residual Strength Prediction Methodology

The goal of this research project is to develop and assess methodologies for the design and analysis of fuselage structures accounting for residual strength. Two primary objectives are included in this research activity: development of structural analysis methodology for predicting residual strength of fuselage shell-type structures; and the development of accurate, efficient analysis, design and optimization tool for fuselage shell structures. Assessment of these tools for robustness, efficient, and usage in a fuselage shell design environment will be integrated with these two primary research objectives.

Knight, Norman F.↗
NASA NTRS1999

On Taylor-Series Approximations of Residual Stress

Although subgrid-scale models of similarity type are insufficiently dissipative for practical applications to large-eddy simulation, in recently published a priori analyses, they perform remarkably well in the sense of correlating highly against exact residual stresses. Here, Taylor-series expansions of residual stress are exploited to explain the observed behavior and "success" of similarity models. Until very recently, little attention has been given to issues related to the convergence of such expansions. Here, we re-express the convergence criterion of Vasilyev [J. Comput. Phys., 146 (1998)] in terms of the transfer function and the wavenumber cutoff of the grid filter.

Pruett, C. David↗
NASA NTRS1998

Analysis of Residual Acceleration Effects on Transport and Segregation During Directional Solidification of Tin-Bismuth in the MEPHISTO Furnace Facility

The research accomplishments summarized in this Final Report during the period from 3/95 to 3/98, which included a 12 months no-cost extension granted at the end of the nominal 2 year period of performance. The report has 5 sections, in section 1 the objectives are presented, a task description is given and the background and significance of the work is outlined. In section 2 the research accomplishments are summarized. In section 3 publications and presentations are listed. Student participation is listed in 4. The work is summarized in section 5. and references for sections 1 and 2 are supplied in section 6. The object of this work, is to approach the problem of determining the transport conditions (and effects of residual acceleration) during the plane-front directional solidification of a tin-bismuth alloy under low gravity conditions. The work involved using a combination of 2- and 3-D numerical models, scaling analyses, ID models and the results of ground-based and low-gravity experiments. The experiments conducted in the MEPHISTO furnace facility during the USW-3 space flight which took place between February 22 through March 6, 199). This experiment represents an unprecedented opportunity to make a quantitative correlation between residual accelerations and the response of an actual experimental solidification

Alexander, J. Iwan D.↗
NASA NTRS2001

Cryogenic Treatment of Al-Li Alloys for Improved Weldability, Repairability, and Reduction of Residual Stresses

Service performance has shown that cryogenic treatment of some metals provides improved strength, fatigue life, and wear resistance to the processed material. Effects such as these were initially discovered by NASA engineers while evaluating spacecraft that had returned from the cold vacuum of space. Factors such as high cost, poor repairability, and poor machinability are currently prohibitive for wide range use of some aerospace aluminum alloys. Application of a cryogenic treatment process to these alloys is expected provide improvements in weldability and weld properties coupled with a reduction in repairs resulting in a significant reduction in the cost to manufacture and life cycle cost of aerospace hardware. The primary purpose of this effort was to evaluate the effects of deep cryogenic treatment of some aluminum alloy plate products, welds, and weld repairs, and optimize a process for the treatment of these materials. The optimized process is being evaluated for improvements in properties of plate and welds, improvements in weldability and repairability of treated materials, and as an alternative technique for the reduction of residual stresses in repaired welds. This paper will present the results of testing and evaluation conducted in this effort. These results will include assessments of changes in strength, toughness, stress corrosion susceptability, weldability, repairability, and reduction in residual stresses of repaired welds.

Malone, Tina W.↗
NASA NTRS2001

Quantifying Residual Stresses by Means of Thermoelastic Stress Analysis

This study focused on the application of the Thermoelastic Stress Analysis (TSA) technique as a tool for assessing the residual stress state of structures. TSA is based on the fact that materials experience small temperature changes when compressed or expanded. When a structure is cyclically loaded, a surface temperature profile results which correlates to the surface stresses. The cyclic surface temperature is measured with an infrared camera. Traditionally, the amplitude of a TSA signal was theoretically defined to be linearly dependent on the cyclic stress amplitude. Recent studies have established that the temperature response is also dependent on the cyclic mean stress (i.e., the static stress state of the structure). In a previous study by the authors, it was shown that mean stresses significantly influenced the TSA results for titanium- and nickel-based alloys. This study continued the effort of accurate direct measurements of the mean stress effect by implementing various experimental modifications. In addition, a more in-depth analysis was conducted which involved analyzing the second harmonic of the temperature response. By obtaining the amplitudes of the first and second harmonics, the stress amplitude and the mean stress at a given point on a structure subjected to a cyclic load can be simultaneously obtained. The experimental results showed good agreement with the theoretical predictions for both the first and second harmonics of the temperature response. As a result, confidence was achieved concerning the ability to simultaneously obtain values for the static stress state as well as the cyclic stress amplitude of structures subjected to cyclic loads using the TSA technique. With continued research, it is now feasible to establish a protocol that would enable the monitoring of residual stresses in structures utilizing TSA.

Gyekenyesi, Andrew L.↗
NASA NTRS2001

The Effect of Heat Treatment on Residual Stress and Machining Distortions in Advanced Nickel Base Disk Alloys

This paper describes an extension of NASA's AST and IDPAT Programs which sought to predict the effect of stabilization heat treatments on residual stress and subsequent machining distortions in the advanced disk alloy, ME-209. Simple "pancake" forgings of ME-209 were produced and given four heat treats: 2075F(SUBSOLVUS)/OIL QUENCH/NO AGE; 2075F/OIL QUENCH/1400F@8HR;2075F/OIL QUENCH/1550F@3HR/l400F@8HR; and 2160F(SUPERSOLVUS)/OIL QUENCH/1550F@3HR/ 1400F@8HR. The forgings were then measured to obtain surface profiles in the heat treated condition. A simple machining plan consisting of face cuts from the top surface followed by measurements of the surface profile opposite the cut were made. This data provided warpage maps which were compared with analytical results. The analysis followed the IDPAT methodology and utilized a 2-D axisymmetric, viscoplastic FEA code. The analytical results accurately tracked the experimental data for each of the four heat treatments. The 1550F stabilization heat treatment was found to significantly reduce residual stresses and subsequent machining distortions for fine grain (subsolvus) ME209, while coarse grain (supersolvus) ME209 would require additional time or higher stabilization temperatures to attain the same degree of stress relief.

Gayda, John↗