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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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NASA NTRS1999

Large Eddy Simulation of Aircraft Wake Vortices in a Homogeneous Atmospheric Turbulence: Vortex Decay and Descent

The effects of ambient turbulence on decay and descent of aircraft wake vortices are studied using a validated, three-dimensional: large-eddy simulation model. Numerical simulations are performed in order to isolate the effect of ambient turbulence on the wake vortex decay rate within a neutrally-stratified atmosphere. Simulations are conducted for a range of turbulence intensities, by injecting wake vortex pairs into an approximately homogeneous and isotropic turbulence field. The decay rate of the vortex circulation increases clearly with increasing ambient turbulence level, which is consistent with field observations. Based on the results from the numerical simulations, simple decay models are proposed as functions of dimensionless ambient turbulence intensity (eta) and dimensionless time (T) for the circulation averaged over a range of radial distances. With good agreement with the numerical results, a Gaussian type of vortex decay model is proposed for weak turbulence: while an exponential type of Tortex decay model can be applied for strong turbulence. A relationship for the vortex descent based on above vortex decay model is also proposed. Although the proposed models are based on simulations assuming neutral stratification, the model predictions are compared to Lidar vortex measurements observed during stable, neutral, and unstable atmospheric conditions. In the neutral and unstable atmosphere, the model predictions appear to be in reasonable agreement with the observational data, while in the stably-stratified atmosphere, they largely underestimate the observed circulation decay with consistent overestimation of the observed vortex descent. The underestimation of vortex decay during stably-stratified conditions suggests that stratification has an important influence on vortex decay when ambient levels of turbulence are weak.

Han, Jongil↗

NASA NTRS1985

Conducting a thermal conductivity survey

A physically transparent approximate theory of phonon decay rates is presented starting from a pair potential model of the interatomic forces in an insulator or semiconductor. The theory applies in the classical regime and relates the 3-phonon decay rate to the third derivative of the pair potential. Phonon dispersion relations do not need to be calculated, as sum rules relate all the needed quantities directly to the pair potential. The Brillouin zone averaged phonon lifetime turns out to involve a dimensionless measure of the anharmonicity multiplied by an effective density of states for 3-phonon decay. Results are given for rare gas and alkali halide crystals. For rare gases, the results are in good agreement with more elaborate perturbation calculations. Comparison to experimental data on phonon linewidths and thermal conductivity are made.

Allen, P. B.↗

NASA NTRS1972

A flight evaluation of methods for predicting vortex wake effects on trailing aircraft

The results of four current analytical methods for predicting wing vortex strength and decay rate are compared with the results of a flight investigation of the wake characteristics of several large jet transport aircraft. An empirical expression defining the strength and decay rate of wake vortices is developed that best represents most of the flight-test data. However, the expression is not applicable to small aircraft that would be immersed in the vortex wake of large aircraft.

Robinson, G. H.↗

NASA NTRS1973

Re-187, recycling r-process elements through stars, and the age of the Galaxy.

The enhanced beta-decay rate of ionized Re-187 in stars has been studied within the context of a detailed numerical model of the production of r-process elements and their recycling through stars during the course of galactic evolution. It is concluded that the enhanced decay rate does not significantly reduce the Re-Os chronometer age for the Galaxy. Consequently, the Re-187 beta-decay half-life and the Os-186/Os-187 neutron cross-section ratio remain as the significant uncertainties in this chronology. Unlike the uncertainties in other chronologies, both are amenable to laboratory measurements.

Talbot, R. J., Jr.↗

NASA NTRS1988

Electrostatic testing of thin plastic materials

Ten thin plastic materials (Velostat, RCAS 1200, Llumalloy, Herculite 80, RCAS 2400, Wrightlon 7000, PVC, Aclar 22A, Mylar, and Polyethylene) were tested for electrostatic properties by four different devices: (1) The static decay meter, (2) the manual triboelectric testing device, (3) the robotic triboelectric testing device, and (4) the resistivity measurement adapter device. The static decay meter measured the electrostatic decay rates in accordance with the Federal Test Method Standard 101B, Method 4046. The manual and the robotic triboelectric devices measured the triboelectric generated peak voltages and the five-second decay voltages in accordance with the criteria for acceptance standards at Kennedy Space Center. The resistivity measurement adapter measured the surface resistivity of each material. An analysis was made to correlate the data among the four testing devices. For the material tested the pass/fail results were compared for the 4046 method and the triboelectric testing devices. For the limited number of materials tested, the relationship between decay rate and surface resistivity was investigated as well as the relationship between triboelectric peak voltage and surface resistivity.

Skinner, S. Ballou↗

NASA NTRS1983

Numerical simulations of the decay of satellite galaxy orbits

A multiple three-body technique is used to study the orbital evolution of satellite galaxies which is similar to the N-body method but neglects two-body forces between stars in the halo of the parent galaxy. It is found that, for satellites orbiting within the halo, Chandrasekhar's (1960) dynamical friction formula accurately describes the orbital decay rate, including its variation with satellite mass and size and with the number density and mass of halo stars. Significant frictional forces are present even outside the halo, and the orbital decay rate, instead of depending on the procedure used to place the satellite in its orbit, is determined only by the current orbital parameters. This semirestricted N-body method is sufficiently fast to have permitted the running of 200 simulations to date, many more than would have been possible by means of the conventional N-body technique.

Lin, D. N. C.↗

NASA NTRS1993

Radiative lifetimes of the 2s2p2(4P) metastable levels of N III

The radiative decay rates of N III 175 nm intersystem lines were measured in the laboratory by recording the time dependence of photon intensities emitted as the 2s2p2(4P) metastable term of N(2+) ions decay to the 2s22p(2P0) ground term. A cylindrical radio frequency ion trap was used to store the electron impact-produced N(2+) ions. The radiative decay signals were analyzed by multiexponential least-squares fits to the data. The measured radiative decay rates to the ground term are 1019(+/- 64)/s for 4P sub 1/2, 74.5(+/- 5.4)/s for 4P sub 3/2, and 308( +/- 22)/s for 4P sub 5/2. Comparisons of the measured values with theoretical values are presented.

Fang, Z.↗

NASA NTRS1984

Quantification of Saint-Venant's Principle for a General Prismatic Member

A semi-analytic finite element method for finding the decay rates of edge effects in anisotropic layered plates was extended to the treatment of prismatic members of an arbitrary cross section composed of any number of anisotropic elements. The displacement fields corresponding to the decay rates with positive real part were determined for a homogenous graphite/epoxy plate two units thick. The behavior of a rectangular beam is also considered.

Source record↗

NASA NTRS1973

Mach stem structure in exothermic systems.

Incident waves having grossly similar characteristics were obtained by passing a shock wave over a ramp mounted on the divergent part of a convergent-divergent nozzle. The range of triple point parameters in both the naturally occurring detonative Mach stems and those generated in the ramp experiments compared favorably, but some differences were observed in the triple point trajectories and the decay rates of the lead waves. Whereas the lead waves in detonations have been found to decay at rates between those of planar and cylindrical blast waves, the decay observed in the ramp experiments was more like that of planar blast waves. The dominant factor in determining a triple point trajectory was found to be the orientation of the lead wave at the triple point. Comparison of reactive and nonreactive cases indicated a faster rate of decay of the lead wave and a steeper triple point trajectory for the reactive cases than for the nonreactive ones. Also, a sensitivity of the trajectory to initial pressure was observed in the former, although none was present in the nonreactive cases.

Liaugminas, R.↗

NASA NTRS1989

Critical fluid light scattering

Thermal fluctuations give rise to a host of thermodynamic anomalies near critical points. The background for these phenomena is presented, and an experimental design on STS flight experiment called Zeno is described. The objective is to measure the decay rates of critical density fluctuations in a simple fluid (xenon) very near its liquid-vapor critical point using laser light scattering and photon correlation spectroscopy. Such experiments have been limited on Earth by the presence of gravity, which causes large density gradients in the sample. Fluctuation decay rates in xenon will be measured at least 100 times closer to the critical point than is possible on Earth. This will require taking data as close as 145 micro K to the critical temperature, after locating it to + or - 10 micro K. The minimum mission time of 100 hours will allow a complete range of temperature points to be covered, limited by the thermal response of the sample. An essential part of the apparatus is the thermostat. Its design principles and key features are given together with some observed performance characteristics of a prototype: + or - 20 micro K stability for 18 hours, and one minute programmed step response for steps less than 10 mK.

Gammon, Robert W.↗

NASA NTRS1987

Critical fluid light scattering

Thermal fluctuations give rise to a host of thermodynamic anomalies near critical points. The background for these phenomena is presented, and an experiment design on STS flight experiment called 'Zeno' described. The objective is to measure the decay rates of critical density fluctuations in a simple fluid (xenon) very near its liquid-vapor critical point using laser light-scattering and photon-correlation spectroscopy. Such experiments have been limited on earth by the presence of gravity, which causes large density gradients in the sample. Fluctuation decay rates in xenon will be measured at least 100 times closer to the critical point than is possible on earth. This will require taking data as close as 145 microK to the critical temperature (Tc = 289.72 K), after locating it to + or - 10 microK. The minimum mission time of 100 hours will allow a complete range of temperature points to be covered, limited by the thermal response of the sample. An essential part of the apparatus is the thermostat. Its design principles and key features are given together with some observed performance characteristics of a prototype: + or - 20 microK stability for 18 hours, and one minute programmed step response for steps less than 10 mK.

Gammon, Robert W.↗

NASA NTRS1996

Orbital Injection of the SEDSAT Satellite: Tethered Systems Dynamics and Flight Data Analysis

This report deals with the following topics which are all related to the orbital injection of the SEDSAT satellite: Dynamics and Stability of Tether Oscillations after the First Cut. The dynamics of the tether after the first cut (i.e., without the Shuttle attached to it) is investigated. The tether oscillations with the free end are analyzed in order to assess the stability of the rectilinear configuration in between the two tether cuts; analysis of Unstable Modes. The unstable modes that appear for high libration angles are further investigated in order to determine their occurrences and the possible transition from bound librations to rotations; Orbital Release Strategies for SEDSAT. A parametric analysis of the orbital decay rate of the SEDSAT satellite after the two tether cuts has been carried out as a function of the following free parameters: libration amplitude at the end of deployment, deviation angle from LV at the first cut, and orbital anomaly at the second cut. The values of these parameters that provide a minimum orbital decay rate of the satellite (after the two cuts) have been computed; and Dynamics and Control of SEDSAT. The deployment control law has been modified to cope with the new ejection velocity of the satellite from the Shuttle cargo bay. New reference profiles have been derived as well as new control parameters. Timing errors at the satellite release as a function of the variations of the initial conditions and the tension model parameters have been estimated for the modified control law.

Lorenzini, Enrico C.↗

NASA NTRS1979

Carbon fiber behavior in an enclosed volume

Tests were performed to evaluate the behavior of single carbon fibers existing in an enclosed space such as a room of a building. Three general phenomena were explored: the concentration decay rate of a fiber-charged room, the degree of uniform mixing of fibers within a room, and the effects of fibers being redisseminated off deposition surfaces within a room. The results were required in understanding the ratio of total indoor fiber exposure to total outdoor fiber exposure, a quantity essential to risk analysis. Results indicate that decay rate is predictable within acceptable limits and that homogeneous mixing can always be assumed. Some factors of redissemination are identified and effects discussed.

Harvey, M. C.↗

NASA NTRS1985

Light Scattering Tests of Fundamental Theories of Transport Properties in the Critical Region

The objective of this program is to measure the decay rates of critical density fluctuations in a simple fluid (xenon) very near its liquid-vapor critical point using laser light scattering and photon correlation spectroscopy. Such experiments have been severely limited on Earth by the presence of gravity which causes large density gradients in the sample when the compressibility diverges approaching the critical point. The goal is to measure decay rates deep in the critical region where the scaled wavevector is the order of 1000. This will require loading the sample to 0.01% of the critical density and taking data as close as 3 microKelvin to the critical temperature (Tc = 289.72 K). Other technical problems have to be addressed such as multiple scattering and the effect of wetting layers. The ability to avoid multiple scattering by using a thin sample (100 microns) was demonstrated, as well as a temperature history which can avoid wetting layers satisfactory temperature control and measurement, and accurate sample loading. Thus the questions of experimental art are solved leaving the important engineering tasks of mounting the experiment to maintain alignment during flight and automating the state-of-the-art temperature bridges for microcomputer control of the experiment.

Gammon, R. W.↗

NASA NTRS1991

Radioactive satellites - Intact reentry and breakup by debris impact

There is a substantial mass of radioactive material in nuclear reactors or radioisotope thermal generators (RTGs) in orbit about the earth. This paper examines the reentry of intact nuclear fuel cores and RTGs and the fragmentation and subsequent radioactive debris cloud deposition and evolution resulting from the impact of orbital debris upon an orbiting reactor, fuel core, or RTG. To assess the intact reentry, decay rates and a predicted decay date using historical and projected orbital decay data, are estimated. The current NASA debris environment model is utilized to estimate impact rates and debris cloud evolution of a fragmentation event. Results of these analyses are compared and concepts are tendered which would tend to minimize the radiological debris hazard to personnel and structures both on the earth's surface and in low earth orbit.

Anz-Meador, P. D.↗

NASA NTRS2004

Signal Processing Variables for Optimization of Flaw Detection in Composites Using Ultrasonic Guided Wave Scanning

This study analyzes the effect of signal processing variables on the ability of the ultrasonic guided wave scan method at NASA Glenn Research Center to distinguish various flaw conditions in ceramic matrix composites samples. In the ultrasonic guided wave scan method, several time- and frequency-domain parameters are calculated from the ultrasonic guided wave signal at each scan location to form images. The parameters include power spectral density, centroid mean time, total energy (zeroth moment), centroid frequency, and ultrasonic decay rate. A number of signal processing variables are available to the user when calculating these parameters. These signal processing variables include 1) the time portion of the time-domain waveform processed, 2) integration type for the properties requiring integrations, 3) bounded versus unbounded integrations, 4) power spectral density window type, 5) and the number of time segments chosen if using the short-time fourier transform to calculate ultrasonic decay rate. Flaw conditions examined included delamination, cracking, and density variation.

Roth, Don J.↗

NASA NTRS1995

Highlights of the Zeno Results from the USMP-2 Mission

The Zeno instrument, a High-precision, light-scattering spectrometer, was built to measure the decay rates of density fluctuations in xenon near its liquid-vapor critical point in the low-gravity environment of the U.S. Space Shuttle. Eliminating the severe density gradients created in a critical fluid by Earth's gravity, we were able to make measurements to within 100 microKelvin of the critical point. The instrument flew for fourteen days in March, 1994 on the Space Shuttle Columbia, STS-62 flight, as part of the very successful USMP-2 payload. We describe the instrument and document its performance on orbit, showing that it comfortably reached the desired 3 microKelvin temperature control of the sample. Locating the critical temperature of the sample on orbit was a scientific challenge; we discuss the advantages and short-comings of the two techniques we used. Finally we discuss problems encountered with making measurements of the turbidity of the sample, and close with the results of the measurement of the decay rates of the critical-point fluctuations.

Gammon, Robert W.↗

NASA NTRS1979

Photochemistry of bromoacetylene - Formation of HBr and quenching of excited Br/4 2P 0 1/2/

The photolysis of bromacetylene is examined by means of vacuum ultraviolet flash photolysis-kinetic spectroscopy, with attention given to the production of Br atoms in the 2P 0 3/2 and 2P 0 1/2 states, the production of HBr, and UV absorption attributable to the ethynyl radical. Although Br(2P 0 1/2) and Br(2P 0 3/2) were both directly observed at the shortest delay time after the flash, strongly indicating the presence of the ethynyl radical, transient absorption by C2H was not detected in the range 125 to 180 nm. Quenching rate constants for the decay of the 2P 0 1/2 state of Br, which is observed to form concurrently with the ground state, are derived for quenching by He, CH2Br, CF4, and D2 from the pseudo-first-order decay rates. HBr was observed as a secondary photolysis product, and a model of its formation by the exothermic reaction of ground-state atoms with C2HBR, coupled with the secondary production of Br atoms, is found to agree well with the experiment.

Laufer, A. H.↗