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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

An upper limit to the atomic carbon abundance in the Orion plateau

Observations made of the atomic carbon line at 492 GHz toward OMC-1 show no evidence for the high velocity dispersion wings observed for many molecular rotational lines. The 3sigma upper limit to the CI column density, NCI, is 6.9 x 10 to the 17th per sq cm for velocities greater than or equal to 4 km per sec from the line center. This upper limit corresponds to a ratio of CI to CO abundances as low as less than 0.13, depending on the assumed CO column density. Atomic carbon is apparently depleted by a factor as large as five in the hot plateau gas, relative to its abundance in other molecular clouds. The lack of CI in the plateau source may mean that the shocks thought to be present in the region are not dissociative in nature and thus do not produce the UV radiation required to convert CO into CI.

Beichman, C. A.↗
NASA NTRS1982

Vibration limiting of rotors by feedback control

Experimental findings of a three mass rotor with four channels of feedback control are reported. The channels are independently controllable with force being proportional to the velocity and/or instantaneous displacement from equilibrium of the shaft at the noncontacting probe locations (arranged in the vertical and horizontal attitudes near the support bearings). The findings suggest that automatic feedback control of rotors is feasible for limiting certain vibration levels. Control of one end of a rotor does afford some predictable vibration limiting of the rotor at the other end.

Lewis, D. W.↗
NASA NTRS1983

Limited flight test experience with a laser transit velocimeter

Limited flight testing of a laser transit velocimeter provided insight into the problems associated with the use of such instruments for flight research. Although the device tested was not designed for flight application, it had certain features such as fiber optics and low laser power which are attractive in the airborne environment. During these tests, operation of the velocimeter was limited by insufficient concentrations of light-scattering particles and background light interference. Normal operation was observed when these conditions were corrected by utilizing cloud particles and flying at night. A comparison between the laser flow velocity measurements and corresponding pressure measurements is presented and shows a coarse correlation. Statistical bias due to turbulence in the flow is suspected to have affected the laser measurements.

Curry, R. E.↗
NASA NTRS1983

Capabilities, limitations, and use of BC SAT-R2 conference software

The computer software developed for the BC SAT-R2 Conference has certain capabilities and limitations which are described. Capabilities of each major program element are addressed with respect to providing the required functions for planning and output reporting. Limitations arise from the inability to exactly represent certain systems that may be examined. Expected use of the software package during the Conference is outlined.

Miller, E. F.↗
NASA NTRS1982

Diffuse galactic gamma-ray line emission from nucleosynthetic Fe-60, Al-26, and Na-22 - Preliminary limits from HEAO 3

Data obtained during a two-week period in the fall of 1979 with the HEAO 3 gamma-ray spectroscopy experiment have been searched for diffuse galactic plane gamma-ray line emission expected t4 result from the decay of nucleosynthetic Fe-60, Al-26, and Na-22. With the possible exception of the 1809 keV line from Al-26 decay, for which a 2.6-omicron cosmic excess of (6.0 + or - 2.3) x 0.0001 photons/sq cm per sec per rad was measured, no positive detection was made. However, new limits ranging from 1.8 to 11 times 0.0001 photons/sq cm per sec per rad, at the 3-omicron level of confidence, have been placed on diffuse emission in these lines from the vicinity of the galactic center (between -30 and 30 deg). These limits are lower than some theories predict and thus place new constraints on the yields of these radionuclides in explosive nucleosynthesis and on the present rate of galactic nucleosynthesis.

Mahoney, W. A.↗
NASA NTRS1983

The Eddington limit and supercritical accretion. II - Time-dependent calculations

Spherically symmetric, time-dependent accretion of an ionized hydrogen plasma onto a neutron star is calculated for accretion rates in excess of the Eddington limit. The coupled hydrodynamic and frequency integrated radiative transfer equations are solved on an Eulerian grid for these supercritical accretion flows. Our results indicate that steady state flows are limited to rates at or below the critical rate, with emergent luminosities equal to or less than the Eddington luminosity. Initially supercritical accretion rates generate a large pulse of radiation which reduces the accretion rate to the critical value and produces an extended quasi-static envelope.

Burger, H. L.↗
NASA NTRS1983

Self-regulating star formation - The rate limit set by ionizing photons

In the present investigation the possibility is explored that during galaxy formation the gas has the capacity to generate stars so rapidly that the back reaction of the young stars on the gas sets the limiting rate. Sources which could supply significant negative feedback include stellar winds, ionizing UV, and supernova explosions. However, only the simplest of these, the heating by ionizing UV, is currently considered. The UV production associated with a given amount of star formation is calculated, and the critical UV production rate above which the UV heating quenches star formation is estimated. It is argued that in the absence of more powerful limiting mechanisms the star formation will stabilize to provide this critical UV emissivity. It is found that this rate is in fact similar to that which galaxy formation models imply has actually occurred.

Cox, D. P.↗
NASA NTRS1984

Lean limit phenomena

The concept of flammability limits in the presence of flame interaction, and the existence of negative flame speeds are discussed. Downstream interaction between two counterflow premixed flames of different stoichiometries are experimentally studied. Various flame configurations are observed and quantified; these include the binary system of two lean or rich flames, the triplet system of a lean and a rich flame separated by a diffusion flame, and single diffusion flames with some degree of premixedness. Extinction limits are determined for methane/air and butane/air mixtures over the entire range of mixture concentrations. The results show that the extent of flame interaction depends on the separation distance between the flames which are functions of the mixtures' concentrations, the stretch rate, and the effective Lewis numbers (Le). In particular, in a positively-stretched flow field Le 1 ( 1) mixtures tend to interact strongly (weakly), while the converse holds for flames in a negatively-stretched flow. Also established was the existence of negative flames whose propagation velocity is in the same general direction as that of the bulk convective flow, being supported by diffusion alone. Their existence demonstrates the tendency of flames to resist extinction, and further emphasizes the possibility of very lean or rich mixtures to undergo combustion.

Law, C. K.↗
NASA NTRS1984

Investigation, development and application of optimal output feedback theory. Volume 2: Development of an optimal, limited state feedback outer-loop digital flight control system for 3-D terminal area operation

This report contains the development of a digital outer-loop three dimensional radio navigation (3-D RNAV) flight control system for a small commercial jet transport. The outer-loop control system is designed using optimal stochastic limited state feedback techniques. Options investigated using the optimal limited state feedback approach include integrated versus hierarchical control loop designs, 20 samples per second versus 5 samples per second outer-loop operation and alternative Type 1 integration command errors. Command generator tracking techniques used in the digital control design enable the jet transport to automatically track arbitrary curved flight paths generated by waypoints. The performance of the design is demonstrated using detailed nonlinear aircraft simulations in the terminal area, frequency domain multi-input sigma plots, frequency domain single-input Bode plots and closed-loop poles. The response of the system to a severe wind shear during a landing approach is also presented.

Broussard, J. R.↗
NASA NTRS1983

Differential versus limiter-discriminator detection of narrow-band FM

The error probability performance of differential detection of narrow-band FM is determined and compared with the analogous results for limiter-discriminator detection of the same modulation. It is shown that over a large class of benign and hostile environments, e.g., Gaussian IF filter, AWGN, partial-band noise jamming, the differential detector offers no theoretical performance advantage over the limiter-discriminator receiver with integrate-and-dump postdetection filtering.

Simon, M. K.↗
NASA NTRS1983

The stability of coronal loops - Finite-length and pressure-profile limits

Results are described from a quickly converging, necessary-and-sufficient, MHD-stability test for coronal-loop models. The primary stabilizing influence arises from magnetic line tying at the photosphere, and this end conditions requires a series expansion of possible loop excitations. The stability boundary is shown to quickly approach a limit as the number of terms increases, providing a critical length for the loop in proportion to its transverse magnetic scale. Several models of force-free-field profiles are tested and the stability behavior of a localized current channel, embedded in an external current-free region, is shown to be superior to that of other, broader, current profiles. Pressure-gradient effects, leading to increased or decreased stability, are shown to be amplified by line tying. Long loops must either conduct low net current, or exhibit an axial-field reversal coexisting with a low-pressure core. The limits on stability depend on the magnetic aspect ratio, the plasma-to-magnetic pressure ratio, and the field orientation at the loop edge. Applications of these results to the structure of coronal loops are described.

Einaudi, G.↗
NASA NTRS1984

A numerical analysis of contact and limit-point behavior in a class of problems of finite elastic deformation

Finite element methods for the analysis of bifurcations, limit-point behavior, and unilateral frictionless contact of elastic bodies undergoing finite deformation are presented. Particular attention is given to the development and application of Riks-type algorithms for the analysis of limit points and exterior penalty methods for handling the unilateral constraints. Applications focus on the problem of finite axisymmetric deformations, snap-through, and inflation of thick rubber spherical shells.

Endo, T.↗
NASA NTRS1984

Limits on the infrared and visual luminosity of the intergalactic H I cloud in Leo

Low surface brightness photometry of the intergalactic neutral hydrogen cloud in Leo sets upper limits on the intensity of this object of 28.0 mag/sq arcsec in the V photometric band and 22.8 mag/sq arcsec in the K band. The corresponding ratio of the H I mass to blue luminosity in the H I emission peaks of the cloud must be greater than 2.9 (solar units) depending on the star formation history of the object. The limits imposed on the star formation rate suggest a strong dependence of this rate on gas density.

Skrutskie, M. F.↗
NASA NTRS1985

Geomorphic classification of Icelandic and Martian volcanoes: Limitations of comparative planetology research from LANDSAT and Viking orbiter images

Some limitations in using orbital images of planetary surfaces for comparative landform analyses are discussed. The principal orbital images used were LANDSAT MSS images of Earth and nominal Viking Orbiter images of Mars. Both are roughly comparable in having a pixel size which corresponds to about 100 m on the planetary surface. A volcanic landform on either planet must have a horizontal dimension of at least 200 m to be discernible on orbital images. A twofold bias is directly introduced into any comparative analysis of volcanic landforms on Mars versus those in Iceland because of this scale limitation. First, the 200-m cutoff of landforms may delete more types of volcanic landforms on Earth than on Mars or vice versa. Second, volcanic landforms in Iceland, too small to be resolved or orbital images, may be represented by larger counterparts on Mars or vice versa.

Williams, R. S., Jr.↗
NASA NTRS1985

Limits to the burster repetition rate as deduced from the 2nd catalog of the interplanetary network

The burster repetition rate is an important parameter in many gamma ray burst models. The localizations of the interplanetary network, which have a relatively small combined surface area, may be used to estimate the average repetition rate. The method consists of (1) estimating the number of random overlaps between error boxes expected in the catalog and comparing this number to that actually observed; (2) modeling the response of the detectors in the network, so that the probability of detecting a burst can be estimated; and (3) simulating the arrival of bursts at the network assuming that burster repetition is governed by a Poisson process. The application of this method for many different burster luminosity functions shows that (1) the lower limit to the burster repetition rate depends strongly upon the assumed luminosity function; (2) the best lower limit to the repetition period obtainable from the data of the network is about 100 months; and (3) that a luminosity function for all bursters similar to that of the 1979 Mar 5 burster is inconsistent with the data.

Atteia, J. L.↗
NASA NTRS1985

An upper limit of muon flux of energies above 100 TeV determined from horizontal air showers observed at Akeno

Muon energy spectrum above 100 TeV was determined by observing the extensive air showers (EAS) from the horizontal direction (HAS). No definite muon originated shower of sizes above 100,000 and zenith angles above 60 deg was observed. The upper limits of HAS intensity is 5x10/12 m/2 s/1 sn/1 above 100,000. It is indicated that the upper limit of muon flux above 100 TeV is about 1.3x10/8 m/2 s/1 sr/1 and is in agreement with that expected from the primary spectrum with a knee assuming scaling in the fragmentation region and 40% protons in the primary beam. The critical energy at which muon flux from prompt processes take over that from the conventional process is higher than 100 Tev at horizontal direction.

Nagano, M.↗
NASA NTRS1985

Limits on deeply penetrating particles in the 10(17) eV cosmic ray flux

Deeply penetrating particles in the 10 to the 17th power eV cosmic ray flux were investigated. No such events were found in 8.2 x 10 to the 6th power sec of running time. Limits were set on the following: quark-matter in the primary cosmic ray flux; long-lived, weakly interacting particles produced in p-air collisions; the astrophysical neutrino flux. In particular, the neutrino flux limit at 10 to the 17th power eV implies that z, the red shift of maximum activity is 10 in the model of Hill and Schramm.

Baltrusaitis, R. M.↗