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At least 235 records · Page 13

All sources NASA NTRS DOE OSTI

NASA NTRS1983

Ultrasonic attenuation of a void-containing medium for very long wavelengths

Ultrasonic longitudinal through-thickness attenuation in an isotropic medium due to scattering by randomly distributed voids is considered analytically. The attenuation is evaluated on the assumption of no interaction between voids. The scattered power is assumed to be entirely lost, thus accounting for the ultrasonic attenuation. The scattered power due to the presence of a void is described in terms of the scattering cross section of the void. An exact solution exists for the scattering cross section of a spherical void. An approximate solution for the scattering cross section of an ellipsoidal void is developed based on the so-called Born approximation commonly used in quantum mechanics. This approximate solution is valid for k sub p a sub i 1, where k sub p is the wave number of the incident longitudinal wave and a sub i is the largest dimension of the void. It is found that the shape of the void has negligible effect on the scattering cross section and that only the volume of the void is important. Thus, it is noted that in cases where k sup p a sub i 1, the exact scattering cross section of a spherical void having the same volume as an arbitrarily shaped void can be used for evaluating ultrasonic attenuation.

Williams, J. H., Jr.↗

NASA NTRS1984

Ultrasonic attenuation of a void-containing medium for very long wavelengths

Ultrasonic longitudinal through-thickness attenuation in an isotropic medium due to scattering by randomly distributed voids is considered analytically. The attenuation is evaluated on the assumption of no interaction between voids. The scattered power is assumed to be entirely lost, thus accounting for the ultrasonic attenuation. The scattered power due to the presence of a void is described in terms of the scattering cross section of the void. An exact solution exists for the scattering cross section of a spherical void. An approximate solution for the scattering cross section of an ellipsoidal void is developed based on the so-called Born approximation commonly used in quantum mechanics. This approximate solution is valid for k sub p a sub i much less than 1, where k sub p is the wave number of the incident longitudinal wave and a sub i is the largest dimension of the void. It is found that the shape of the void has negligible effect on the scattering cross section and that only the volume of the void is important. Thus, it is noted that in cases where k sub p a sub i is much less than 1, the exact scattering cross section of a spherical void having the same volume as an arbitrarily shaped void can be used for evaluating ultrasonic attenuation. Previously announced in STAR as N83-28466

Williams, J. H., Jr.↗

NASA NTRS1985

Analytical prediction of labyrinth-seal-flow-induced arotor excitation forces

An analytical method to calculate the rotor excitation forces arising from labyrinth seals is presented. The objective is to model the gas flow through the seal clearance passages and cavities when the rotor is positioned eccentricly relative to the stator center. The seal flow model used in the analysis yields solutions which validate the experimentally observed influence of seal parameters on seal forces reported in the literature. The analytically predicted seal pressure distributions and forces were compared with published experimental results.

Rajakumar, C.↗

NASA NTRS1985

The structure of high-temperature solar flare plasma in non-thermal flare models

Analytic differential emission measure distributions have been derived for coronal plasma in flare loops heated both by collisions of high-energy suprathermal electrons with background plasma, and by ohmic heating by the beam-normalizing return current. For low densities, reverse current heating predominates, while for higher densities collisional heating predominates. There is thus a minimum peak temperature in an electron-heated loop. In contrast to previous approximate analyses, it is found that a stable reverse current can dominate the heating rate in a flare loop, especially in the low corona. Two 'scaling laws' are found which relate the peak temperature in the loop to the suprathermal electron flux. These laws are testable observationally and constitute a new diagnostic procedure for examining modes of energy transport in flaring loops.

Emslie, A. G.↗

NASA NTRS1989

Lifting-surface theory for propfan vortices impinging on a downstream wing

Retrofitment of commercial aircraft with propfans could introduce undesirable aerodynamic sources of structure-borne noise that are absent for current turbojet powerplants. This paper theoretically examines the whipping action of the vortex wake from a generic propeller on the downstream rigid wing that supports it. The model addresses the high-frequency/compressible regime of most anticipated propfan implementations and produces an analytic solution for the distributed wing airload due to the periodic vortex impingement. The analysis also yields an expression for the local unsteady lift obtained from integration over an arbitrary internal patch of wing surface, for the purpose of applying a practical number of such forces at the nodes of a finite-element model for the corresponding structure (wing response results are not included in the present paper). Reported estimates of induced wing loads for a conventional-propeller example of demonstration appear to be in the reasonable range of expectation.

Martinez, R.↗

NASA NTRS1989

Infall patterns around rich clusters of galaxies

The pattern of infall velocities induced by a rich cluster of galaxies is considered, using an infall model based on the Friedmann solution to determine the exact implicit dependence of the peculiar velocity on the density enhancment and the mean cosmological mass density, Omega(0). An analytic model for the distribution of galaxies around a cluster core in redshift space is developed. The high-density caustics in redshift space are shown to appear as envelopes around rich clusters. Assuming that the galaxies trace the matter distribution, low Omega(0) models can explain observational data obtained for four clusters. The present results support the prediction that light traces mass in the infall region.

Regos, Eniko↗

NASA NTRS1989

Coordination in a hierarchical multi-actuator controller

A hierarchical multi-actuator controller is represented as a multi-resolutional information (knowledge) system utilizing a number of intelligent modules with decision making capabilities. The laws of multi-resolutional information (knowledge) organization and processing are presumed to be satisfied including the rules of dealing with redundant knowledge. A general case is considered in which a process to be controlled by a multiplicity of actuators is a distributed one and the condition of distribution can be formulated analytically. Operation of a lumped multi-actuator process is a particular case which has a broad practical application.

Meystel, Alex↗

NASA NTRS1990

Three-dimensional analytical model for the spatial variation of the foreshock electron distribution function - Systematics and comparisons with ISEE observations

A model which is consistent with the solar wind and shock surface boundary conditions for the foreshock electron distribution in the absence of wave-particle effects is formulated for an arbitrary location behind the magnetic tangent to the earth's bow shock. Variations of the gyrophase-averaged velocity distribution are compared and contrasted with in situ ISEE observations. It is found that magnetic mirroring of solar wind electrons is the most important process by which nonmonotonic reduced electron distributions in the foreshock are produced. Leakage of particles from the magnetosheath is shown to be relatively unimportant in determining reduced distributions that are nonmonotonic. The two-dimensional distribution function off the magnetic field direction is the crucial contribution in producing reduced distributions which have beams. The time scale for modification of the electron velocity distribution in velocity space can be significantly influenced by steady state spatial gradients in the background imposed by the curved shock geometry.

Fitzenreiter, R. J.↗

NASA NTRS1991

Continuous wave coherent laser radar - Calculation of measurement location and volume

The SNR equation of a CW, monostatic, coherent laser radar system is examined for the case of a distributed aerosol target. General analytic equations are derived which explicitly describe the contribution to total SNR from each range interval along the optic axis, the range resolution of the measurement, and the atmospheric volume contributing to the measurement. Plots are presented showing the location in range and the volume of the laser radar measurement.

Kavaya, Michael J.↗

NASA NTRS1950

The application of the statistical theory of extreme values to gust-load problems

An analysis is presented which indicates that the statistical theory of extreme values is applicable to the problems of predicting the frequency of encountering the larger gust loads and gust velocities for both specific test conditions as well as commercial transport operations. The extreme-value theory provides an analytic form for the distributions of maximum values of gust load and velocity. Methods of fitting the distribution are given along with a method of estimating the reliability of the predictions. The theory of extreme values is applied to available load data from commercial transport operations. The results indicate that the estimates of the frequency of encountering the larger loads are more consistent with the data and more reliable than those obtained in previous analyses. (author)

Press, Harry↗

NASA NTRS1993

The role of NASTRAN in the preliminary design cycle

This paper explains how NASTRAN can be utilized advantageously in the preliminary design cycle. The initial portion of the preliminary design process lends itself to programs that can produce multiple configurations or variations on a particular design with minimal cost or effort. The latter portion of the process encompasses refining the design and adding more detailed analyses (particularly for other disciplines). A method for quickly generating balanced spacecraft loading conditions for use in preliminary design and analysis also is explained. The following additional sections are included: Background, Symbols, Analytical Process, Aerodynamic Load Distributions, NASTRAN Applications, Conclusion and References.

Grooms, H. R.↗

NASA NTRS1999

Ion Transport Modeling in Non-Collisional RF Sheaths

A reactor scale ICP plasma model does not adequately resolve the submillimeter scale rf sheaths due to severe disparities in length and times scales. These non-collisional sheaths determine the spread in the ion bombardment energy distribution on the wafer. Analytical models based on "damped potential" has been used by various authors to simplify sheath modeling. However, inherent in these models are some assumptions that may be invalid. In this work we show that simplifying assumptions such as a uniform ion flux may be in error by as much as 200% at 13.56MHz. This work involves solving ion transport in non-collisional rf sheaths. A comprehensive study of the dependence of ion flux and energy on uncertainties in the sheath-presheath (i.e. sheath thickness) boundary will also be done.

Meyyappan, M.↗

NASA NTRS1979

Comparative study of solar optics for paraboloidal concentrators

Different analytical methods for computing the flux distribution on the focal plane of a paraboloidal solar concentrator are reviewed. An analytical solution in algebraic form is also derived for an idealized model. The effects resulting from using different assumptions in the definition of optical parameters used in these methodologies are compared and discussed in detail. These parameters include solar irradiance distribution (limb darkening and circumsolar), reflector surface specular spreading, surface slope error, and concentrator pointing inaccuracy. The type of computational method selected for use depends on the maturity of the design and the data available at the time the analysis is made.

Wen, L.↗

NASA NTRS1986

Global distribution and migration of subsurface ice on Mars

An analytical model is developed for the factors which changed the initial distribution and state of H2O, mainly trapped as subsurface ice, on Mars. A uniform initial ice distribution is assumed, and changes in ice layer thickness are calculated at discrete latitudes of Mars. The thermal/diffusive model used for the calculations accounts for variations in the solar luminosity, the planetary obliquity and eccentricity, with obliquity starting 3.5 Gyr ago, and insolation at various latitudes. Attention is given to variations in the albedo due to seasonal CO2 variations, in the regolith temperature, and in atmospheric pressure. Migration of H2O into and out of the regolith is a function of the H2O vapor temperature at the subsurface ice boundary and the annual average H2O at the base of the atmosphere. Projections of the ice thickness over 100 obliquity periods indicate that the intensity of the obliquity effects are latitudinally dependent. H2O atmospheric concentrations are projected to vary up to six orders of magnitude over an obliquity cycle.

Fanale, F. P.↗

DOE OSTI2026

Third integer resonant extraction transit time simulation studies

In this work, we present the investigation of transit time of particles in the non-linear third-integer resonant extraction process. Transit time is defined as the number of turns a particle takes to get extracted once it is in the unstable region in the phase space, i.e., outside the triangular separatrix in case of third-integer resonance. The study of transit time is important because transit time directly contributes to the beam response time during resonant extraction and thus knowing it apriori would be practically useful in designing of the extraction system. In this work, we shall investigate the analytical derivation of the transit time of particles (to the first order Kobayashi Hamiltonian) in different parts of the phase space distribution and compare against the analytical results. We also compare the simulation result of the transit time of particles (with higher statistics) for the static as well as dynamic extraction conditions cases, particularly in the context of resonant extraction parameters for Mu2e experiment at Fermilab.

Narayanan, Aakaash [Fermilab]↗

DOE OSTI2025

Third Integer Resonant Extraction Transit Time Simulation Studies

Narayanan, Aakaash [Fermilab] (ORCID:0000000157944↗

NASA NTRS1976

FLEXSTAB analysis

The wing differential pressure distributions of two vehicles are predicted using FLEXSTAB. Three space shuttle configurations were investigated. Comparisons were made of wind tunnel and analytical data for a selected range of angle of attack, angle of sideslip, and Mach number. A drone-type vehicle was investigated and comparisons were made between flight and analytical data of wing pressure distributions and static longitudinal stability derivatives.

Lowder, H. E., Jr.↗

DOE OSTI2020

Comparative investigations of multi-fidelity modeling on performance of electrostatically-actuated cracked micro-beams

Silicon is a commonly used material for the fabrication of beams for use in micro-electrical-mechanical systems (MEMS). Although silicon is a brittle material, it has been shown to accumulate fatigue damage at the micro-scale. Understanding the effect this has on the overall device performance is critical to the design of reliable devices. Analytical methods for modeling damage provide expedient results but are limited by broad modeling assumptions. Numerical models account for more detailed physical phenomena but can be computationally intensive. In this work, two different crack scenarios are modeled using both analytical techniques and 3D computational simulations. First, the effects of a single surface crack on the static deflection and natural frequency of an electrostatically actuated micro-beam are formulated and compared. Then, a new method for approximating damage associated with realistic distributed crack networks is formulated for use in an analytical model and numerical simulations. A method for utilizing experimentally derived crack statistics to inform the analytical and numerical distributed crack models is developed. Good agreement between the analytical and numerical models is obtained for both crack scenarios. Altogether, these models can be used to effectively simulate a variety of damage and fatigue behaviors in silicon-based MEMS devices.

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