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Results for “theoretical developments”

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 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Thermionic converter studies at Thermo Electron

The Thermionic Energy Converter (TEC) development program includes theoretical and experimental surface studies, diode evaluation, enhanced mode converter investigations and system studies, as well as materials fabrication and testing. The primary effort has been concentrated on increasing converter performance via improved electrodes. Diodes with oxide collectors (i.e., tungsten oxide, titanium oxide, zinc oxide and barium oxide) have provided the best performance. Emitters requiring reduced cesium pressure (such as oxygenated tungsten, lanthanum hexaboride, platinum and graphite) are being tested. Converter configurations using particle spacing are under investigation. Experiments with auxiliary electrode thermionic converters are in progress. Both silicon carbide and alloy hot shells (i.e., barriers to isolate the converter from the combustion environment) have been fabricated and tested.

Huffman, F. N.↗
NASA NTRS1977

Analysis of simple 2-D and 3-D metal structures subjected to fragment impact

Theoretical methods were developed for predicting the large-deflection elastic-plastic transient structural responses of metal containment or deflector (C/D) structures to cope with rotor burst fragment impact attack. For two-dimensional C/D structures both, finite element and finite difference analysis methods were employed to analyze structural response produced by either prescribed transient loads or fragment impact. For the latter category, two time-wise step-by-step analysis procedures were devised to predict the structural responses resulting from a succession of fragment impacts: the collision force method (CFM) which utilizes an approximate prediction of the force applied to the attacked structure during fragment impact, and the collision imparted velocity method (CIVM) in which the impact-induced velocity increment acquired by a region of the impacted structure near the impact point is computed. The merits and limitations of these approaches are discussed. For the analysis of 3-d responses of C/D structures, only the CIVM approach was investigated.

Witmer, E. A.↗
NASA NTRS1978

Jet pump assisted arterial heat pipe

This paper discusses the concept of an arterial heat pipe with a capillary driven jet pump. The jet pump generates a suction which pumps vapor and noncondensible gas from the artery. The suction also forces liquid into the artery and maintains it in a primed condition. A theoretical model was developed which predicts the existence of two stable ranges. Up to a certain tilt the artery will prime by itself once a heat load is applied to the heat pipe. At higher tilts, the jet pump can maintain the artery in a primed condition but self-priming is not possible. A prototype heat pipe was tested which self-primed up to a tilt of 1.9 cm, with a heat load of 500 watts. The heat pipe continued to prime reliably when operated as a VCHP, i.e., after a large amount of noncondensible gas was introduced.

Bienert, W. B.↗
NASA NTRS1978

Prediction of plasma properties in mercury ion thrusters

A simplified theoretical model was developed which obtains to first order the plasma properties in the discharge chamber of a mercury ion thruster from basic thruster design and controllable operating parameters. The basic operation and design of ion thrusters is discussed, and the important processes which influence the plasma properties are described in terms of the design and control parameters. The conservation for mass, charge and energy were applied to the ion production region, which was defined as the region of the discharge chamber having as its outer boundary the surface of revolution of the innermost field line to intersect the anode. Mass conservation and the equations describing the various processes involved with mass addition and removal from the ion production region are satisfied by a Maxwellian electron density spatial distribution in that region.

Longhurst, G. R.↗
NASA NTRS1978

Blown flap noise prediction

Theoretical and experimental developments of flow-surface interaction noise with a particular emphasis on blown-flap noise were reviewed. Several blown-flap noise prediction methods were evaluated by comparing predicted acoustic levels, directivity, and spectra with a recently obtained data base. A prediction method was selected and a detailed step-by-step description of this method was provided to develop a computer module to calculate one-third octave band frequency spectra at any given location in the far-field for under-the-wing and upper surface blown configurations as a function of geometric and operational parameters.

Reddy, N. N.↗
NASA NTRS1979

Statistical characterization of the fatigue behavior of composite lamina

A theoretical model was developed to predict statistically the effects of constant and variable amplitude fatigue loadings on the residual strength and fatigue life of composite lamina. The parameters in the model were established from the results of a series of static tensile tests and a fatigue scan and a number of verification tests were performed. Abstracts for two other papers on the effect of load sequence on the statistical fatigue of composites are also presented.

Yang, J. N.↗
NASA NTRS1979

Numerical optimization techniques for bound circulation distribution for minimum induced drag of nonplanar wings: Basic formulations

A theoretical method was developed for determining the optimum span load distribution for minimum induced drag for subsonic nonplanar configurations. The undistorted wing wake is assumed to have piecewise linear variation of shed vortex sheet strength, resulting in a quadratic variation of bound circulation and span load. The optimum loading is obtained either through a direct technique, whereby derivatives of the drag expression are calculated analytically in terms of the unknown wake vortex sheet strengths. Both techniques agree well with each other and with available exact solutions for minimum induced drag.

Kuhlman, J.↗
NASA NTRS1979

Radial diffusion of Io-injected plasma

The paper reexamines the problem of particle injection by Io and subsequent radial diffusion by flux tube interchange using a proper phase space density formulation. The mathematical formalism is developed, and the theoretical results are compared with the observations, taking into account the pitch angle and energy coverage of the detector on Pioneer 10. Two objectives are pursued: (1) to test the hypothesis of Siscoe and Chen (1977) that Io is the source for all of the plasma observed by Frank et al. (1976) inside 10 R(J); and (2) to describe a simple but flexible method intended to analyze the observations of any other plasma instrument flown through the inner Jovian magnetosphere, e.g., those on Voyager 1 and 2 and Galileo.

Goertz, C. K.↗
NASA NTRS1979

On solar cosmic rays

The report summarizes the major results obtained over the last four years in the following areas: (1) transport in interplanetary space which includes solar particle observations combined with theoretical efforts to develop models which simulate the dynamics of the event; (2) transport in the solar corona and release of particles in the interplanetary medium; and (3) characteristics of source spectra and acceleration models. Modelling the propagation of solar cosmic rays through interplanetary space and establishing the magnitude and the radial and energy dependence of the diffusion coefficient K are considered. An important result due to statistical observation is that the coronal propagation must be independent of or weakly dependent on both rigidity and energy. Increasing evidence is found for prolonged injection at the sun.

Van Hollebeke, M. A. I.↗
NASA NTRS1980

Calculation of laminar heating rates on three-dimensional configurations using the axisymmetric analogue

A theoretical method was developed for computing approximate laminar heating rates on three dimensional configurations at angle of attack. The method is based on the axisymmetric analogue which is used to reduce the three dimensional boundary layer equations along surface streamlines to an equivalent axisymmetric form by using the metric coefficient which describes streamline divergence (or convergence). The method was coupled with a three dimensional inviscid flow field program for computing surface streamline paths, metric coefficients, and boundary layer edge conditions.

Hamilton, H. H., II↗
NASA NTRS1980

Acoustic plane waves incident on an oblique clamped panel in a rectangular duct

The theory of acoustic plane waves incident on an oblique clamped panel in a rectangular duct was developed from basic theoretical concepts. The coupling theory between the elastic vibrations of the panel (plate) and the oblique incident acoustic plane wave in infinite space was considered in detail, and was used for the oblique clamped panel in the rectangular duct. The partial differential equation which governs the vibrations of the clamped panel (plate) was modified by adding to it stiffness (spring) forces and damping forces. The Transmission Loss coefficient and the Noise Reduction coefficient for oblique incidence were defined and derived in detail. The resonance frequencies excited by the free vibrations of the oblique finite clamped panel (plate) were derived and calculated in detail for the present case.

Unz, H.↗
NASA NTRS1979

Electron diffusion through the baffle aperture of a hollow cathode thruster

The use of a hollow cathode in place of an oxide cathode to increase thruster operating lifetimes requires, among other things, the addition of a baffle to restrict the flow of electrons from the hollow cathode. A theoretical model is developed which relates the baffle aperture area of a hollow-cathode thruster to the magnetic flux density and plasma properties in the aperture region, with the result that this model could be used as an aid in thruster design. Extensive Langmuir probing is undertaken to verify the validity of the model and demonstrate its capability. It is shown that the model can be used to calculate the aperture area required to effect discharge operation at a specified discharge voltage and arc current.

Brophy, J. R.↗
NASA NTRS1981

Study of noise reduction characteristics of multilayered panels and dual pane windows with Helmholtz resonators

The experimental noise attenuation characteristics of flat, general aviation type, multilayered panels are discussed. Experimental results of stiffened panels, damping tape, honeycomb materials and sound absorption materials are presented. Single degree of freedom theoretical models were developed for sandwich type panels with both shear resistant and non-shear resistant core material. The concept of Helmholtz resonators used in conjunction with dual panel windows in increasing the noise reduction around a small range of frequency was tested. It is concluded that the stiffening of the panels either by stiffeners or by sandwich construction increases the low frequency noise reduction.

Navaneethan, R.↗
NASA NTRS1981

Detection of gratings and small features in speckle imagery

The extent of picture degradation of speckle, in particular in synthetic aperture radar pictures, has been investigated in the cases where an observer has to detect (1) a small feature immersed in a darker background, and (2) a square wave grating. In each case, a theoretical model is developed for the observer's detection mechanism, and the probability of correct decision is related to relevant picture parameters such as contrast, looks per pixel, and size. These calculations are verified by psychophysical experiments using computer-simulated pictures. Detectability of gratings as a criterion for characterizing picture quality is shown to be far inferior to feature detectability.

Korwar, V. N.↗
NASA NTRS1981

Discrimination of form in images corrupted by speckle

The problem investigated is that of a human observer having to distinguish between certain specified geometrical forms corrupted by speckle - an idealization of the problem of a scientist studying a synthetic aperture radar map. Specifically, the cases of two simple alternative forms and of two and four orientations of a simple form have been considered. A theoretical model is developed for the observer's decision process by analogy with optimal receiver theory, and the probability of a correct decision is related to form parameters like size, contrast, and looks. These calculations are verified by psychophysical experiments using computer-simulated pictures.

Korwar, V. N.↗
NASA NTRS1981

Vortex-flow aerodynamics - An emerging design capability

Promising current theoretical and simulational developments in the field of leading edge vortex-generating delta, arrow ogival wings are reported, along with the history of theory and experiment leading to them. The effects of wing slenderness, leading edge nose radius, Mach number and incidence variations, and planform on the onset of vortex generation and redistribution of aerodynamic loads are considered. The range of design possibilities in this field are consequential for the future development of strategic aircraft, supersonic transports and commercial cargo aircraft which will possess low-speed, high-lift capability by virtue of leading edge vortex generation and control without recourse to heavy and expensive leading edge high-lift devices and compound airfoils. Attention is given to interactive graphics simulation devices recently developed.

Campbell, J. F.↗
NASA NTRS1981

Edge delamination in angle-ply composite laminates, part 5

A theoretical method was developed for describing the edge delamination stress intensity characteristics in angle-ply composite laminates. The method is based on the theory of anisotropic elasticity. The edge delamination problem is formulated using Lekhnitskii's complex-variable stress potentials and an especially developed eigenfunction expansion method. The method predicts exact orders of the three-dimensional stress singularity in a delamination crack tip region. With the aid of boundary collocation, the method predicts the complete stress and displacement fields in a finite-dimensional, delaminated composite. Fracture mechanics parameters such as the mixed-mode stress intensity factors and associated energy release rates for edge delamination can be calculated explicity. Solutions are obtained for edge delaminated (theta/-theta theta/-theta) angle-ply composites under uniform axial extension. Effects of delamination lengths, fiber orientations, lamination and geometric variables are studied.

Wang, S. S.↗
NASA NTRS1981

On-line optimization of aircraft altitude and flight path angle dynamics

The optimization of aircraft altitude and flight path angle dynamics is addressed in a form suitable for on-line computation and control. The approach here is a direct extension of the work reported by Calise (1979), where singular perturbation methods were used to optimize position, energy and heading dynamics; it thus represents an optimal control solution that models all of the primary trajectory related dynamics. It is pointed out that the resulting algorithm can be regarded as a nonlinear feedback control law. The minimum time intercept of a fixed terminal point is used in setting the framework in which the analytical results are developed. The main theoretical result is that the dynamics, while not completely separable, can be approximated by singular perturbation methods when the control model includes relative position dynamics.

Calise, A. J.↗