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At least 235 records · Page 13

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NASA NTRS1974

A dust-shell model of the infrared object HD 45677

A dust-shell model of the infrared star HD-45677 was constructed by means of a generalization of the mathematical techniques of Huang. The model shows that an isotropically scattering, uniform-density spherical circumstellar dust shell is able to account for the observed emergent energy distribution. The extinction properties of the grains required depend on the energy distribution incident on the inner boundary of the shell such that gray grains are needed if the stellar temperature is about 13,000 K and a reciprocal wavelength extinction law is indicated for a star of 20,000 K. This model for HD 45677 is compared with that of Dyck and Milkey, who propose free-free emission from a circumstellar gas shell to account for the observed infrared excess.

Apruzese, J. P.↗

NASA NTRS1974

Shell deformation studies using holographic interferometry

The buckling of shallow spherical shells under pressure has been the subject of many theoretical and experimental papers. Experimental data above the theoretical buckling load of Huang have given rise to speculation that shallow shell theory may not adequately predict the stability of nonsymmetric modes in higher-rise shells which are normally classified as shallow by the Reissner criterion. This article considers holographic interferometry as a noncontact, high-resolution method of measuring prebuckling deformations. Prebuckling deformations of a lambda = 9, h/b = 0.038 shell are Fourier-analyzed. Buckling is found to occur in an N = 5 mode as predicted by Huang's theory. The N = 4 mode was unusually stable, suggesting that even at this low value of h/b, stabilizing effects may be at work.

Parmerter, R. R.↗

NASA NTRS1972

Large amplitude flexural vibration of thin elastic flat plates and shells

The general equations governing the large amplitude flexural vibration of any thin elastic shell using curvilinear orthogonal coordinates are derived and consist of two coupled, nonlinear, partial differential equations in the normal displacement w and the stress function F. From these equations, the governing equations for the case of shells of revolution or flat plates can be readily obtained as special cases. The material of the shell or plate is isotropic and homogeneous and Hooke's law for the two-dimensional case is valid. It is suggested that the difference between the hardening type of nonlinearity in the case of flat plates and straight beams and the softening type of nonlinearity in the case of shells and rings can, in general, be traced to the amount of curvature present in the underformed median surface of the structure concerned.

Pandalia, K. A. V.↗

NASA NTRS1974

The non-existence of the Oort cometary shell

The procedures adopted as theory for a shell of comets are shown to be invalid. Any plot of numbers of long-period comets against 1/a will automatically exhibit a peak at small values of this parameter, and cannot be inverted to demonstrate a high volume-density of aphelia in space. The positions of actual aphelion-points show no sign of any concentration at any range. Further, the aphelion-distance undergoes large almost random changes owing to planetary perturbations at each return, and present values can yield no indication of original positions. A recent attempt to save the shell-theory by making use solely of comets with large perihelion-distance is shown to rest on exactly the same errors as have done all the earlier presentations. The plain conclusions emerge that the shell-theory is devoid of any support by facts, and that the alleged shell of comets is non-existent.

Lyttleton, R. A.↗

NASA NTRS1976

Finite element representations for thin shell instability analysis

The development of finite element calculational procedures for thin-shell instability analysis has involved the definition of appropriate element representations (i.e. the geometric form of the element and the approximation of displacement and/or stress), constitutive expressions, and computational algorithms. Among these, the status of thin-shell finite element representations remains unsettled. This paper, therefore, emphasizes recent developments in the basic aspects of thin-shell finite element analysis and discusses one simplified approach in more detail. Formulative and computational procedures for elastic instability analysis are then summarized, and numerical results are shown for the simplified shell element formulation.

Gallagher, R. H.↗

NASA NTRS1976

Design of efficient stiffened shells of revolution

A method to produce efficient piecewise uniform stiffened shells of revolution is presented. The approach uses a first order differential equation formulation for the shell prebuckling and buckling analyses and the necessary conditions for an optimum design are derived by a variational approach. A variety of local yielding and buckling constraints and the general buckling constraint are included in the design process. The local constraints are treated by means of an interior penalty function and the general buckling load is treated by means of an exterior penalty function. This allows the general buckling constraint to be included in the design process only when it is violated. The self-adjoint nature of the prebuckling and buckling formulations is used to reduce the computational effort. Results for four conical shells and one spherical shell are given.

Majumder, D. K.↗

NASA NTRS1976

Recent advances in shell theory

The results reviewed are divided into two categories: those that relate two-dimensional shell theory to three-dimensional elasticity theory and those concerned with shell theory per se. In the second category results for general elastic systems that carry over, by specialization or analogy, to shells and results that are unique to shell theory itself are considered.

Simmonds, J. G.↗

NASA NTRS1976

Si II lines in the shell of Zeta Tau

Non-LTE strengths of Si II lines were computed in an effort to reproduce the observed strengths of Si II lines in the shell spectrum of Zeta Tau. This effort was unsuccessful, probably because it assumed that the shell could be represented by a finite, plane-parallel slab. The main conclusion of this study, then, is that in order to understand the shell spectra of Be stars, a more realistic geometry must be assumed, in which the shell is represented by a thick disk.

Heap, S. R.↗

NASA NTRS1977

Determination of the critical loads of shells by nondestructive methods

Two methods for determining the location of and load level to produce instability of compressed cylindrical shells are presented. The first relates the variation in the wall normal stiffness as a function of applied compressive force to the critical load. It uses the distribution of stiffness over the surface of the shell as a guide to buckle location. The second method associates the local dynamic mass with instability behavior. The test data presented show that either method will give excellent prediction capability from low-load-level data for shells of orthodox form. Neither method appears to apply to spirally stiffened shells. This is thought to be due to the fact that there is a substantial difference between the buckle pattern under axial compression and the imperfection shape induced by the normal displacement which is used to ascertain the wall stiffness and the dynamic mass.

Horton, W. H.↗

NASA NTRS1977

Imperfection surveys on a 10-ft-diameter shell structure

The results of an extensive imperfection survey on a 10-ft-diameter integrally stiffened cylindrical shell are presented. The shape of the measured initial imperfections is clearly influenced by details of the shell construction. The modal components of the measured imperfection surface as a function of the circumferential and of the axial wave numbers are calculated. The discrete axial power spectral density functions and the corresponding root-mean-square values of the imperfections are also determined for given circumferential wave numbers. Using the Fourier coefficients of the measured initial imperfections, buckling loads are calculated by solving the nonlinear Donnell-type imperfect shell equations iteratively. The calculated lowest buckling load compares favorably with the values usually recommended for similar shell structures.

Arbocz, J.↗

NASA NTRS1978

The transfer of cyclotron lines through the magnetospheric shell of Hercules X-1

Cyclotron emission from a strong magnetic field, has been proposed to identify line feature observed in the hard X-ray spectrum of the neutron star Hercules X-1. If this interpretation is indeed correct, and if the model wherein the 1.24 second hard X-ray pulse is due to the occultation of a more isotropic X-ray flux by an opaque magnetospheric shell is accepted then these observations can be used to place strict limits on the total source luminosity of the cyclotron emission. The observational fate of cyclotron line photons that have emerged from the region near the neutron star surface, but have yet to encounter an opaque shell at the magnetosphere is discussed. It is shown that if a narrow line is incident on such a shell, then a residual narrow feature can emerge from the shell.

Weave, R. P.↗

NASA NTRS1978

FASOR - A second generation shell of revolution code

An integrated computer program entitled Field Analysis of Shells of Revolution (FASOR) currently under development for NASA is described. When completed, this code will treat prebuckling, buckling, initial postbuckling and vibrations under axisymmetric static loads as well as linear response and bifurcation under asymmetric static loads. Although these modes of response are treated by existing programs, FASOR extends the class of problems treated to include general anisotropy and transverse shear deformations of stiffened laminated shells. At the same time, a primary goal is to develop a program which is free of the usual problems of modeling, numerical convergence and ill-conditioning, laborious problem setup, limitations on problem size and interpretation of output. The field method is briefly described, the shell differential equations are cast in a suitable form for solution by this method and essential aspects of the input format are presented. Numerical results are given for both unstiffened and stiffened anisotropic cylindrical shells and compared with previously published analytical solutions.

Cohen, G. A.↗

NASA NTRS1979

Radiative transfer in spherical dust shells using a generalized two-stream Eddington approximation

Application of a generalized two-stream Eddington approximation to the problem of radiative transfer in extended, spherically symmetric dust shells is presented. It is assumed that the radiation field can be characterized by the mean intensity, the flux, and a positionally dependent direction cosine specifying the division into two solid-angle ranges. The direction cosine is not specified a priori and is a function of the geometry, opacity, and emissivity in the dust shell. A multiple-grain-size multiple-temperature-distribution dust shell is postulated in which isotropic and anisotropic scattering as well as absorption and thermal reemission are allowed. A program has been developed that solves for the multiple temperature distributions by applying the constraint of radiative equilibrium to each grain size, and then calculates emergent fluxes. Results of one such calculation are presented for a model dust shell having a maximum optical depth (approximately 41) in the visible, clearly showing large optical extinction and a moderate infrared excess.

Haisch, B. M.↗

NASA NTRS1979

Computer analysis of shells of revolution using asymptotic results

It is suggested that asymptotic results for the behavior of thin shells can be incorporated in a general computer code for the analysis of a complex shell structure. The advantage when compared to existing finite difference or finite element codes is a substantial reduction in computational labor with the capability of working to a specified level of accuracy. A reduction in user preparation time and dependance on user judgment is also gained, since mesh spacing can be internally generated. The general theory is described in this paper, as well as the implementation in the computer code FAST 1 (Functional Algorithm for Shell Theory) for the analysis of the general axisymmetric shell structure with axisymmetric loading.

Steele, C. R.↗

NASA NTRS1981

Application of the line-spring model to a cylindrical shell containing a circumferential or axial part-through crack

An approximate solution was obtained for a cylindrical shell containing a part-through surface crack. It was assumed that the shell contains a circumferential or axial semi-elliptic internal or external surface crack and was subjected to a uniform membrane loading or a uniform bending moment away from the crack region. A Reissner type theory was used to account for the effects of the transverse shear deformations. The stress intensity factor at the deepest penetration point of the crack was tabulated for bending and membrane loading by varying three dimensionless length parameters of the problem formed from the shell radius, the shell thickness, the crack length, and the crack depth. The upper bounds of the stress intensity factors are provided by the results of the elasticity solution obtained from the axisymmetric crack problem for the circumferential crack, and that found from the plane strain problem for a circular ring having a radial crack for the axial crack. The line-spring model gives the expected results in comparison with the elasticity solutions. Results also compare well with the existing finite element solution of the pressurized cylinder containing an internal semi-elliptic surface crack.

Delale, F.↗

NASA NTRS1982

On the shape and orientation control of an orbiting shallow spherical shell structure

The dynamics of orbiting shallow flexible spherical shell structures under the influence of control actuators was studied. Control laws are developed to provide both attitude and shape control of the structure. The elastic modal frequencies for the fundamental and lower modes are closely grouped due to the effect of the shell curvature. The shell is gravity stabilized by a spring loaded dumbbell type damper attached at its apex. Control laws are developed based on the pole clustering techniques. Savings in fuel consumption can be realized by using the hybrid shell dumbbell system together with point actuators. It is indicated that instability may result by not including the orbital and first order gravity gradient effects in the plant prior to control law design.

Bainum, P. M.↗

NASA NTRS1982

Liquid drop technique for generation of organic glass and metal shells

It was found that liquid drop techniques are very useful in several diverse areas. For producing very uniform metallic, organic, inorganic and, on particular, glassy shells, the liquid jet method is the most reproducible and exceptionally useful of all the techniques studied. The technique of capillary wave synchronization of the break-up of single and multiple component jets was utilized to produce uniform sized liquid drops and solid particles, and hollow liquid and solid shells. The technique was also used to encapsulate a number of liquids in impermeable spherical shells. Highly uniform glass shells were made by generating uniform drops of glass forming materials in an aqueous solution, subsequently evaporating the water, and then fusing and blowing the remaining solids in a high temperature vertical tube furnace. Experimental results are presented and the critical problems in further research in this field are discussed.

Hendricks, C. D.↗

NASA NTRS1981

Intergalactic shells at large redshift

The intergalactic shells produced by galactic explosions at large redshift, whose interiors cool by inverse Compton scattering off the cosmic background radiation, have a characteristic angular size of about 1 arcmin at peak brightness. At z values lower than 2, the shells typically have a radius of 0.5 Mpc, a velocity of about 50 km/sec, a metal abundance of about 0.0001 of cosmic values, and strong radiation in H I(Lyman-alpha), He II 304 A, and the IR fine-structure lines of C II and Si II. The predicted extragalactic background emission from many shells, strongly peaked toward the UV, sets an upper limit to the number of exploding sources at z values of about 10. Shell absorption lines of H I, C II, Si II, and Fe II, which may be seen at more recent epochs in quasar spectra, may probe otherwise invisible explosions in the early universe.

Shull, J. M.↗