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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Stability of a spherical shell distribution of pickup ions

The stability of a spherical shell (complete or incomplete) is investigated. The study is motivated by the research of cometary pickup ions. The present discussion is restricted to low-frequency electromagnetic waves propagating along the ambient magnetic field. It is found that a complete shell is stable, whereas an incomplete shell can be unstable. The numerical results are reported and discussed.

Freund, H. P.↗
NASA NTRS1988

Effects of compressibility on the temperature jump at the interface of layered, spherical-shell convection

Large temperature jumps at the interface of layered convection are important to the argument used against the likelihood of separate circulations in the upper and lower mantles. This problem was studied within the framework of a compressible, constant viscosity spherical-shell model. Both mechanical and thermal coupling configurations are considered. Although the temperature jumps are reduced by compressibility, their magnitudes remain quite large, in the case of mechanical coupling. For thermal coupling, the temperature jumps become smaller but still are substantial, between 500 to 1000 C. In layered spherical-shell convection, flows in the lower mantle are several times greater than the surface velocities.

Yen, David A.↗
NASA NTRS1972

Fracture of cylindrical and spherical shells containing a crack.

Fatigue and fracture of cylindrical and spherical shells containing a through crack and subjected to internal pressure or torsion are considered. The stress intensity factor ratios giving the effect of curvature are obtained as functions of a dimensionless shell parameter. By using the conventional plastic strip model the plastic zone size around the crack tip and the crack opening displacement are calculated. The calculated COD values are shown to be in good agreement with the experimental results. The fracture criterion of critical COD is verified by using the results of the burst tests in titanium and aluminum alloy cryogenic pressure vessels. Modified fatigue crack propagation models taking into account the bending effect in shells are introduced and are applied to the analysis of experimental data obtained from flat plates, and cylindrical shells under axial tension, internal pressure, and torsion. Finally, the paper includes the effect of humidity on the crack growth rate and the effect of load biaxiality on the rupture strength.

Erdogan, F.↗
NASA NTRS1972

Nonlinear axisymmetric flexural vibration of spherical shells

Axisymmetric responses are presented of a nonshallow thin-walled spherical shell on the basis of nonlinear bending theory. An ordinary differential equation with nonlinearity of quadratic as well as cubic terms associated with variable time is derived. The derivation is based on the assumption that the deflection mode is the sum of four Legendre polynomials, and the Galerkin procedure is applied. The equation is solved by asymptotic expansion, and a first approximate solution is adopted. Unstable regions of this solution are discussed.

Kunieda, H.↗
NASA NTRS1985

Convective motions in a spherical shell

We compute the axisymmetric convective motions that exist in a spherical shell heated from below with inner to outer radius ratio equal to 0.5. The boundaries are stress-free and gravity is directly proportional to radius. Accurate solutions at large Rayleigh numbers, O(100000), are made feasible by a spectral method that employs diagonal-mode truncation. By examining the stability of axisymmetric motions it is inferred that the preferred form of convection varies dramatically according to the value of the Rayleigh number. While axisymmetric motions with different patterns may exist for modestly nonlinear convection, only a single motion persists at sufficiently large values of the Rayleigh number. This circulation is symmetric about the equator and has two meridional cells with rising motion at the poles. Instability of this single axisymmetric motion determines that the preferred pattern of three-dimensional convection has one azimuthal wave.

Zebib, A.↗
NASA NTRS1988

Electromagnetic-capillary instabilities of a hollow liquid cylinder - Production of spherical shells under microgravity conditions

An electrical device is proposed for manufacturing spherical shells out of hollow metallic cylinders in a microgravity environment. Such a cylinder is melted by passing a strong axial current through it. The resulting liquid column is unstable to axisymmetric disturbances and breaks up to form hollow spheres after solidification. The stability analysis presented in this paper, which is based on an inviscid magnetohydrodynamic formulation, shows that the size of the spheres may be controlled by varying the strength of the current passed through the column.

Chow, C.-Y.↗
NASA NTRS1987

Deformation of the core-mantle boundary induced by spherical-shell, compressible convection

Single-mode, mean-field equations, for spherical-shell convection are derived, with variable viscosity and compressibility included. Core-mantle boundary (CMB) topography produced by whole mantle convection is computed for various rheologies and different thermal boundary conditions at CMB. Results for long-wavelength undulations show the extreme sensitivity of these signatures to rheological and equation of state parameters. In order to obtain excess ellipticity of about 1 km, the activation energy and volume in the lower mantle should be low, and the temperatures at CMB should be around 4500 K, in accord with recent experimental findings.

Zhang, Shuxia↗
NASA NTRS1962

ON THE INFLUENCE OF NON-SYMMETRICAL MODES ON THE BUCKLING OF SHALLOW SPHERICAL SHELLS UNDER UNIFORM PRESSURE

In view of the wide discrepancy between theoretical and experimental results that still exists, the problem of buckling of shallow spherical shells under uniform external pressure is reexamined with the introduction of non-symmetrical deflection modes. The basic idea is that, for a symmetrical shell under gradually increasing symmetrical loading, the deformation will be symmetrical up to and after buckling, but it is feasible that a non-symmetrical mode participates in the snap-through process. Including this possibility in the governing equations may lower the theoretical buckling load. Some analytical results are presented and compared with experiment.

SPHERICAL SHELL↗
NASA NTRS1971

Forced axisymmetric response of fluid filled spherical shells.

A general solution for the forced, linear axisymmetric response of a fluid filled spherical shell is derived with the effects of shell transverse shear and rotational inertia included. The fluid is assumed to be inviscid and compressible. Expressions for the shell radial displacement are computed for the problem of a uniform radial Heaviside load on a cap of the shell surface. In addition, the time history of the shell inner fiber stresses at the impact pole is obtained. Some comparisons with the in vacuo shell theory are also given.

Lee, Y. C.↗