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At least 289 records · Page 16

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NASA NTRS1981

Singular perturbation, state aggregation and nonlinear filtering

Consideration is given to a state process evolving in R(n), whose motion is that of a pure jump process in R(n) in the 0(1) time scale, upon which is superimposed a continuous motion along the orbits of a gradient-like vector field g in R(n) in the 0(1/epsilon) time scale. The infinitesimal generator of the state process is, in other words, of the form L + (1/epsilon)g. It follows from the main results presented that the projected filters converge to the finite state Wonham filter corresponding to the problem of estimating the finite state process in the presence of additive white noise.

Hijab, O.↗

NASA NTRS1983

Observational evidence of Z and L-O mode waves as the origin of auroral kilometric radiation from the Jikiken (EXOS-B) satellite

The present investigation has the objective to present new information on the polarization of the auroral kilometric radiation detected by the Jikiken (EXOS-B) satellite. On September 16, 1978, the satellite had been launched into an orbit with altitudes of the initial apogee and perigee of 30,050 km and 250 km, respectively, and an inclination of -31 deg. This orbit provides the conditions for suitable observations of the spectra of auroral kilometric radiation (AKR) in the proximity of the source regions. A study has been conducted of the discovered Z mode waves. These waves are found to be refracted to a nearly perpendicular direction of propagation with respect to the magnetic field vector within a very short distance of the source position. AKR spectra have been observed at positions where the ambient plasma defines a lower edge of the spectra by the cutoff effects of the waves propagating in the local plasma.

Oya, H.↗

NASA NTRS1983

Nonlinear scattering from a plasma column. I - Theory. II Special cases

The scattered signal excited by nonlinear mixing of two plane waves normally incident on an infinitely long column of plasma is investigated. A general solution is obtained for the polarization in which the electric field vectors of the waves are perpendicular to the column axis and the column is assumed to be radically inhomogeneous. This general theory is then applied to the special cases of the inhomogeneous column in the long-wavelength limit, and the homogeneous column both for the general case and in the long-wavelength limit. It is determined that dipole and quadrupole components should predominate in the polar radiation pattern for the long-wavelength case. The special case of second harmonic generation due to a single incident wave is analyzed in detail. Nonlinear scattering coefficients are computed, and the corresponding polar radiation patterns are determined. The findings of this study are employed to evaluate the feasibility of observing nonlinear scattering from meteor trails.

Crawford, F. W.↗

NASA NTRS1984

Flow visualization and interpretation of visualization data for deflected thrust V/STOL nozzles

Flow visualization studies were made for four deflected thrust nozzle models at subsonic speeds. Based on topological rules and the assumption that observed streaks constitute continuous vector fields, available visualization pictures are interpreted and flow patterns on interior surfaces of the nozzles are synthesized. In particular, three dimensional flow structure and separations are discussed. From the synthesized patterns, the overall features of the flow field in a given nozzle can be approximately perceived.

Kao, H. C.↗

NASA NTRS1984

Flow visualization and interpretation of visualization data for deflected thrust V/STOL nozzles

Kao, H. C.↗

NASA NTRS1984

Observation of plasma deceleration at a rotational magnetopause discontinuity

This letter presents a detailed experimental analysis of the electric fields and electron flow velocities at a rotational magnetopause discontinuity. It is shown for this event that the magnetosheath plasma velocity is sharply decelerated on entry into the magnetosphere in very good agreement with a theoretical magnetohydrodynamic fit. This type of event is remarkable because the product of the electric-field vector and the Chapman-Ferraro magnetopause current is negative during the magnetopause crossing. This case represents one of two types of valid solution to the MHD boundary conditions. It differs from previously published examples of reconnective magnetopause geometries in that energy is being transferred locally from kinetic energy into electromagnetic energy at the time of these measurements.

Aggson, T. L.↗

NASA NTRS1985

The Hanle effect applied to magnetic field measurements

The Hanle effect is the modification by a local magnetic field of the polarization due to coherent scattering in spectral lines. It results from the precession of a classical oscillator about the magnetic field direction. The sophisticated quantum-mechanical treatment, which is required to compute the polarization parameters of scattered light, was developed. The main features of the Hanle effect concerning magnetic field measurements are: (1) a good sensitivity within the approximate range 0.1 B gamma rho to 10 B gamma rho where B gamma rho is the field strength yielding a Larmor period equal to the radiative lifetime, (2) there is no Hanle effect for field vectors parallel to the excitating beam, (3) the Hanle effect refers essentially to the linear polarization in a spectral line, (4) various points in the line profile are affected in the same way by change of linear polarization so that polarization parameters can be measured on the integrated line profile.

Leroy, J. L.↗

NASA NTRS1985

Generation and transfer of polarized radiation in the solar atmosphere: Physical mechanisms and magnetic-field diagnostics

The main physical mechanisms responsible for the generation and transfer of polarized radiation in the solar atmosphere can be classified in a suitable bidimensional diagram with an indicator of the magnetic field strength on its vertical axis and an indicator of the radiation field anisotropy on its horizontal axis. The various polarimetric observations performed on solar spectral lines are interpreted with different theoretical schemes according to their classification in the diagram and to the optical depths involved. These theoretical schemes, and the associated diagnostic tools for inferring the magnetic field vector from observations are reviewed. In particular, the role of magneto-optical effects in determining the direction of the observed linear polarization in active regions is discussed in some detail.

Deglinnocenti, E. L.↗

NASA NTRS1985

The Use of a Laser Doppler Velocimeter in a Standard Flammability Tube

The use of the Laser Doppler Velocimeter, (LDV), to measure the flow associated with the passage of a flame through a standard flammability limit tube (SFLT) was studied. Four major results are presented: (1) it is shown that by using standard ray tracing calculations, the displacement of the LDV volume and the fringe rotation within the experimental error of measurement can be predicted; (2) the flow velocity vector field associated with passage of an upward propagating flame in an SFLT is determined; (3) it is determined that the use of a light interruption technique to track particles is not feasible; and (4) it is shown that a 25 mW laser is adequate for LDV measurements in the Shuttle or Spacelab.

Strehlow, R. A.↗

NASA NTRS1985

Shock drift acceleration in the presence of waves

Attention is given to the initial results of a model designed to study the modification of the scatter-free, shock drift acceleration of energetic test particles by wave activity in the vicinity of a quasi-perpendicular, fast-mode MHD shock. It is emphasized that the concept of magnetic moment conservation is a valid approximation only in the perpendicular and nearly perpendicular regimes, when the angle theta-Bn between the shock normal and the upstream magnetic field vector is in the range from 70 deg to 90 deg. The present investigation is concerned with one step in a program which is being developed to combine the shock drift and diffusive processes at a shock of arbitrary theta-Bn.

Decker, R. B.↗

NASA NTRS1985

Alignment of velocity and field changes across tangential discontinuities in the solar wind

Three-dimensional IMP 8 and Voyager 2 plasma and field data are used to investigate the relative changes in direction of the velocity and magnetic field vectors across tangential discontinuities in the solar wind. It is found that Delta-v and Delta-B/rho exp 1/2 are closely aligned either parallel or antiparallel to each other in the sense associated with the propagation of Alfven waves or rotational discontinuities outward from the sun. This alignment is observed at all solar distances betwen 1 and 2.2 AU and is independent of the solar wind stream structure. Several possible causes of the effect are briefly discussed, including interplanetary turbulence, the propagation of large-amplitude Alfvenic fluctuations traveling independently through the solar wind on both sides of the discontinuities, and the propagation of surface waves on tangential discontinuities.

Neugebauer, M.↗

NASA NTRS1986

The construction and use of divergence free vector expansions for incompressible fluid flow calculations

For incompressible fluids the law of mass conservation reduces to a constraint on the velocity vector, namely that it be divergence free. This constraint has long been a source of great difficulty to the numericist seeking to discretize the Navier-Stokes and Euler equations. A spectral method is discussed which overcomes this difficulty. Its efficacy is demonstrated on some simple problems. The velocity is approximated by a finite sum of divergence free vectors, each of which satisfies the same boundary conditions as the velocity. Projecting the governing equation onto the space of inviscid vector fields eliminates the pressure term and produces a set of ordinary differential equations that must be solved for the coefficents in the velocity. The pressure can then be recovered if it is needed.

Mhuiris, N. M. G.↗

NASA NTRS1986

Approximating linearizations for nonlinear systems

The following problem is examined: given a nonlinear control system dot-x(t) = f(x(t)) + the sum to m terms(i=1) u sub i (t)g sub i (x(t)) on R(n) and a point x(0) in R(n), approximate the system near x(0) by a linear system. One approach is to use the usual Taylor series linearization. However, the controllability properties of both the nonlinear and linear systems depend on certain Lie brackets of the vector field under consideration. This suggests that a linear approximation based on Lie bracket matching should be constructed at x(0). In general, the linearizations based on the Taylor method and the Lie bracket approach are different. However, under certain mild assumptions, it is shown that there is a coordinate system for R(n) near x(0) in which these two types of linearizations agree. The importance of this agreement is indicated by examining the time responses of the nonlinear system and its linear approximation and comparing the lower order kernels in Volterra expansions of each.

Hunt, L. R.↗

NASA NTRS1985

Preparation for microgravity science investigation of compound semiconductor crystal growth

Preparatory work on Bridgman directional solidification (BDS) of PbSnTe crystals prior to microgravity crystal growth experiments on Shuttle flights are reported. Gravitational effects become important in crystal growth when density gradients are present. The situation is critical in BDS of PbSnTe because of the necessity of obtaining homogeneous compositional distributions, which can be disturbed when convective processes occur. Numerical models have been defined which quantify the effects of convection in the crystal growth solution. The models were verified by earth-based crystal-growth tests in a two-zone furnace using equal concentrations of each of the elements. Data are provided to demonstrate the differences in composition among crystals grown at different orientations to the gravitational field vector.

Fripp, A. L.↗

NASA NTRS1986

Electromagnetic scattering theory

Electromagnetic scattering theory is discussed with emphasis on the general stochastic variational principle (SVP) and its applications. The stochastic version of the Schwinger-type variational principle is presented, and explicit expressions for its integrals are considered. Results are summarized for scalar wave scattering from a classic rough-surface model and for vector wave scattering from a random dielectric-body model. Also considered are the selection of trial functions and the variational improvement of the Kirchhoff short-wave approximation appropriate to large size-parameters. Other applications of vector field theory discussed include a general vision theory and the analysis of hydromagnetism induced by ocean motion across the geomagnetic field. Levitational force-torque in the magnetic suspension of the disturbance compensation system (DISCOS), now deployed in NOVA satellites, is also analyzed using the developed theory.

Bird, J. F.↗

NASA NTRS1986

A canonical expansion for nonlinear systems

The importance of differential geometry, particularly Lie brackets of vector fields, in the study of nonlinear systems is well established. Under very mild assumptions, it is shown that a real-analytic nonlinear system has an expansion in which the coefficients are computed in terms of Lie brackets. This expansion occurs in a special coordinate system. The concept of a pure feedback system is also explained. For control design involving a nonlinear system, one approach is to put the system in its canonical expansion and approximate by that part having only feedback paths.

Su, R.↗

NASA NTRS1987

Algebra and topology for applications to physics

The principal concepts of algebra and topology are examined with emphasis on applications to physics. In particular, attention is given to sets and mapping; topological spaces and continuous mapping; manifolds; and topological groups and Lie groups. The discussion also covers the tangential spaces of the differential manifolds, including Lie algebras, vector fields, and differential forms, properties of differential forms, mapping of tangential spaces, and integration of differential forms.

Rozhkov, S. S.↗

NASA NTRS1986

Tangential discontinuities in the solar wind - Correlated field and velocity changes and the Kelvin-Helmholtz instability

Three-dimensional Helios plasma and field data are used to investigate the relative changes in direction of the velocity and magnetic field vectors across tangential discontinuities (TDs) in the solar wind at solar distances of 0.29-0.50 AU. It is found for TDs with large Delta-v and (Delta-B)/B that Delta-v and Delta-B are closely aligned with each other, in agreement with the unexpected results of previous studies of TDs observed at 1 AU and beyond. It is shown that this effect probably results from the destruction by the Kelvin-Helmholtz instability of TDs for which Delta-v and Delta-B are not aligned. The observed decrease in the number of interplanetary discontinuities with increasing solar distance may be associated with the growth of the Kelvin-Helmholtz instability with decreasing Alfven speed.

Neugebauer, M.↗