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NASA NTRS1990

CFD spinoff - Computational electromagnetics for radar cross section (RCS) studies

A finite-volume discretization procedure derived from proven CFD methods is used to solve the conservation form of the time-domain Maxwell's equations, in order to compute EM scattering from layered objects. This time-domain approach handles both single-frequency/continuous wave and broadband-frequency/pulse incident excitation. Arbitrarily shaped objects are modeled by means of a body-fitted coordinate transformation; complex internal/external structures with many material layers are treated through the implementation of a multizone framework capable of handling any type of zonal boundary condition. Results are presented for various two- and three-dimensional problems.

Shankar, Vijaya↗

NASA NTRS1985

Electromagnetic wave scattering in a two-layer anisotropic random medium

For electromagnetic wave propagation and scattering in an anisotropic random medium, the Dyson equation for the mean field and the Bethe-Salpeter equation for the correlation or the covariance of the field were derived. With the random permittivity expressed in a general anisotropic form, the bilocal and the nonlinear approximations are employed to solve the Dyson equation, and the ladder approximation to solve the Bethe-Salpeter equation. The mean dyadic Green's function for a two-layer anisotropic random medium with arbitrary three-dimensional correlation functions has been investigated with the zeroth-order solutions to the Dyson equation under the nonlinear approximation. The effective propagation constants are calculated for the four characteristic waves associated with the coherent vector fields, propagating in an anisotropic random-medium layer, which are the ordinary and extraordinary waves with upward- and downward-propagating vectors.

Lee, J. K.↗

NASA NTRS1987

Ram ion scattering caused by Space Shuttle v x B induced differential charging

Observations of secondary, high-inclination ions streams have been reported in the literature. The authors of these previous papers attributed the source of the secondary ions to a disturbed region in the plasma about 10 m from the Space Shuttle Orbiter. A new theory has been developed which shows how v x B induced differential charging on the plasma diagnostics package (PDP) can scatter the ram ion flux. Some of these ions are reflected back to the PDP and may be the sorce of the observed ion distributions. The effect is unique to large spacecraft; it occurs only when the magnitude of the induced v x B potentials are much larger than the electron thermal energy and of the order of the ion ram energy. That the ion streams observed at large angles must have been reflected from the PDP surface is demonstrated with three-dimensional sheath and particle trajectory calculations using the low earth orbit version of the NASA Charging Analyzer Program (NASCAP/LEO).

Katz, I.↗

NASA NTRS1983

Correlated particle and magnetic field observations of a large-scale magnetic loop structure behind an interplanetary shock

From a survey of observations on ISEE-3, an example of correlated particle and magnetic field observations of a large-scale magnetic loop structure is presented. Bidirectional proton fluxes were observed for a period of 40 hours in the energy range 35-1600 keV approximately 12 hours after the passage of the interplanetary shock of December 11, 1980, and directly after the passage of a discontinuity. For each of the eight logarithmically spaced energy channels, a three-dimensional anisotropy analysis reveals streaming along both directions of the magnetic field. The magnetic field rotated slowly but steadily through approximately 180 deg during this same 40-hour period; this is consistent with the existence of a large-scale loop with extent greater than 0.5 AU. The observations suggest that the particles are being injected into the loop sunward of the spacecraft; they appear as bidirectional fluxes in the outermost regions of the loop arising from a combination of focusing and near scatter-free transport.

Sanderson, T. R.↗

NASA NTRS1976

Particle and flow field measurements by laser holography

Holograms make transmission recordings of aerodynamic events which are real-illuminated with ground-glass light diffusers and with no requirements for precision. The three-dimensional interferograms can be recorded by stored-beam, double-exposure, and double-plate techniques. Weak traveling shock waves, generated by rotating blade rows of an aircraft fan compressor, can be visualized by rapid double-exposure holograms recorded with a ruby laser. The same laser technique can also determine flow velocities from holograms of small flow-entrained particles. Particles larger than one-quarter millimeter are recorded under rear diffuse illumination conditions. Smaller particles can be recorded by their own forward scattering of laser light through the sensitivity of the holographic process for weak signals. Since the holographic apparatus can be path-matched, both particle and flow interferograms can be recorded with lasers of short coherence length.

Wuerker, R. F.↗

NASA NTRS1986

Field experiment for measurement of the radiative characteristics of a hazy atmosphere

Theoretical two-dimensional and three-dimensional solutions to the radiative-transfer equation have been applied to the earth-atmosphere system. A field experiment was conducted to test this theory. In the experiment the upward radiance was measured above and below a haze layer during simultaneous measurements of the haze characteristics. The measurements were conducted at a narrow near-IR channel (773 + or - 22 nm) which represents the visible and near-IR spectral region. The aerosol vertical optical thickness at eight wavelengths, as well as the vertical and horizontal profiles of the scattering coefficient, the temperature, and dew point were measured at several locations. These measurements quantified the vertical and spatial structure of the atmospheric haze and the atmospheric radiation. The result was a well-defined radiative-transfer experiment. The experimental data set is used to quantify the haze effect on upward radiance, including the adjacency effect (the effect of a bright area on the upward radiance measured above a dark adjacent area), and to test radiative-transfer models for a plane-parallel atmosphere above a nonuniform surface. A comparison is given between the theoretical prediction of upward radiance above the haze and the measurement. Agreement between theory and the experiment is discussed.

Kaufman, Y. J.↗

NASA NTRS1987

A low-frequency synthesis array in earth orbit

The scientific objectives and design concept of a space-based VLBI array for high-resolution astronomical observations at 1-30 MHz are discussed. The types of investigations calling for such an array include radio spectroscopy of individual objects, measurement of the effects of scattering and refraction by the interplanetary medium (IPM) and the ISM, mapping the distribution of low-energy cosmic-ray electrons, and determining the extent of the Galactic halo. Consideration is given to the limitations imposed on an LF VLBI array by the ionosphere, the IPM, and the ISM; the calibration advantages offered by circular polar orbits of slightly differing ascending-node longitude for the array satellites; and collection of the IF data streams from the array satellites by one 'master' satellite prior to transmission to the ground. It is shown that determination of the three-dimensional array geometry by means of intersatellite radio links is feasible if there are at least seven spacecraft in the array.

Jones, Dayton L.↗

NASA NTRS1988

Erupting prominences and the geometry of coronal mass ejections

Studies of the origins of coronal mass ejections (CMEs) show a strong association between CMEs and erupting prominences. 'Loop' CMEs form the most common morphological class for both Skylab and Solar Maximum Mission (SMM) and the class most likely to be associated with erupting prominences. In the optically thin corona, the geometry of such structures is interpreted as either planar (two-dimensional), shell-like, or bubblelike (three-dimensional) structures. Trottet and MacQueen (1980) (hereinafter referred to as TM) first examined the orientations of prominences associated with Skylab CMEs and argued that looplike CMEs were planar structures in the plane of the sky. In this work, the initial orientations of prominences identified with SMM CMEs in 1980 were measured in order to examine the overall geometry of CMEs. Contrary to TM's result, no preferred orientation for prominences associated with SMM CMEs nor any difference between that population and the population of all disk filaments were found. In addition, plots comparing CME/filament orientation angles and the angular widths of the CME envelopes show large scatter. Taken together, these results support a shell-like rather than a planar geometry for at least looplike CMEs.

Webb, David F.↗

NASA NTRS1975

Calculations of the radiative and dynamical state of the Venus atmosphere

Results are reported for accurate multiple-scattering calculations to determine the solar-energy deposition profile in the atmosphere of Venus. It is found that most of the absorbed energy is deposited in the main cloud-layer region, located at altitudes above 35 km, and that the ground receives approximately 3% of the energy absorbed in toto by Venus. Using these results, vertical temperature profiles are computed under conditions of pure radiative equilibrium and radiative-convective equilibrium. Since the latter results satisfactorily match the temperature structure determined from various spacecraft observations, it is inferred that the greenhouse effect can account for the high surface temperature. Aerosols make an important contribution to the infrared opacity in these calculations. Preliminary three-dimensional calculations of the general circulation of the atmosphere are discussed which incorporate the results of the radiative calculations.

Pollack, J. B.↗

NASA NTRS1990

Time-dependent wave-packet method for the complete determination of S-matrix elements for reactive molecular collisions in three dimensions

An alternative time-dependent wave-packet method for treating three-dimensional gas phase reactive atom-diatom collisions is presented. The method employs a nonreactive body-frame wave packet propagation procedure, made possible by judicious use of absorbing optical potentials, a novel scheme for interpolating the wave function from coordinates in one arrangement to those in another and the fact that the time-dependent Schroedinger equation is an initial-value problem. The last feature makes possible a computationally viable and accurate procedure for changing from one arrangement's coordinates to another. In addition, the method allows the determination of S-matrix elements over a wide range of energies from a single wave-packet propagation. The method is illustrated by carrying out detailed calculations of inelastic and reactive scattering in the H + H2 system using the Liu-Siegbahn-Truhlar-Horowitz potential surface.

Judson, Richard S.↗

NASA NTRS1977

Scattering in discrete random media with implications to propagation through rain

The multiple scattering effects on wave propagation through a volume of discrete scatterers were investigated. The mean field and intensity for a distribution of scatterers was developed using a discrete random media formulation, and second order series expansions for the mean field and total intensity derived for one-dimensional and three-dimensional configurations. The volume distribution results were shown to proceed directly from the one-dimensional results. The multiple scattering intensity expansion was compared to the classical single scattering intensity and the classical result was found to represent only the first three terms in the total intensity expansion. The Foldy approximation to the mean field was applied to develop the coherent intensity, and was found to exactly represent all coherent terms of the total intensity.

Ippolito, L. J., Jr.↗

NASA NTRS1977

An experimental investigation of a three-dimensional wall jet

An experimental investigation of the effects of a curved and flat wall surface on the flow development of an axisymmetric jet exhausting into a moving airstream has been made. Of primary concern was the comparison of the one-point statistical properties of the unconfined jet to the quantities measured in the flows over a flat plate and a curved wall surface. Quantities measured in this investigation included mean velocities in all three directions, turbulent intensities, autocorrelations, power spectral densities, and intermittencies. The investigation was confined to the near field of the turbulent jet. The curved wall (flap) was found to have a very large effect on both the mean velocity and the turbulent velocity components of the flowfield. The potential core region of the jet was found to break up most rapidly for the flow over the flap. A laser-Doppler velocimeter, using a phase locked-loop processor, was used to make the desired velocity field measurements. To determine the intermittency profiles, a laser light scattering technique was employed.

Catalano, G. D.↗

NASA NTRS1980

Measurements of the sampling volume of an infrared laser doppler velocimeter for the study of feasibility of simultaneous three-dimensional velocity profile mapping

High resolution incoherent mapping of the sampling volume was performed to determine the three dimensional profile of the volume of the beam from which information can be obtained by scattering. The incoherent sampling volume of the infrared laser Doppler velocimeter was found to be heavily dependent upon the configuration of the outgoing laser beam. In general when only half the beam is allowed to focus at the focal plane, the x, y traverses indicate a broader beam with nominal side lobes. However, if the total beam is permitted to impinge on the focal plane there is substantial narrowing of the beam with increased maximum intensity and an increase in side lobes. The double maximum indicated by the coherent sampling volume appears to be an artifact of the scanning process not a fundamental entity of the sampling volume.

Bond, R. L.↗

NASA NTRS1984

Effects of boundary layer refraction and fuselage scattering on fuselage surface noise from advanced turboprop propellers

An acoustic disturbance's propagation through a boundary layer is discussed with a view to the analysis of the acoustic field generated by a propfan rotor incident to the fuselage of an aircraft. Applying the parallel flow assumption, the resulting partial differential equations are reduced to an ordinary acoustic pressure differential equation by means of the Fourier transform. The methods used for the solution of this equation include those of Frobenius and of analytic continuation; both yield exact solutions in series form. Two models of the aircraft fuselage-boundary layer system are considered, in the first of which the fuselage is replaced by a flat plate and the acoustic field is assumed to be two-dimensional, while in the second the fuselage is a cylinder in a fully three-dimensional acoustic field. It is shown that the boundary layer correction improves theory-data comparisons over simple application of a pressure-doubling rule at the fuselage.

Mcaninch, G. L.↗

NASA NTRS1986

Radar scene generation for tactical decision aids

The Mueller matrix and polarization covariance matrix for polarimetric radar systems was studied. The clutter was modeled by a layer of random permittivity, described by a three-dimensional correlation function, with variance, and horizontal and vertical correlation lengths. This model was applied, using the wave theory with Born approximations carried to the second order, to find the backscattering elements of the polarimetric matrices. Theoretical predictions are matched with experimental data for vegetation fields. The strong fluctuation theory was used to derive the backscattering cross sections. A two-layer anisotropic random medium model was developed for the active and passive microwave remote sensing of ice fields. A three-layer random medium model was adopted to study the volume scattering effects for the active and passive microwave remote sensing of snow-covered ice fields.

Kong, J. A.↗

NASA NTRS1988

Three-dimensional trajectory analyses of two drop sizing instruments: PMS OAP and PMS FSSP

Flow induced distortions of water drop fluxes and speeds seen by the instruments were predicted by use of three dimensional flow and trajectory calculation methods. Sensitivities were determined for the instruments, in isolation and mounted under the wing of an airplane, to: water drop diameter (2 to 1000 microns), angle of attack and free stream air speed. For the optical array probe in isolation and on the airplane at 0 deg angle of attack, flux distortions of practical consequence are not found. At 4 deg airplane angle of attack, partial flow stagnation under the uptilted wing causes significant decreases in both flux and speed for cloud size droplets. For the forward scattering spectrometer probe in isolation, only marginally significant sensitivities to free stream air speed are found, and no sensitivity is found to angle of attack. Both speed and flux of cloud size droplets are predicted to be undermeasured by from 12 to 24 percent depending on airplane angle of attack. For the wing-mounted instruments, effects of flow about the instruments themselves are found to be equal in importance to effects of flow about the airplane. Preferred orientation (canting) angles of distorted water drops are found to be functions of drop size, angle of attack and air speed.

Norment, Hillyer G.↗

NASA NTRS1988

Three-dimensional trajectory analyses of two drop sizing instruments - PMS OAP and PMS FSSP

Norment, Hillyer G.↗

NASA NTRS1991

Backscattering by nonspherical particles - A review of methods and suggested new approaches

Scattering of electromagnetic radiation near the backward direction is more sensitive to particle shape than scattering near the forward direction. Mie theory is therefore of dubious applicability to predicting backscattering by atmospheric particles known to be irregular or to inverting measurements on such particles. An irregular particle is one with an uncertain shape. In the face of uncertainty one must adopt a statistical approach in which scattering properties of ensembles are determined. To obtain ensemble averages, a basis is needed for averaging over a set of electromagnetic microstates. Ensemble averages based on the Rayleigh theory for small ellipsoids and on the T matrix method for spheroids agree better with measurements than Mie theory does. The coupled-dipole method also provides a basis for ensemble averaging. This method also leads to a simple physical interpretation of why backscattering is so sensitive to particle shape and can be used to calculate scattering by one- and two-dimensional analogs to three-dimensional irregular particles.

Bohren, Craig F.↗