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At least 775 records · Page 43

Orbital topography and other astrophysical consequences of Rosen's bimetric theory of gravity

Since Rosen's bimetric theory of gravity provides at present a worthy devil's advocate for the black hole hypothesis, it is important for eventual observational work to elaborate the astrophysical consequences and possibilities peculiar to it. This work is begun by deriving the orbital topography of the spherically symmetric solution to Rosen's field equations - which is relevant to the behavior of relativistic axisymmetric accretion flows - and calculating predicted accretion disk efficiencies, which can be as much as 2.5 times higher than for a disk in Schwarzschild. Thereafter, a brief treatment of the shortest kinematic time scale and the time dilations for in-falling material is given. Finally it is shown that Birkhoff's theorem does not hold in Rosen's theory, and, therefore, that genuine gravitational monopole radiation is possible. The energy it carries, however, is not positive definite.

Stoeger, W. R.↗

Point source moving above a finite impedance reflecting plane - Experiment and theory

A widely used experimental version of the acoustic monopole consists of an acoustic driver of restricted opening forced by a discrete frequency oscillator. To investigate the effects of forward motion on this source, it was mounted above an automobile and driven over an asphalt surface at constant speed past a microphone array. The shapes of the received signal were compared to results computed from an analysis of a fluctuating-mass-type point source moving above a finite impedance reflecting plane. Good agreement was found between experiment and theory when a complex normal impedance representative of a fairly hard acoustic surface was used in the analysis.

Norum, T. D.↗

Aircraft measurement of radio frequency noise at 121.5 MHz, 243MHz and 406MHz

An airborne survey measurement of terrestrial radio-frequency noise over U.S. metropolitan areas has been made at 121.5, 243 and 406 MHz with horizontal-polarization monopole antennas. Flights were at 25,000 feet altitude during the period from December 30, 1976 to January 8, 1977. Radio-noise measurements, expressed in equivalent antenna-noise temperature, indicate a steady-background noise temperature of 572,000 K, at 121.5 MHz, during daylight over New York City. This data is helpful in compiling radio-noise temperature maps; in turn useful for designing satellite-aided, emergency-distress search and rescue communication systems.

Taylor, R. E.↗

Measurements of acoustic sources in motion

Results of the far-field pressures measured from three different types of moving sources are presented. These acoustic sources consist of a point monopole, a small model jet, and an aircraft. Results for the pressure time history produced by the point source show good agreement with those predicted analytically. Both actual and simulated forward motion of the model jet show reductions in noise levels with forward speed at all angles between the source and observer. Measurement with the aircraft over both an anechoic floor and over the ground yields a method for evaluating the transfer function for ground reflections at various angles between the moving aircraft and measurement position.

Maestrello, L.↗

Attenuation of sound in a low Mach number nozzle flow

The energy conversion mechanisms which govern the emission of low frequency sound from an axisymmetric jet pipe of arbitrary nozzle contraction ratio in the case of low Mach number nozzle flow are discussed. The energy of the incident sound which flows through the nozzle is used to maintain two distinct characteristic disturbances in the exterior fluid. First, there is the emitted radiation which has the directivity equivalent to that of a monopole-dipole combination. Second, essentially incompressible vortex waves are induced on the jet by vortex shedding from the lip of the nozzle and may involve the excitation of instability modes. Two linearized analytical models are examined to determine the partition of the emitted energy between the radiation field and the vortex waves. One of these is an exact linear theory in which the jet boundary is treated as a vortex sheet. The second model assumes that the width of the mean shear layer of the jet cannot be neglected. The results are discussed with reference to recent nozzle attenuation measurements.

Howe, M. S.↗

Observable phase factors and symmetry of electric and magnetic charges

The observable phase factor is taken as a basic concept for the description of electromagnetism. Generalization of this concept to SU(2) and SU(2) x U(1) groups is carried out in such a way that the monopoles with quantized charges appear naturally and that the symmetry between the electric and magnetic phenomena is preserved. Some physical implications are discussed.

Hsu, J. P.↗

Radiation of an acoustic source near the trailing edge of a wing in forward motion

The acoustic sources simulating engine or jet noise are located near the trailing edge. During landing or take-off operations the flap angle is not small. Solutions for the scattering of the acoustic sources, monopoles, dipoles or quadrupoles, by the wing with the flap at finite angle are constructed for the case that the acoustic wave length is much larger than the length of the flap but may be of the same order as the chord. For each given geometry of the flap and the position of the singularities, critical orientations of the dipoles and the quadrupoles are defined along which the noise becomes 'suppressed', i.e., the far field pressure becomes one order smaller.

Ting, L.↗

Attenuation of sound in a low Mach number nozzle flow

This paper examines the energy conversion mechanisms which govern the emission of low frequency sound from an axisymmetric jet pipe of arbitrary nozzle contraction ratio in the case of low Mach number nozzle flow. The incident acoustic energy which escapes from the nozzle is partitioned between two distinct disturbances in the exterior fluid. The first of these is the free-space radiation, whose directivity is equivalent to that produced by monopole and dipole sources. Second, essentially incompressible vortex waves are excited by the shedding of vorticity from the nozzle lip, and may be associated with the large-scale instabilities of the jet. Two linearized theoretical models are discussed. One of these is an exact linear theory in which the boundary of the jet is treated as an unstable vortex sheet. The second assumes that the finite width of the mean shear layer of the real jet cannot be neglected. The analytical results are shown to compare favourably with recent attenuation measurements.

Howe, M. S.↗

Airborne East Coast/Midwest urban survey at 121.5/243 MHz

Results of an airborne RF interference measurement survey of the emergency-distress, Search and Rescue frequency bands at 121.5 and 243 MHz at an altitude of 25,000 ft over the U.S. East Coast and Midwest urban areas are presented. Included are a series of minutely median profile plots of antenna-noise temperature computed from RF interference power measurements at the output terminal of a quarter-wavelength monopole antenna, for both daytime and nighttime observations. The greatest value, 450,000 K, of minutely median temperature was recorded over New York City at 121.5 MHz during the day. East Coast urban areas exhibited a day-night effect at 121.5 MHz, where nighttime observations had a median value of 8.6-dB below daytime values; Midwest counterparts did not exhibit a significant day-night effect at 121.5 MHz.

Taylor, R. E.↗

Experimental measurements of moving noise sources

The far-field pressure was measured from three different types of moving sources: a point monopole, a small model jet, and an aircraft. Results for the pressure time history produced by the point source show good agreement with those predicted analytically. Both actual and simulated forward motion of the model jet show reductions in noise levels with forward speed at all angles between the source and observer. Measurement with the aircraft over both an anechoic floor and over the ground yields a method for evaluating the transfer function for ground reflections at various angles between the moving aircraft and measurement position.

Maestrello, L.↗

Direct and inverse problems in radiation of sound from discrete random sources on two coaxial rings

An analytical model consisting of two ring sources of sound is developed to study the direct radiation in terms of correlation, coherence, and phase and also to aid in solving the inverse-radiation problem of determining the noise source in terms of farfield measurements. The rings consist of discrete sources which are either monopoles or quadrupoles with Gaussian autocorrelations. Only adjacent sources, both within and between the rings, are correlated. Results show that from the farfield information one can determine when the sources are compact or noncompact with respect to the acoustic wavelength and distinguish between the types of sources. In addition, from the inverse-radiation approach one can recover the center of mass, the location and separation distance of the ring, and the respective diameters.

Maestrello, L.↗

Gravitational scattering of electromagnetic radiation

The scattering of electromagnetic radiation by linearized gravitational fields is studied to second order in a perturbation expansion. The incoming electromagnetic radiation can be of arbitrary multipole structure, and the gravitational fields are also taken to be advanced fields of arbitrary multipole structure. All electromagnetic multipole radiation is found to be scattered by gravitational monopole and time-varying dipole fields. No case has been found, however, in which any electromagnetic multipole radiation is scattered by gravitational fields of quadrupole or higher-order multipole structure. This lack of scattering is established for infinite classes of special cases, and is conjectured to hold in general. The results of the scattering analysis are applied to the case of electromagnetic radiation scattered by a moving mass. It is shown how the mass and velocity may be determined by a knowledge of the incident and scattered radiation.

Brooker, J. T.↗

Coaxial phased array antenna

A coaxial antenna array for communicating circularly polarized electromagnetic radiation is disclosed. A pair of open ended antenna cavities is coaxially constructed and operates by excitation of linear radiation elements arranged within each of the cavities. A pair of crossed dipole radiation devices is centered within the inner cavity and operated by means of a phase shifting network circuit to transmit as well as receive circularly polarized radiation. Four monopole radiation devices are symmetrically arranged to operate in the outer cavity in phase quadrature by means of the phase shifting network circuit to both transmit and receive circularly polarized electromagnetic radiation. Combined operation of the two antenna cavities with a 180 deg phase differential between the fields related to the two antenna cavities provides a broad beam, relatively wide frequency bandwidth communication capability. Particular embodiments disclosed feature a generally square cavity array as well as a circular cavity array.

Ellis, H., Jr.↗

Some effects of motion and plane reflecting surfaces on the radiation from acoustic sources

The effects of translatory and rotational motion and of the proximity of ground surface on the radiation from acoustic monopole and dipole sources are discussed together with the key equations. It is shown that interference patterns are produced by reflection and absorption at a plane surface and that consequently, the far-field directivity of an acoustic source near the ground can be substantially altered from its free-field value. The kinematic effects discussed include the beaming of sound in the direction of source motion and the shifting of the observer source frequency in a directionally dependent manner. The effects are illustrated by experimental data.

Lansing, D. L.↗

Determination of two-stroke engine exhaust noise by the method of characteristics

A computational technique was developed for the method of characteristics solution of a one-dimensional flow in a duct as applied to the wave action in an engine exhaust system. By using the method, it was possible to compute the unsteady flow in both straight pipe and tuned expansion chamber exhaust systems as matched to the flow from the cylinder of a small two-stroke engine. The radiated exhaust noise was then determined by assuming monopole radiation from the tailpipe outlet. Very good agreement with experiment on an operation engine was achieved in the calculation of both the third octave radiated noise and the associated pressure cycles at several locations in the different exhaust systems. Of particular interest is the significance of nonlinear behavior which results in wave steepening and shock wave formation. The method computes the precise paths on the x-t plane of a finite number of C(sub +), C(sub -) and P characteristics, thereby obtaining high accuracy in determining the tailpipe outlet velocity and the radiated noise.

Jones, A. D.↗

UTD analysis of electromagnetic scattering by flat structures

The different scattering mechanisms that contribute to the radar cross of finite flat plates were identified and analyzed. The geometrical theory of diffraction, the equivalent current and the corner diffraction are used for this study. A study of the cross polarized field for a monopole mounted on a plate is presented, using novel edge wave mechanism in the analysis. The results are compared with moment method solutions as well as measured data.

Sikta, F. A.↗

GTD analysis of airborne antennas radiating in the presence of lossy dielectric layers

The patterns of monopole or aperture antennas mounted on a perfectly conducting convex surface radiating in the presence of a dielectric or metal plate are computed. The geometrical theory of diffraction is used to analyze the radiating system and extended here to include diffraction by flat dielectric slabs. Modified edge diffraction coefficients valid for wedges whose walls are lossy or lossless thin dielectric or perfectly conducting plates are developed. The width of the dielectric plates cannot exceed a quarter of a wavelength in free space, and the interior angle of the wedge is assumed to be close to 0 deg or 180 deg. Systematic methods for computing the individual components of the total high frequency field are discussed. The accuracy of the solutions is demonstrated by comparisons with measured results, where a 2 lambda by 4 lambda prolate spheroid is used as the convex surface. A jump or kink appears in the calculated pattern when higher order terms that are important are not included in the final solution. The most immediate application of the results presented here is in the modelling of structures such as aircraft which are composed of nonmetallic parts that play a significant role in the pattern.

Rojas-Teran, R. G.↗

Highly ionizing cosmic rays at mountain altitude

Large arrays of CR-39 plastic track detectors are being used at various depths in the atmosphere to study energetic, highly ionizing particles. The first measurements are made of the energy spectra of He and Li nuclei at mountain altitude (603 g/sq cm) and the first observations of relativistic heavy nuclei (Z/beta greater than or equal to 12; Z ranging from 8 to 14) at subsonic aircraft altitude (186 g/sq cm). A 95 percent CL upper limit of 0.4/sq m-yr was determined for the flux at 603 g/sq cm of particles with Z/beta greater than or equal to 30 and of monopoles with velocity as low as 0.01 c.

Price, P. B.↗