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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

The Influence of Finite-size Sources in Acousto-ultrasonics

This work explores the effects that the finite normal axisymmetric traction loading of an infinite isotropic plate has on wave propagation in acousto-ultrasonics (AU), in which guided waves are created using two normal incidence transducers. Although the work also addresses the effects of the transducer pressure distribution and pulse shape, this thesis concentrates on two main questions: how does the transducer's diameter control the phase velocity and frequency spectrum of the response, and how does the plate thickness relate to the plate's excitability? The mathematics of the time-harmonic solution and the physical principles and the practical considerations for AU wave generation are explained. Transient sources are modeled by the linear superposition of the time-harmonic solutions found using the Hankel transform and they are then compared to experimental data to provide insight into the relation between the size of the transducer and the preferred phase velocity.

Pavlakovic, Brian N.↗
NASA NTRSDate not supplied

Where Lies the Quantumness Behind Detecting Electromagnetic Waves for Frequencies from Infrared and Up?

The synthesis of Newtonian concept of corpuscles during emission with Huygens’ concept of secondary-wavelets during propagation implies that all EM radiations from quantized atoms and molecules are released as discrete amount of energies. However, they propagate out as time-finite Maxwellian light pulses. Huygens also underscored that his secondary wavelets keep propagating as independent pulses in the absence of any interacting medium, or until intercepted by an interface with a medium or a detector. Then we use the Superposition Principle and the coherence theory to derive Einstein’s photoelectric equation by summing innumerable random time-finite pulses. This process driven approach should yield the characteristic statistical variations of photoelectron current pulses, as generated by photodetectors for different kinds of light sources. Lamb & Scully originally proposed this semiclassical approach without assuming that light actually consists of time finite pulses. The quantumness remains confined within the excitation and de-excitations processes in the material particles.

Electromagnetic Waves↗
NASA NTRS1976

Aeroacoustics

A review of the acoustics of moving media is provided, taking into account a derivation of basic equations, wave-like solutions of acoustic equations, the solutions of acoustic equations by superposition of elementary sources, the source distribution in unbounded regions, the radiation field, energy relations, and moving sound sources. The characteristics of aerodynamic sound are considered along with the effect of solid boundaries, the effect of uniform flow, acoustic field equations, and the effects of nonuniform mean flow on the generation of sound. Attention is given to Lighthill's acoustic analogy, the solution of Lighthill's equation when no solid boundaries are present, the application of Lighthill's theory to turbulent flows, and the physics of jet noise.

Goldstein, M. E.↗
NASA NTRS1998

Interference of Backscatter from Two Droplets in a Focused Continuous Wave CO2 Doppler Lidar Beam

Using a focused continuous wave CO2 Doppler lidar at wavelength 9.1 microns the interference of the superposition of backscatter from two approx. 14.12 microns diameter silicone oil droplets in the lidar beam was observed. This interference results in a single backscatter pulse from the two droplets with a distinct periodic structure. The interference is due to the phase difference in backscatter from the two droplets while they are traversing the lidar beam because they are moving at different speeds and, thus, the droplet separation is not constant. The complete cycle of interference, with periodicity of 2(Pi), gives excellent agreement between measurements and lidar theory.

Jarzembski, Maurice A.↗
NASA NTRS2007

A Deeper Look at the Fundamentals of Heterodyne Detection Requirements

We generally accept the experimentally observed criteria for heterodyne detections that the two waves that are mixed must (i) be collinear, (ii) have matched wave fronts and (iii) cannot be orthogonally polarized. We have not found in the literature adequate physical explanations for these requirements. The purpose of this paper is to find deeper physical understanding of the coherent heterodyne detection processes that could lead to better coherent laser radar system designs1. We find that there are a number of unresolved paradoxes in classical and quantum optics regarding the definitions and understanding of the "interference" and "coherence" properties of light, which are attributed as essentially due to inherent properties of the EM waves. A deeper exploration indicates that it is the various quantum mechanical properties of the detecting material dipoles that make light detectable (visible, or measurable) to us. Accordingly, all the properties that we generally attribute to only light, are in reality manifestations of collective properties of dipole-light interactions. "Interference" and "coherence" can be better understood in terms of this mutual interaction, followed by energy absorption by the dipoles from EM wave fields, manifesting in some measurable transformation of the detecting dipoles. Light beams do not interfere by themselves. The superposition effects due to light beams become manifest through the response characteristics of the detecting dipoles. In this paper, we will show some preliminary expe rimental results that clearly demonstrate that the heterodyning wave fronts have quantitative degradation in signal generation as the angle between them deviates from perfect collinearity. Subsequently, we will propose a hypothesis for this behavior. We will present experimental data establishing that the so called incoherent light can be detected through heterodyne mixing as long as the pulse length contained in the "incoherent" light is longer than the response time of the detector. We will also present a correspondingly better interpretation of two distinguishable coherence properties, temporal coherence and spectral coherence. Our investigation provides a deeper insight into how to rela x various system requirements for heterodyne detection and accordingly develop systems that are simpler, more reliable and lower in cost. Also, we believe that engineering of detector architecture by appropriately modifying dipole behavior using emerging nanotechnology to optimize heterodyne efficiency will be advantageous.

Roychoudhuri, Chandrasekhar↗
NASA NTRS1990

Simplified calculation of wave-coupling between lines in high-speed integrated circuits

The construction of time-domain solutions by the superposition of modes is explained for the general case, which may not have any symmetry in the lines or terminations. This simplified method requires that the logic-gate terminal impedances be approximated by linear effective resistances that can be adequately determined by SPICE analysis of the actual logic gates. Several examples of results of crosstalk calculations made by the simplified method are presented and compared with corresponding results obtained using the SPICE transmission-line methods of Tripathi and Rettig (1985) along with full SPICE models of the logic gates. The simplified approach is shown to give satisfactory agreement for most purposes.

Matthaei, George L.↗
NASA NTRS1975

Plasma wave interactions with energetic ions near the magnetic equator

An intense band of electromagnetic noise is frequently observed near the magnetic equatorial plane at radial distance from about 2 to 5 Re. Recent wideband wave-form measurements with the IMP-6 and Hawkeye-1 satellites have shown that the equatorial noise consists of a complex superposition of many harmonically spaced lines. Several distinctly different frequency spacings are often evident in the same spectrum. The frequency spacing typically ranges from a few Hz to a few tens of Hz. It is suggested that these waves are interacting with energetic protons, alpha particles, and other heavy ions trapped near the magnetic equator. The possible role these waves play in controlling the distribution of the energetic ions is considered.

Gurnett, D. A.↗
NASA NTRS2018

Direct Observation of Electron Distributions Inside Millisecond Duration Electron Holes

Despite the importance of millisecond duration spatial structures [chorus wave nonlinearity or time domain structures (TDS)] to plasma dynamics, there have been no direct observations of the generation and interaction of these waves and TDS with electrons at the millisecond timescale required for their understanding. Through superposition of 0.195 ms Magnetospheric Multiscale Satellite electron measurements inside 37 superposed, millisecond duration electron holes, the first observations of electron spectra and pitch angle distributions on a submillisecond timescale have been obtained. They show that keV electrons inside the superposed electron hole are accelerated by several hundred volts and that the spectrum of electrons inside the electron hole contain several maxima and minima that are explained by a model of electron energy changes on entering the holes. We report the first observation of trapped electrons inside the TDS, in accordance with the theoretical requirement that such electrons must be present in order to form the phase space holes. Mechanisms of electron acceleration by electron holes (through perpendicular energy gain as the TDS moves into a converging magnetic field) and scattering (due to the perpendicular electric field) are discussed.

Mozer, F. S.↗
NASA NTRS1976

Plasma wave interactions with energetic ions near the magnetic equator

An intense band of electromagnetic noise is frequently observed near the magnetic equatorial plane at radial distance from about 2 to 9 earth radii. Recent wide band wave form measurements with the Imp 6 and Hawkeye 1 satellites have shown that the equatorial noise consists of a complex superposition of many harmonically spaced lines. Several distinctly different frequency spacings are often evident in the same spectrum. The frequency spacing typically ranges from a few hertz to a few tens of hertz. The purpose of this paper is to suggest that these waves are interacting with energetic protons, alpha particles, and other heavy ions trapped near the magnetic equator. The possible role that these waves play in controlling the distribution of the energetic ions is considered.

Gurnett, D. A.↗
NASA NTRS1980

A model theory for the fibrous absorber. Part 1: Regular fibre arrangements

The axial propagation of sound waves in a model consisting of parallel fibers is calculated. The viscous forces and the thermal conduction are taken into account. This leads to viscous waves and to thermal waves besides the usual acoustic compression wave. The potential function for the total field near a fiber is treated as the superposition of the radiated field from the fiber itself and of the scattered fields from all the other fibers. The explicit field equations for a regular square fiber arrangement is derived and the influence of the order of symmetry of the arrangement is discussed. This leads to simplifications in the field equations and to field equations for the case of a homogeneous fiber distribution.

Mechel, F. P.↗
NASA NTRS1972

Diffraction of a plane pulse by a three-dimensional corner.

The conical solutions for the incidence of a plane pulse on a three-dimensional corner are presented. The corner is represented by a trihedron with one edge perpendicular to the other two. Both the boundary condition of the first kind and that of the second kind are considered. Outside the characteristic sphere of the vertex of the corner, the solution is represented by the well-known conical solutions in two variables. Inside the characteristic sphere, the problem involves three conical variables. By the separation of variables, the problem is reduced to that of an eigenvalue problem with an irregular boundary which is in turn reduced to a system of homogeneous algebraic equations. The eigenvalues are then determined numerically. By the superposition of the conical solutions for plane pulses, the solution for the incidence of a plane wave is obtained. Numerical examples simulating the incidence of a sonic boom on the corner of a structure are presented.

Ting, L.↗
NASA NTRS1995

Nonclassical light in interferometric measurements

It is shown that the even and odd coherent light and other nonclassical states of light like superposition of coherent states with different phases may replace the squeezed light in an interferometric gravitational wave detector to increase its sensitivity.

Ansari, N. A.↗
NASA NTRS1998

Root-growth behavior of the Arabidopsis mutant rgr1. Roles of gravitropism and circumnutation in the waving/coiling phenomenon

In this study we investigated the kinetics of the gravitropic response of the Arabidopsis mutant rgr1 (reduced root gravitropism). Although the rate of curvature in rgr1, which is allelic to axr4, was smaller than in the wild type (ecotype Wassilewskija), curvature was initiated in the same region of the root, the distal elongation zone. The time lag for the response was unaffected in the mutant; however, the gravitropic response of rgr1 contained a feature not found in the wild type: when roots growing along the surface of an agar plate were gravistimulated, there was often an upward curvature that initiated in the central elongation zone. Because this response was dependent on the tactile environment of the root, it most likely resulted from the superposition of the waving/coiling phenomenon onto the gravitropic response. We found that the frequency of the waving pattern and circumnutation, a cyclic endogenous pattern of root growth, was the same in rgr1 and in the wild type, so the waving/coiling phenomenon is likely governed by circumnutation patterns. The amplitudes of these oscillations may then be selectively amplified by tactile stimulation to provide a directional preference to the slanting.

NASA Discipline Plant Biology↗
NASA NTRS2016

Scattering of Gaussian Beams by Disordered Particulate Media

A frequently observed characteristic of electromagnetic scattering by a disordered particulate medium is the absence of pronounced speckles in angular patterns of the scattered light. It is known that such diffuse speckle-free scattering patterns can be caused by averaging over randomly changing particle positions and/or over a finite spectral range. To get further insight into the possible physical causes of the absence of speckles, we use the numerically exact superposition T-matrix solver of the Maxwell equations and analyze the scattering of plane-wave and Gaussian beams by representative multi-sphere groups. We show that phase and amplitude variations across an incident Gaussian beam do not serve to extinguish the pronounced speckle pattern typical of plane-wave illumination of a fixed multi-particle group. Averaging over random particle positions and/or over a finite spectral range is still required to generate the classical diffuse speckle-free regime.

radiative transfer theory↗
NASA NTRS1984

Line sources of instability waves in a Blasius boundary layer

Numerical solutions of the instability wave pattern behind a harmonic point source in a Blasius boundary layer are used to form line sources by superposition. For infinite-length spanwise line sources of constant amplitude and phase, the result is just the two-dimensional normal mode of the same frequency; for a sinusoidal amplitude or linear phase distribution, the result is an oblique normal mode of the same spanwise wavenumber. A finite-length spanwise source simulates a vibrating ribbon. In a study of the influence of the source length on the downstream amplitude, the effective field of view is found to have a half-angle of about 16 deg. If the source tips are within this field, the amplitude may be either greater than or less than the comparable normal-mode amplitude, depending on the distance from the source and the spanwise location. For an oblique line source, the downstream wave development at each spanwise location is found to be close to, but not identical with, that of an oblique normal mode which originates at the source with the initial wave angle of the source and satisfies the irrotationality condition on the wavenumber vector.

Mack, L. M.↗
NASA NTRS1989

Interplanetary energetic particle observations of the March 1989 events

The IMP-8 spacecraft placed in an elongated orbit of approximately R(sub E) x R(sub E) orbit around the Earth was the only monitor of the energetic particle environment of the near interplanetary space during the period of the solar particle events associated with the Active Region 5395 in March 1989. Measurements of energetic ion and electron intensities were obtained in a series of channels within the energy range: 0.3 to 440 MeV for photons, 0.6 to 52 MeV/nuc for alpha particles, 0.7 to 3.3 MeV/nuc for nuclei with Z greater than or equal to 3, 3 to 9 MeV/nuc with Z greater than or equal to 20, and 0.2 to 2.5 MeV for electrons. The responses of selected energy channels during the period 5 to 23 March 1989 are displayed. It is clearly noted that the most prominent energetic ion intensity enhancements in that time interval were associated with the interplanetary shock wave of March 13 (07:42 UT) as well as that of March 8 (17:56 UT), which have distinct particle acceleration signatures. These shock waves play a major role in determining the near Earth energetic ion intensities during the above period by accelerating and modulating the ambient solar energetic particle population, which was already present in high intensities in the interplanetary medium due to the superposition of a series of solar flare particle events originating in AR 5395. The differential ion intensities at the lowest energy channel of the CPME experiment, which were associated with the March 13 shock wave, reached the highest level in the life of the IMP-8 spacecraft at this energy. At high energies, the shock associated intensity peak was smaller by less than a factor of 3 than the maxima of solar flare particle intensities from some other major flares, in particular from those with sites well connected to the Earth's magnetic flux tubes.

Sarris, E. T.↗
NASA NTRS1992

Intermodulation in the oscillatory magnetoresistance of a two-dimensional electron gas

The oscillatory magnetoresistance wave form of a 2-D electron gas shows multiple structures when two subbands are populated. In addition to high-field oscillations at a frequency equal to the sum of the two frequencies corresponding to the concentrations of the subbands, and to a superposition at intermediate fields, we observed oscillations at the difference frequency at low fields and higher temperatures. The field range at which the frequency difference is observed increases with increasing temperature. The crossover from superposition to frequency difference is accompanied by a beat. Similar beats, whose field location shows identical temperature dependence, can be observed in data obtained by other groups on different structures. The various components of the wave form can be attributed to different phase relations between the diagonal and off-diagonal elements of the conductivity tensor. It is shown how the intermodulation term, when inserted into the extended oscillatory equation, can give rise to all three structures.

Schacham, S. E.↗