Search NASA⌕ Search

SEARCH · Search NASA

Results for “AMPLITUDES”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

A complex T-matrix derivation of a resonance amplitude

Time-dependent perturbation-theory techniques are used to derive a compact expression, valid to arbitrary order and displaying time dependence explicitly, for a quantum-mechanical transition amplitude applicable to the description of resonances. A solution representing a complex-energy generalization of the usual real-energy (nonresonant) amplitude is obtained and generalized to all orders by introducing a complex-energy T matrix. Applications to physical problems such as the extension of the Fermi golden rules to resonances (Norbury and Deutchman, 1984); pion production in relativistic nucleus-nucleus collisions (Norbury et al., 1985); and the determination of resonant cross sections for nuclear, atomic, or molecular processes involving the formation and decay of intermediate discrete or continuum states are indicated.

Norbury, J. W.↗
NASA NTRS1985

Elastic scattering of alpha particles and the phase of the nucleon-nucleon scattering amplitude

Glauber theory can describe elastic scattering of alpha particles by He-4, He-3, H-2, and H-1 at 7 GeV/c if the phase of the nucleon-nucleon elastic-scattering amplitude varies with momentum transfer. The phase variation leads to diffraction patterns differing markedly from those typical of constant-phase calculations and greatly affects the magnitudes of the intensities. These changes are mainly due to changes in the interference between amplitudes for different orders of multiple scattering and to a decrease in their moduli.

Franco, V.↗
NASA NTRS1985

Brewster-plate spoiler - A novel method for reducing the amplitude of interference fringes that limit tunable-laser absorption sensitivities

A simple method is described for substantially reducing the amplitude of interference fringes that limit the sensitivities of tunable-laser high-resolution absorption spectrometers. A lead-salt diode laser operating in the 7-micron region is used with a single Brewster-plate spoiler to reduce the fringe amplitude by a factor of 30 and also to allow the detection of absorptances 0.001 percent in a single laser scan without subtraction techniques, without complex frequency modulation, and without distortion of the molecular line-shape signals. Application to multipass-cell spectrometers is described.

Webster, C. R.↗
NASA NTRS1985

The role of amplitude-to-phase conversion in the generation of oscillator flicker phase noise

The role of amplitude-to-phase conversion as a factor in feedback oscillator flicker phase noise is examined. A limiting stage consisting of parallel-connected opposite polarity diodes operating in a circuit environment contining reactance is shown to exhibit amplitude-to-phase conversion. This mechanism coupled with resistive upconversion provides an indirect route for very low frequency flicker noise to be transferred into the phase of an oscillator signal. It is concluded that this effect is more significant in the lower frequency regimes where the onlinear reactances associated with active devices are overwhelmed by linear reactive elements.

Hearn, C. P.↗
NASA NTRS1985

Finite-amplitude steady waves in plane viscous shear flows

Computations of two-dimensional solutions of the Navier-Stokes equations are carried out for finite-amplitude waves on steady unidirectional flow. Several cases are considered. The numerical method employs pseudospectral techniques in the streamwise direction and finite differences on a stretched grid in the transverse direction, with matching to asymptotic solutions when unbounded. Earlier results for Poiseuille flow in a channel are re-obtained, except that attention is drawn to the dependence of the minimum Reynolds number on the physical constraint of constant flux or constant pressure gradient. Attempts to calculate waves in Couette flow by continuation in the velocity of a channel wall fail. The asymptotic suction boundary layer is shown to possess finite-amplitude waves at Reynolds numbers orders of magnitude less than the critical Reynolds number for linear instability. Waves in the Blasius boundary layer and unsteady Rayleigh profile are calculated by employing the artifice of adding a body force to cancel the spatial or temporal growth. The results are verified by comparison with perturbation analysis in the vicinity of the linear-instability critical Reynolds numbers.

Milinazzo, F. A.↗
NASA NTRS1986

The motion of ions specularly reflected off a quasi-parallel shock in the presence of large-amplitude, monochromatic MHD waves

A model is used to examine the motion of specularly reflected ions in the presence of large-amplitude, monochromatic, transverse MHD waves. The calculations of ion trajectories are described. The heating downstream from the quasi-parallel bow shock is analyzed. The relationship between the specularly reflected ions and their gyrospeeds and guiding center speeds is studied. The data reveal that the characteristics of the motion depend on the frequency, wavelength, phase, and amplitude of the wave that is converted into the shock.

Fuselier, S. A.↗
NASA NTRS1986

Excitation of nearly steady finite-amplitude barotropic waves

An exact nonlinear stationary solution is obtained for barotropic waves in a beta-plane channel and it is shown that it can be excited under a range of initial conditions. Results show that a finite-amplitude wave in a constant shear flow, given an initial phase tilt against the shear and a sufficient initial amplitude, interacts with the mean flow to produce a nearly steady state close to the exact stationary solution. This equilibration process involves nonlinear transients; in particular, as the flow equilibrates, the emergence of critical levels is accompanied by the neutralization of local mean vorticity gradients at these levels, thus allowing the solution to attain a nonsingular modal structure.

Hou, A. Y.↗
NASA NTRS1986

Large amplitude wave packets observed in the ionosphere in association with transverse ion acceleration

Very short duration, large amplitude bursts of monochromatic waves ('spikelets') were detected by the electric field experiment on the sounding rocket MARIE, launched in February 1985 from Churchill, Manitoba. About 35 events were detected, with an average time scale of 5 ms and an average amplitude of 100-150 mV/m. Their frequency varied between 7 and 18 kHz, and there is some evidence that the frequency is a decreasing function of altitude. The bursts are not correlated with any events on the payload, and their occurrence is not related to the rocket's spin or coning. The events were confined to the altitude range 450-650 kilometers. This coincides exactly with the altitude range for which perpendicular (90 deg) ion conics were detected by the particle experiment on the same payload. The 'spikelets' were also correlated one-to-one with small (10-100 mV/m) double-layerlike or shocklike features of similar time scale in the dc electric field data.

Labelle, J.↗
NASA NTRS1986

Observations of fatigue crack growth rate and closure behavior in 7475-T731 aluminum under variable amplitude loading

Simultaneous measures of fatigue crack propagation rate and crack closure were taken from 7475-T731 aluminum alloy center cracked tensile specimens under variable amplitude loading conditions. Results indicate the presence of load interaction in a variety of repeated variable amplitude loading blocks. Load interaction results in retardation of the propagation rate for most loading blocks examined in the study. Closure did not explain observed delay behavior associated with high stress ratio and single peak overloads. Primary influences on the crack opening stress intensity level include the maximum stress intensity of the repeated loading block, the minimum stress intensity of the block, and the number of fatigue cycles between overloads of the block.

Turner, Charles C.↗
NASA NTRS1987

Simulation of amplitude-modulated circularly polarized Alfven waves for beta less than one

The nonlinear properties of the amplitude-modulated circularly polarized Alfven wave are studied for beta less than one. The temporal behavior of the wave packet of the electromagnetic hybrid simulation is compared with a numerical solution of the derivative nonlinear Schroedinger (DNLS) equation. It is shown that the left-hand-polarized mode evolves into a shocklike structure due to the modulational instability. However, both cyclotron damping and a snowplow effect near the steepened wave packet suppress its further steepening, contrary to the predictions of the DNLS equation. For the right-hand mode, formation of the shock does not take place, and the initial time development is well described by the DNLS equation. The daughter Alfven wave and ion acoustic waves are excited due to the decay instability at a later time. Heating or acceleration of the particles takes place for both left- and right-hand waves. Energy transfer from the wave to the particles occurs effectively when substantial modulation in the wave amplitude is present.

Machida, S.↗
NASA NTRS1988

Spectral estimation of received phase in the presence of amplitude scintillation

A technique is demonstrated for obtaining the spectral parameters of the received carrier phase in the presence of carrier amplitude scintillation, by means of a digital phased locked loop. Since the random amplitude fluctuations generate time-varying loop characteristics, straightforward processing of the phase detector output does not provide accurate results. The method developed here performs a time-varying inverse filtering operation on the corrupted observables, thus recovering the original phase process and enabling accurate estimation of its underlying parameters.

Vilnrotter, V. A.↗
NASA NTRS1987

The effect of decay of the amplitude of oscillation on random process models for QPO X-ray stars

Random process models provide a useful mathematical framework for analysis of the quasi-periodic oscillations (QPO) recently discovered in the X-ray emission from certain luminous Galactic X-ray stars. In this paper, consideration is given to the effects of the power spectrum of the decay of the amplitude of oscillation that is expected in some physical models for the QPO phenomena. The resulting changes in the power spectrum depend on the ratio of the decay time of the amplitude of oscillation to the lifetime of the shot envelope. For values of this ratio less than about 10, it is found that incoherent QPO peaks in the power spectrum are significantly reduced in height and total power and are broadened in width, making them more difficult to detect. Coherent terms in the power spectrum, if present, are reduced in height and total power, but their width is unaffected. The red noise component remains unchanged. The present results are applied to the beat-frequency modulated accretion model in order to set constraints on the physical properties of the boundary layer between the inner edge of the accretion disk and the neutron star magnetosphere.

Shibazaki, N.↗
NASA NTRS1988

Modeling of large-amplitude high-angle-of-attack maneuvers

Results from a simulation study to characterize and model rapid, large-amplitude high-angle-of-attack maneuvers of high performance aircraft are summarized. There is concern that conventional aerodynamic math models combining static and small-amplitude demping data may be deficient in properly representing the associated aerodynamics to allow accurate prediction of these types of motion. The objectives of this investigation were: (1) to characterize these maneuvers in order to establish the aerodynamic modeling requirements, (2) to define refinements to address these requirements, and (3) to examine the impact of these refinements on the calculated motions. The results show that accurate modeling of the dynamic terms is vital due to the very high angular rates generated during these maneuvers. It was found that the addition of dynamic terms to include the lateral acceleration derivatives and the rotary aerodynamics can in some cases significantly affect the simulated flight motions.

Angle of attack↗
NASA NTRS1988

Predicting the maximum amplitude for the sunspot cycle from the rate of rise in sunspot number

The paper examines the relationship between the maximum amplitude for the sunspot cycle and the rate of rise during the ascending phase. It is found that, prior to about 2 yr into the cycle, the rate of rise is not a reliable predictor for maximum amplitude. Only during the second half of the ascent do the fits display strong linearity, having a correlation coefficient of r = 0.9 and a standard error Syx less than about 20.

Wilson, Robert M.↗
NASA NTRS1989

A finite-element method for large-amplitude, two-dimensional panel flutter at hypersonic speeds

The nonlinear flutter behavior of a two-dimensional panel in hypersonic flow is investigated analytically. An FEM formulation based unsteady third-order piston theory (Ashley and Zartarian, 1956; McIntosh, 1970) and taking nonlinear structural and aerodynamic phenomena into account is derived; the solution procedure is outlined; and typical results are presented in extensive tables and graphs. A 12-element finite-element solution obtained using an alternative method for linearizing the assumed limit-cycle time function is shown to give predictions in good agreement with classical analytical results for large-amplitude vibration in a vacuum and large-amplitude panel flutter, using linear aerodynamics.

Mei, Chuh↗
NASA NTRS1989

Subharmonic and fundamental high amplitude excitation of an axisymmetric jet

The effect of simultaneous excitation at the fundamental and subharmonic frequencies on the behavior of a circular jet shear layer is studied. Attention is given to the effect of the initial phase difference, the Strouhal number pair, and amplitudes of the fundamental and subharmonic tones. High-amplitude excitation devices which can provide a wide range of forcing conditions when used in conjunction with equipment that produces complex waveforms are used.

Raman, Ganesh↗
NASA NTRS1960

Experimental Determination of the Effects of Frequency and Amplitude of Oscillation on the Roll-Stability Derivatives for a 60 Degree Delta-Wing Airplane Model

A 60' delta-wing airplane model was oscillated in roll for several frequencies and amplitudes of oscillation to determine the effects of the oscillatory motion on the roll-stability derivatives for the model. The derivatives were measured at a Reynolds number of 1,600,000 for the wing alone, the wing-fuselage combination, and the complete model which included a triangular-plan-form vertical tail. Both rolling and yawing moments due to rolling velocity exhibited large frequency effects for angles of attack higher than 16 degrees. Variations in these derivatives were measured for the lowest frequencies of oscillation; as the frequency increased, the derivatives because more nearly linear with angle of attack. Both velocity derivatives were considerably different at high angles of attack from the corresponding derivatives measured by the steady-state rolling-flow technique. Rolling and yawing moments due to rolling acceleration were measured and similarly found to be highly dependent on frequency at high angles of attack. Some period and time-to-damp computations, which were made to reveal the significance of the acceleration derivatives, indicated that inclusion of the measured derivatives in the equations of motion lengthened the period of the lateral oscillation by 10 percent for a typical delta-wing airplane and increased the time to damp to one-half amplitude by 50 percent.

Fisher, Lewis R.↗
NASA NTRS1990

The amplitudes of interplanetary fluctuations - Stream structure, heliocentric distance, and frequency dependence

A study is presented of the heliocentric distance, frequency, and stream structure dependence of the amplitudes of interplanetary fluctuations in the velocity and magnetic field from 0.3 to nearly 20 AU and for spacecraft-frame periods of 10 days to a few hours. Evidence is presented that, at a given heliocentric distance, the amplitude of the magnetic field fluctuations is proportional to the magnitude of the field, nearly independently of the solar wind speed. The radial evolution of magnetic fluctuations is shown to be nearly consistent with WKB expectations except at smaller scales in the inner heliosphere and at the largest scales in the outer heliosphere. While the large-scale velocity fluctuations are kinetic energy-dominated in the inner heliosphere due to the presence of streams, the magnetic fluctuation energy eventually comes to be slightly dominant over the kinetic energy at all scales. The theoretical implications of the results are considered.

Roberts, D. A.↗