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NASA NTRS1986

Crash energy absorbing composite sub-floor structure

Static crushing tests were conducted on four different beam concepts; honeycomb sandwich, sine-wave and two integrally stiffened designs. The sine-wave beams, depending upon specimen geometry, has the highest energy absorption potential of the four concepts evaluated. All beam designs produced a progressive crushing mode similar to tube specimens. The energy absorption capability of sine-wave beam specimens were predictable from results of circular cross section tubes. A comparison of energy absorption capability was made between integrally stiffened beams fabricated from graphite/epoxy, Kevlar-49/epoxy and aluminum. The energy absorption capability of the graphite/epoxy integrally stiffened beams exceeded both the Kevlar-49/epoxy and aluminum integrally stiffened beams. The energy absorption potential of composite structures is between five and ten times that of comparable metallic structure.

Farley, G. L.↗

NASA NTRS1989

The oblique behavior of low-frequency electromagnetic waves excited by newborn cometary ions

The free energy in oxygen or hydrogen ions freshly created in the solar wind stimulates low-frequency electromagnetic waves whose growth does not always maximize at parallel propagation. Exploration of the wave vector plane discloses the frequent occurrence of islets of oblique growth unconnected to the unstable parallel modes. Contour plots of the growth rate, real frequency, polarization, and magnetic compression characterize the oblique wave behavior for large values of the initial pitch angle of the cometary particles. Although wave-particle (Landau and cyclotron) resonances feed most of the surveyed oblique instabilities, some are seemingly fluidlike. The results, obtained from the numerical solution of the kinetic dispersion and wave equations, imply that newborn ions can easily excite significant oblique hydromagnetic wave activity. Cometary environments provide the adopted plasma model, but the study is helpful in the interpretation of other low-frequency wave observations in space.

Brinca, Armando L.↗

NASA NTRS2012

Envelope Solitons in Acoustically Dispersive Vitreous Silica

Acoustic radiation-induced static strains, displacements, and stresses are manifested as rectified or dc waveforms linked to the energy density of an acoustic wave or vibrational mode via the mode nonlinearity parameter of the material. An analytical model is developed for acoustically dispersive media that predicts the evolution of the energy density of an initial waveform into a series of energy solitons that generates a corresponding series of radiation-induced static strains (envelope solitons). The evolutionary characteristics of the envelope solitons are confirmed experimentally in Suprasil W1 vitreous silica. The value (-11.9 plus or minus 1.43) for the nonlinearity parameter, determined from displacement measurements of the envelope solitons via a capacitive transducer, is in good agreement with the value (-11.6 plus or minus 1.16) obtained independently from acoustic harmonic generation measurements. The agreement provides strong, quantitative evidence for the validity of the model.

Cantrell, John H.↗

NASA NTRS1971

Surface deviations and imaging performance

Telescope wave optical imaging performance, deriving point spread function, radial energy integral and modulation transfer functions for different wave front surface deviations

Tschunko, H. F. A.↗

NASA NTRS2007

New and Improved T-wave Morphology Parameters to Differentiate Healthy Individuals from those with Cardiomyopathy and Coronary Artery Disease

We investigated the ability of several known as well as new ECG repolarization parameters to differentiate healthy individuals from patients with obstructive coronary artery disease (CAD) and cardiomyopathy (CM). Advanced high-fidelity 12-lead ECG tests (approx. 5-min supine) were first performed on a "training set" of 99 individuals: 33 with ischemic or dilated CM and low ejection fraction (EF less than 40%); 33 with catheterization-proven obstructive CAD but normal EF; and 33 age-/gender-matched healthy controls. The following multiple parameters of T-wave morphology (TWM) were derived via signal averaging and singular value decomposition (SVD, which yields 8 eigenvalues, rho(sub 1) greater than rho(sub 2)...greater than rho(sub 8) and studied for their retrospective accuracy in detecting underlying disease: 1) Principal component analysis ratio of the T wave (T-PCA) = 100*rho(sub 2)/rho(sub 1); 2) Relative T-wave residuum (rTWR) = 100* SIGMA (rho(sub 4)(sup 2) +...+ rho(sub 8)(sup 2)); 3) Modified complexity ratio of the T wave (T-mCR) = 100*SIGMA(rho(sub 3)(sup 2) +...+rho(sb 8) (sup 2)); and 4) Normalized 3-dimensional volume of the T wave (nTV) = 100*(rho(sub 2)*rho(sub 3)/rho(sub 1)(sup 2). All TWM parameters significantly differentiated CAD from controls (p less than 0.0001), and also CM from controls (p less than 0.0001). Retrospective areas under the ROC curve were 0.77, 0.81, 0.82, and 0.83 (CAD vs. controls) and 0.93, 0.89, 0.95 and 0.96 (CM vs. controls) for T-PCA, rTWR, T-mCR and nTV respectively. The newer TWM parameters (T-mCR and nTV) thus outperformed the more established parameters (T-PCA and rTWR), presumably by putting a greater emphasis on the third T-wave eigenvalue, which in most healthy subjects has little energy compared to the first two eigenvalues. Subsequent prospective analyses have also yielded similar results, such that we conclude that diagnostic differentiation of pathology from non-pathology may be especially aided by detecting the transference of energy from the first and second T-wave eigenvalues into the third T-wave eigenvalue.

Greco, E. C.↗

NASA NTRS1982

Internal gravity waves in the solar atmosphere. II - Effects of radiative damping

In the solar photosphere, temperature fluctuations associated with acoustic-gravity waves may be rapidly smoothed by the transfer mechanism of radiation between hotter and cooler regions. The present investigation of the radiative effects on internal gravity waves takes into account the parameterization of the radiative energy, employing the Newtonian cooling approximation. A linear analysis of the propagation of internal gravity waves is carried out in a model of the solar atmosphere which is taken to be homogeneous in the horizontal coordinates. Linear wave properties both with and without radiative cooling are summarized, and the variation with height of energy fluxes and of nonlinearities in the waves is discussed. Attention is given to the significance of the obtained results in terms of energy balance in the chromosphere and in relation to spectral line observations.

Mihalas, B. W.↗

NASA NTRS1988

Microwave background anisotropy induced by gravitational waves

A cosmological background of gravitational waves induces redshift perturbations in light transversing it. Calculations of this Sachs-Wolfe effect on the microwave background are presented in an Omega = 1 Friedmann universe as a function of angular scale and gravitational wave spectrum. Blurriness of the last-scattering surface can cause nonnegligible dilution of the anisotropy for wavelengths less than about 100 Mpc. The limit implied for the energy density of the gravitational waves is given. A difficulty in associating a linear scale with an angular anisotropy, due to the clumpiness of the universe, is also pointed out.

Linder, Eric V.↗

NASA NTRS1984

Alfvenic resonances on solar spicules

It is suggested that twisting and heating of solar spicules can be produced by Alfven waves which enter the spicule from below. The spicule is treated as a region of constant Alfven speed which is bounded above by a region of much higher Alfven speed (the corona) and below by a region of exponentially increasing Alfven speed (the photosphere and chromosphere). It is shown how the spicule can act as a resonant cavity. The transmission of the waves into the cavity is analytically determined to be enhanced at certain resonant frequencies. With reasonable spicule parameters, and assuming the spicule damping to be moderately large, it is found that twisting velocities of approximately 20-30 km/s can be induced on the spicule. It is suggested that the Alfven waves are dissipated via a turbulent cascade of their energy to higher wavenumbers. It is shown that the waves can thereby heat the spicules to the observed temperatures. It is further suggested that the continued input of energy can explain why H-alpha spicules fade, since the predicted heating rate is sufficient to heat the spicules to temperatures at which the hydrogen is fully ionized; thus H-alpha spicules may evolve into EUV spicules.

Sterling, A. C.↗

NASA NTRS2007

Binary Black Holes, Numerical Relativity, and Gravitational Waves

The final merger of two black holes releases a tremendous amount of energy, more than the combined light from all the stars in the visible universe. This energy is emitted in the form of gravitational waves, and observing these sources with gravitational wave detectors such as LISA requires that we know the pattern or fingerprint of the radiation emitted. Since black hole mergers take place in regions of extreme gravitational fields, we need to solve Einstein's equations of general relativity on a computer in order to calculate these wave patterns. For more than 30 years, scientists have tried to compute these wave patterns. However, their computer codes have been plagued by problems that caused them to crash. This situation has changed dramatically in the past 2 years, with a series of amazing breakthroughs. This talk will take you on this quest for these gravitational wave patterns, showing how a spacetime is constructed on a computer to build a simulation laboratory for binary black hole mergers. We will focus on the recent advances that are revealing these waveforms, and the dramatic new potential for discoveries that arises when these sources will be observed by LISA

Centrella, Joan↗

NASA NTRS2007

Cosmic Messengers: Binary Black Holes and Gravitational Waves

The final merger of two black holes releases a tremendous amount of energy, more than the combined light from all the stars in the visible universe. This energy is emitted in the form of gravitational waves, and observing these sources with gravitational wave detectors such as LISA requires that we know the pattern or fingerprint of the radiation emitted. Since black hole mergers take place in regions of extreme gravitational fields, we need to solve Einstein s equations of general relativity on a computer in order to calculate these wave patterns. For more than 30 years, scientists have tried to compute these wave patterns. However, their computer codes have been plagued by problems that caused them to crash. . This situation has changed dramatically in the past 2 years, with a series of amazing breakthroughs. This talk will take you on this quest for these gravitational wave patterns, showing how a spacetime is constructed on a computer to build a simulation laboratory for binary black hole mergers. We will focus on the recent advances that are revealing these waveforms, and the dramatic new potential for discoveries that arises when these sources will. be observed by LISA.

Centrella, Joan↗

NASA NTRS2007

Binary Black Holes and Gravitational Waves

The final merger of two black holes releases a tremendous amount of energy, more than the combined light from all the stars in the visible universe. This energy is emitted in the form of gravitational waves, and observing these sources with gravitational wave detectors such as LIGO and LISA requires that we know the pattern or fingerprint of the radiation emitted. Since black hole mergers take place in regions of extreme gravitational fields, we need to solve Einstein's equations of general relativity on a computer in order to calculate these wave patterns. For more than 30 years, scientists have tried to compute these wave patterns. However, their computer codes have been plagued by problems that caused them to crash. This situation has changed dramatically in the past 2 years, with a series of amazing breakthroughs. This discussion examines these gravitational patterns, showing how a spacetime is constructed on a computer to build a simulation laboratory for binary black hole mergers. The focus is on recent advances that are revealing these waveforms, and the dramatic new potential for discoveries that arises when these sources will be observed by the space-based gravitational wave detector LISA.

Centrella, Joan↗

NASA NTRS2007

Binary Black Holes and Gravitational Waves

Centrella, Joan↗

NASA NTRS1990

Stochastic Fermi acceleration in solar flares

Proton spectra, valid from non- to ultra-relativistic energies, resulting from stochastic Fermi acceleration in solar flares are calculated. These spectra were obtained by numerically solving the Fokker-Planck equation, in which the escape of the particles from the acceleration region is characterized by an energy-independent escape time. In addition to equilibrium spectra, time-dependent energy spectra showing the approach to equilibrium are also presented. These numerical equilibrium spectra are compared with previous results which were obtained either by Monte Carlo simulations or approximate analytical treatments. There are no analytic solutions valid in the transrelativistic regime, which is very important for the production of pions and neutrons in solar flares. The acceleration efficiency is related to physical parameters, in particular the energy density in either magnetosonic or Alfven waves, and a lower limit is placed on either of these energy densities from acceleration times implied by gamma-ray observations. Also discussed is the physical interpretation of the escape time.

Miller, James A.↗

NASA NTRS1991

Equilibrium wave spectrum and sea state bias in satellite altimetry

For a well-developed sea at equilibrium with a constant wind, the energy-containing range of the wavenumber spectrum for wind-generated gravity waves is approximated by a generalized power law involving the angular spread function and mu, interpreted as a fractal codimension of a small surface patch. Dependence of mu on the wave age is estimated, and the 'Phillips constant', beta, along with the low-wavenumber boundary, k0, of the inertial subrange are analyzed on the basis of the wave action and energy conservation principles. The resulting expressions are employed to evaluate various non-Gaussian statistics of a weakly nonlinear sea surface, which determine the sea state bias in satellite altimetry. The locally accelerated decay of the spectral density function in a high-wavenumber dissipation subrange is pointed out as an important factor of wave dynamics and the geometrical optics treatment of the sea state bias. The analysis is carried out in the approximation of a unidirectional wave field and confined to the case of a well-developed sea.

Glazman, Roman E.↗

NASA NTRS1978

An instability of finite amplitude circularly polarized Alfven waves

A demonstration is presented that a finite-amplitude circularly-polarized Alfven wave is generally unstable in a MHD fluid. The wave decays by a four-wave coupling process in which the daughter waves are forward propagating random density and magnetic fluctuations and a backward-propagating magnetic wave. For parameters typical of the solar corona and the solar wind (thermal to magnetic energy density ratios between 0.1 and 1, and values between 0.1 and 0.9 for the ratio of magnetic energy density of the initial Alfven wave to that of the background magnetic field), large decay rates are found.

Goldstein, M. L.↗

NASA NTRS1996

Spectra of Baroclinic Inertia-Gravity Wave Turbulence

Baroclinic inertia-gravity (IG) waves form a persistent background of thermocline depth and sea surface height oscillations. They also contribute to the kinetic energy of horizontal motions in the subsurface layer. Measured by the ratio of water particle velocity to wave phase speed, the wave nonlinearity may be rather high. Given a continuous supply of energy from external sources, nonlinear wave-wave interactions among IG waves would result in inertial cascades of energy, momentum, and wave action. Based on a recently developed theory of wave turbulence in scale-dependent systems, these cascades are investigated and IG wave spectra are derived for an arbitrary degree of wave nonlinearity. Comparisons with satellite-altimetry-based spectra of surface height variations and with energy spectra of horizontal velocity fluctuations show good agreement. The well-known spectral peak at the inertial frequency is thus explained as a result of the inverse cascade. Finally, we discuss a possibility of inferring the internal Rossby radius of deformation and other dynamical properties of the upper thermocline from the spectra of SSH (sea surface height) variations based on altimeter measurements.

Glazman, Roman E.↗

NASA NTRS1983

Quasi-relativistic electron precipitation due to interactions with coherent VLF waves in the magnetosphere

The equations of motion for the cyclotron resonance interaction between coherent whistler mode waves and energetic particles are rederived with the inclusion of relativistic effects. The pitch angle scattering of the near-loss-cone quasi-relativistic electrons trapped in the magnetosphere is studied using a test particle method employing these relativistic equations, and the precipitated energy spectrum due to the wave-induced perturbations of a full distribution of particles is computed. Results show that the full width at half maximum peak width of the rms scattering pattern of the near-loss-cone particles would give an upper bound to the peak width of the associated precipitated energy spectrum under the conditions of moderate wave intensities in the low L shell region. In addition, it is found that the peak widths are within the upper limit values measured by recent satellite experiments. It is concluded that interactions of inner radiation belt particles with monochromatic waves could produce precipitated fluxes with relatively sharp spectral widths, and that therefore the L-dependent narrow peaks observed by low altitude satellite particle detectors could be caused by such interactions.

Chang, H. C.↗

NASA NTRS1985

Electrostatic ion cyclotron waves in a plasma with an ion beam and counterstreaming bulk electrons - Waves in the zero-frequency band

A common feature of the auroral plasma in the region above field-aligned (parallel) potential drops are electrostatic hydrogen cyclotron (EHC) waves. The present paper has the objective to show that wave excitation in the zero-frequency band can occur when the ion beams and the current-carrying bulk electrons counterstream. The instability mechanism involves the Landau interaction of the slow (negative energy) ion-beam-cyclotron waves with the drifting electrons and also with the target (background) ions. Only the latter resonant interaction between the beam and the target ions was considered by Okuda and Nishikawa (1984). In this study, it is shown that an electron drift makes an additional unstable contribution to the waves in the zero-frequency band, including those discussed by Okuda and Nishikawa.

Singh, N.↗