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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Nonlinear acoustic propagation in rectangular ducts

The method of multiple scales is used to obtain a second-order uniformly valid expansion for nonlinear acoustic wave propagation in a rectangular duct whose walls are treated with a nonlinear acoustic material. The wave propagation in the duct is characterized by the unsteady nonlinear Euler equations. The results show that nonlinear materials attenuate sound more than linear materials except at high acoustic frequencies. The nonlinear materials produce higher and combination tones which have higher attenuation rates than the fundamentals. Moreover, the attenuation rates of the fundamentals increase with increasing amplitude.

Nayfeh, A. H.↗
NASA NTRS1991

Atmospheric Propagation

Reviewed here is the current state of knowledge with respect to each basic mechanism of sound propagation in the atmosphere and how each mechanism changes the spectral or temporal characteristics of the sound received at a distance from the source. Some of the basic processes affecting sound wave propagation which are present in any situation are discussed. They are geometrical spreading, molecular absorption, and turbulent scattering. In geometrical spreading, sound levels decrease with increasing distance from the source; there is no frequency dependence. In molecular absorption, sound energy is converted into heat as the sound wave propagates through the air; there is a strong dependence on frequency. In turbulent scattering, local variations in wind velocity and temperature induce fluctuations in phase and amplitude of the sound waves as they propagate through an inhomogeneous medium; there is a moderate dependence on frequency.

Embleton, Tony F. W.↗
NASA NTRS1990

On resonance in linear conservation laws without source terms

A phenomena is described occuring in wave propagation for systems of conservation laws which are not hyperbolic. The nature of the wave propagation for linear systems of conservation laws which are degenerate in the sense that they are not equivalent to diagonal systems is examined. It is concluded that in the degenerate case, 'resonance' occurs, that is, the solutions are combinations of traveling waves and 'resonance waves'. Here resonance means that the solution may become unbounded even if the initial values are bounded. The solutions (waves) can be described as a superposition of 'packets' (groups) of 'resonance waves' traveling with the same speed. These are linear systems with constant coefficients which are self-exciting, which is what one would expect to encounter in nonlinear systems.

Gingold, H.↗
NASA NTRS1966

Breakup of Droplets in an Accelerating Gas Flow

A study of droplet breakup phenomena by an accelerating gas flow is described. The phenomena are similar to what propellant droplets experience when exposed to accelerating combustion gas flow in a rocket engine combustion zone. Groups of several dozen droplets in the 100-10 750-micron-diameter range were injected into a flowing inert gas in a transparent rectangular nozzle. Motion photography of the behavior of the droplets at various locations in the accelerating gas flow has supplied quantitative and qualitative data on the breakup phenomena which occur under conditions similar to those found in large rocket engine combustors. A blowgun injection device, used to inject very small amounts of liquid at velocities of several hundred feet per second into a moving gas stream, is described. The injection device was used to inject small amounts of liquid RP-1 and water into the gas stream at a velocity essentially equal to the gas velocity where the group of droplets was allowed to stabilize its formation in a constant area section before entering the convergent section of the transparent nozzle. Favorable comparison with the work of previous investigators who have used nonaccelerating gas flow is found with the data obtained from this study with accelerating gas flow. The criterion for the conditions of minimum severity required to produce shear-type droplet breakup in an accelerating gas flow is found to agree well with the criterion previously established at Rocketdyne for breakup in nonaccelerating flow. An extension of the theory of capillary surface wave effects during droplet breakup is also presented. Capillary surface waves propagating in the surface of the droplet, according to classical hydrodynamical laws, are considered. The waves propagate tangentially over the surface of the droplet from the forward stagnation point to the major diameter. Consideration of the effects of relative gas velocity on the amplitude growth of these waves allows conclusions to be made regarding rates of the droplet breakup processes and total time of the breakup process.

Dickerson, R. A.↗
NASA NTRS1971

Probing atmospheric water vapor profiles via multiple scattering of electromagnetic waves

A theoretical analysis on the multiple scattering of electromagnetic waves propagating in a finite inhomogeneous medium is presented and applied to the study of wave propagation in a clear atmosphere (fine weather conditions). It is shown that the analysis offers a method of synthesizing the water vapor density profile in a clear atmosphere by measuring the resultant reflections from the density profile at several different frequencies. It is also shown that the resultant reflection emerges as the consequence of multiple scattering of partial reflections from various parts of the inhomogeneous medium. The solutions of the multiple scattering approach are shown to be more accurate than those of the WKB approach, which neglects the multiple scattering effects.

Tang, C. C. H.↗
NASA NTRS1985

Propagation of waves in a bounded random layer with laminar structure

A closed form solution has been developed to obtain the intensity propagating in a bounded layer with laminar structure. Then, the brightness temperature due to an arbitrary temperature profile has been derived. Results are specialized to a half space to compare with those reported in the literature.

Karam, M. A.↗
DOE OSTI2022

Writable spin wave nanochannels in an artificial-spin-ice-mediated ferromagnetic thin film

We report magnonics, which employs spin-waves to transmit and process information, is a promising venue for low-power data processing. One of the major challenges is the local control of the spin-wave propagation path. Here, we introduce the concept of writable magnonics by taking advantage of the highly flexible reconfigurability and rewritability of artificial spin ice systems. Using micromagnetic simulations, we show that globally switchable spin-wave propagation and locally writable spin-wave nanochannels can be realized in a ferromagnetic thin film underlying an artificial pinwheel spin ice. The rewritable magnonics enabled by reconfigurable spin wave nanochannels provides a unique setting to design programmable magnonic circuits and logic devices for ultra-low power applications.

42 ENGINEERING↗
NASA NTRS1974

Nonlinear acoustic propagation in two-dimensional ducts

The method of multiple scales is used to obtain a second-order uniformly valid expansion for the nonlinear acoustic wave propagation in a two-dimensional duct whose walls are treated with a nonlinear acoustic material. The wave propagation in the duct is characterized by the unsteady nonlinear Euler equations. The results show that nonlinear effects tend to flatten and broaden the absorption versus frequency curve, in qualitative agreement with the experimental observations. Moreover, the effect of the gas nonlinearity increases with increasing sound frequency, whereas the effect of the material nonlinearity decreases with increasing sound frequency.

Nayfeh, A. H.↗
DOE OSTI2021

Helicon full-wave modeling with scrape-off-layer turbulence on the DIII-D tokamak

Helicon waves have been recently proposed as an off-axis current drive actuator due to their expected high current drive efficiency in the mid-radius region in high beta tokamaks. This paper focuses on a numerical study to better understand effects of scrape-off-layer (SOL) turbulence on helicon wave propagation and absorption on the DIII-D tokamak using a recently developed helicon full-wave model with turbulent density inputs from synthetic single wavelength SOL turbulence and first-principles HERMES multi-wavelength turbulence models. With both input turbulence models, three key effects are observed: the helicon wave can scatter to undesirable locations in the plasma, large helicon wave electric fields can form in localized regions near the SOL turbulence, and the helicon wave can mode convert to slow waves in the SOL. This is shown to cause helicon wave refraction to undesirable locations and strong helicon wave absorption in the SOL resulting in significantly less helicon wave power in the core plasma. Using synthetic SOL turbulence, the simulations additionally show that high amplitudes and long wavelengths greater than a few cm on average have the largest effect on modifying the helicon wave propagation and absorption; the modeling predicts, for example, that approximately 60% of helicon power can be absorbed in the SOL for $\tilde{n}$ /n ~ 0.8 and lambda_perp ~ 0.05 m. Here, several potential physical mechanisms that may explain the interaction of helicon waves with SOL turbulence in these simulations are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1993

Propagation of electromagnetic waves parallel to the magnetic field in the nightside Venus ionosphere

The propagation of electromagnetic waves parallel to the magnetic field in the nightside Venus ionosphere is presented in a theoretical and numerical analysis. The model assumes a source of electromagnetic radiation in the Venus atmosphere, such as that produced by lightning. Specifically addressed is wave propagation in the altitude range z = 130-160 km at the four frequencies detectable by the Pioneer Venus Orbiter Electric Field Detector: 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz. Parameterizations of the wave intensities, peak electron density, and Poynting flux as a function of magnetic field are presented. The waves are found to propagate most easily in conditions of low electron density and high magnetic field. The results of the model are consistent with observational data.

Huba, J. D.↗
NASA NTRS1993

Propagation characteristics of waves upstream and downstream of quasi-parallel shocks

The propagation characteristics of waves upstream and downstream of quasi-parallel shocks are investigated by using 2D hybrid simulations. At low Alfven Mach numbers, M(A) below about 2, the shock is initially associated with upstream phase-standing whistlers. At later times, backstreaming ions excite longer-wavelength whistlers via the right-hand resonant ion/ion instability. These waves propagate along the magnetic field at a group velocity no smaller than the upstream flow speed, so that the waves remain in the upstream region. At higher MA (above about 3), these waves are convected back into the shock, causing its reformation and downstream perturbations. Shock transmitted waves mode-convert into Alfven/ion-cyclotron waves which have a wave vector along the shock normal (pointing upstream) and convect downstream. The 2D simulation results confirm our earlier suggestion that the upstream waves should be field aligned, and that their convection into the downstream is associated with linear mode conversion into the Alfven/ion-cyclotron branch.

Krauss-Varban, D.↗
DOE OSTI2023

A Fast Butterfly-Compressed Hadamard–Babich Integrator for High-Frequency Helmholtz Equations in Inhomogeneous Media with Arbitrary Sources

Here we present a butterfly-compressed representation of the Hadamard-Babich (HB) ansatz for the Green's function of the high-frequency Helmholtz equation in smooth inhomogeneous media. For a computational domain discretized with Nv discretization cells, the proposed algorithm first solves and tabulates the phase and HB coefficients via eikonal and transport equations with observation points and point sources located at the Chebyshev nodes using a set of much coarser computation grids, and then butterfly compresses the resulting HB interactions from all Nv cell centers to each other. The overall CPU time and memory requirement scale as O(Nv log2 Nv) for any bounded two-dimensional (2D) domains with arbitrary excitation sources. A direct extension of this scheme to bounded 3D domains yields an O(Nv4/3) CPU complexity, which can be further reduced to quasi-linear complexities with proposed remedies. The scheme can also efficiently handle scattering problems involving inclusions in inhomogeneous media. Although the current construction of our HB integrator does not accommodate caustics, the resulting HB integrator itself can be applied to certain sources, such as concave-shaped sources, to produce caustic effects. Compared to finite-difference frequency domain methods, the proposed HB integrator is free of numerical dispersion and requires fewer discretization points per wavelength. As a result, it can solve wave propagation problems well beyond the capability of existing solvers. Remarkably, the proposed scheme can accurately model wave propagation in 2D domains with 640 wavelengths per direction and in 3D domains with 54 wavelengths per direction on a state-of-the-art supercomputer at Lawrence Berkeley National Laboratory.

Hadamard--Babich ansatz↗
DOE OSTI2020

Cold-hot coupled waves in a flowing magnetized plasma

Abstract Nonlinear coupling of cold and hot waves in a flowing magnetized plasma is analyzed with the Vlasov equation. An analytical solution is obtained for cold waves of a small amplitude (weak flow) and a long wavelength. The distribution function is obtained by integrating the kinetic equation along a perturbed phase-space trajectory for a time-varying plasma flow. The kinetic description presents a generalized dispersion relation that involves resonances depending on cold and hot wave dispersions. Coherent fluid motion leads to radiation peaks in addition to the cyclotron harmonics, where the wavenumber of the cold wave determines the peak frequencies. The peaks appear narrow when the wave propagates perpendicular to the time-averaged flow while they become broad due to the Doppler effect when the wave propagates parallel to the flow. Fully kinetic particle-in-cell simulations corroborate the theoretical predictions. The dispersion relation and resulting wave spectra provide information about plasma parameters and flow properties.

Physics↗
NASA NTRS1987

An analysis of the shocklike electrostatic noise observed during AMPTE solar wind ion releases

Electrostatic waves produced by the interaction of the ion cloud with the solar wind are analyzed in order to explain the shocklike noise detected by the Ion Release Module instruments during the Active Magnetospheric Particle Tracer Explorers solar wind ion releases. The results indicate that the shocklike noise can be generated by two instabilities, an ion-electron instability that occurs for waves propagating parallel to the solar wind velocity and an ion-ion instability that occurs for waves propagating at a large angle to the solar wind direction. Both instabilities give high growth rates for both the lithium and barium releases. The calculated results agree with the observed ones in all main features. In the earlier stage of the ion cloud expansion, both instabilities are likely to occur, while in the later stage the ion-ion instability is more likely to occur.

Ma, T. Z.↗
NASA NTRS1987

Wave-measurement capabilities of the surface contour radar and the airborne oceanographic lidar

The 36-gigahertz surface contour radar and the airborne oceanographic lidar were used in the SIR-B underflight mission off the coast of Chile in October 1984. The two systems and some of their wave-measurement capabilities are described. The surface contour radar can determine the directional wave spectrum and eliminate the 180-degree ambiguity in wave propagation direction that is inherent in some other techniques such as stereophotography and the radar ocean wave spectrometer. The Airborne Oceanographic Lidar can acquire profile data on the waves and produce a spectrum that is close to the nondirectional ocean-wave spectrum for ground tracks parallel to the wave propagation direction.

Walsh, Edward J.↗
NASA NTRS1994

Motion of the heliospheric termination shock 3: Incident interplanetary shocks

In this paper the response of the heliospheric termination shock to an incident interplanetary shock is examined. This paper is an extension of a recent study by Barnes (1993), which treated the analogous problem for an incident contact discontinuity. The termination shock is treated as a strong gas-dynamic shock. The post-interaction configuration consists of a moving termination shock, a postshock contact discontinuity, and either a shock or rarefaction wave propagating the disturbance signal into the downstream medium. For a decrease in dynamic pressure a rarefaction wave propagates downstream, and the new termination shock propagates inward, while, for an enhancement of dynamic pressure, the termination shock moves outwards and a weak outer shock propagates into the downstream medium; speeds of motion of the termination shock are typically of the order of approximately 100 km/s. The results are similar to those presented by Barnes (1993) indicating that the results of that paper are robust within the gas-dynamic model, in the sense of being independent of the details of the initial disturbance.

Naidu, Kamcilla↗