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At least 523 records · Page 29

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NASA NTRS1986

Numerical simulation of synthetic aperture radar image spectra for ocean waves

A numerical model for predicting the synthetic aperture radar (SAR) image of a moving ocean surface is described, and results are presented for two SIR-B data sets collected off the coast of Chile. Wave height spectra measured by the NASA radar ocean wave spectrometer (ROWS) were used as inputs to this model, and results are compared with actual SIR-B image spectra from orbits 91 and 106. Additional parametric variations are presented to illustrate the effects of nonlinearities in the imaging process.

Lyzenga, D. R.↗

NASA NTRS1998

Computational Modeling of Semiconductor Dynamics at Femtosecond Time Scales

The Interchange No. NCC2-5149 deals with the emerging technology of photonic (or optoelectronic) integrated circuits (PICs or OEICs). In PICs, optical and electronic components are grown together on the same chip. To build such devices and subsystems, one needs to model the entire chip. PICs are useful for building components for integrated optical transmitters, integrated optical receivers, optical data storage systems, optical interconnects, and optical computers. For example, the current commercial rate for optical data transmission is 2.5 gigabits per second, whereas the use of shorter pulses to improve optical transmission rates would yield an increase of 400 to 1000 times. The improved optical data transmitters would be used in telecommunications networks and computer local-area networks. Also, these components can be applied to activities in space, such as satellite to satellite communications, when the data transmissions are made at optical frequencies. The research project consisted of developing accurate computer modeling of electromagnetic wave propagation in semiconductors. Such modeling is necessary for the successful development of PICs. More specifically, these computer codes would enable the modeling of such devices, including their subsystems, such as semiconductor lasers and semiconductor amplifiers in which there is femtosecond pulse propagation. Presently, there are no computer codes that could provide this modeling. Current codes do not solve the full vector, nonlinear, Maxwell's equations, which are required for these short pulses and also current codes do not solve the semiconductor Bloch equations, which are required to accurately describe the material's interaction with femtosecond pulses. The research performed under NCC2-5149 solves the combined Maxwell's and Bloch's equations.

Agrawal, Govind P.↗

NASA NTRS1986

Influence of transient baroclinic eddies on planetary-scale waves

Low-order models with a mean zonal flow forced by zonally symmetric heating and one planetary-scale wave forced by topography have multiple flow equilibria. Two stable equilibria are characterized by strong zonal flow with low wave amplitude (high index circulation) and weak zonal flow with a high wave amplitude (low index circulation) fixed in phase with the topography. These two states presumably represent normal zonal circulation and a blocking configuration, respectively. When a shorter, baroclinically unstable wave is dded to this low-order model, the planetary-scale wave no longer stays in stable equilibrium states. Instead, the long wave remains in weather regimes or preferred regions in phase space. These regimes are, in general, different from the equilibria of the model with the planetary-scale wave only. Thus, the short unstable wave adds some randomness to planetary-scale circulation and changes its position with respect to the planetary-scale topography. The Reinhold and Pierrehumbert model is further explored here by adding a long-wave in the thermal forcing and exploring wider parameter space. When the symmetric thermal forcing is weak so that the short wave is stable, the planetary scale wave is fixed in amplitude and phase by the asymmetric forcing. However, when the symmetric forcing is increased to an unstable level, the planetary-scale wave becomes less organized. With asymmetric thermal forcing only (i.e., no topography) the long-wave is randomly distributed through phase space. Thermal forcing seems to be less effective in organizing weather regimes than topographic forcing.

Branscome, L. E.↗

NASA NTRS1987

Revised NASA axially symmetric ring model for coupled-cavity traveling-wave tubes

A versatile large-signal, two-dimensional computer program is used by NASA to model coupled-cavity travelling-wave tubes (TWTs). In this model, the electron beam is divided into a series of disks, each of which is further divided into axially symmetric rings which can expand and contract. The trajectories of the electron rings and the radiofrequency (RF) fields are determined from the calculated axial and radial space-charge, RF, and magnetic forces as the rings pass through a sequence of cavities. By varying electrical and geometric properties of individual cavities, the model is capable of simulating severs, velocity tapers, and voltage jumps. The calculated electron ring trajectories can be used in designing magnetic focusing and multidepressed collectors. The details of using the program are presented, and results are compared with experimental data.

Wilson, Jeffrey D.↗

NASA NTRS1983

Seismic properties of the crust and uppermost mantle of North America

Seismic refraction profiles for the North American continent were compiled. The crustal models compiled data on the upper mantle seismic velocity (P sub n), the crustal thickness (H sub c) and the average seismic velocity of the crystalline crust (V sub p). Compressional wave parameters were compared with shear wave data derived from surface wave dispersion models and indicate an average value for Poisson's ratio of 0.252 for the crust and of 0.273 for the uppermost mantle. Contour maps illustrate lateral variations in crustal thickness, upper mantle velocity and average seismic velocity of the crystalline crust. The distribution of seismic parameters are compared with a smoothed free air anomaly map of North America and indicate that a complidated mechanism of isostatic compensation exists for the North American continent. Several features on the seismic contour maps also correlate with regional magnetic anomalies.

Braile, L. W.↗

NASA NTRS1992

Weakly nonlinear models for turbulent mixing in a plane mixing layer

New closure models for turbulent free shear flows are presented in this paper. They are based on a weakly nonlinear theory with a description of the dominant large-scale structures as instability waves. Two models are presented that describe the evolution of the free shear flows in terms of the time-averaged mean flow and the dominant large-scale turbulent structure. The local characteristics of the large-scale motions are described using linear theory. Their amplitude is determined from an energy integral analysis. The models have been applied to the study of an incompressible mixing layer. For both models, predictions of the mean flow developed are made. In the second model, predictions of the time-dependent motion of the large-scale structures in the mixing layer are made. The predictions show good agreement with experimental observations.

Liou, William W.↗

NASA NTRS1990

Two-component model of strong Langmuir turbulence - Scalings, spectra, and statistics of Langmuir waves

A simple two-component model of strong turbulence that makes clear predictions for the scalings, spectra, and statistics of Langmuir waves is developed. Scalings of quantities such as energy density, power input, dissipation power wave collapse, and number density of collapsing objects are investigated in detail and found to agree well with model predictions. The nucleation model of wave-packet formation is strongly supported by the results. Nucleation proceeds with energy flowing from background to localized states even in the absence of a driver. Modulational instabilities play little or no role in maintaining the turbulent state when significant density nonuniformities are present.

Robinson, P. A.↗

NASA NTRS1989

Observations of gravity waves in the mesosphere with the MU radar

Wind motions were observed at 60 to 90 km altitudes with the MU radar during daylight hours (0800 to 1600 LT) from 13 to 31 October 1986. Quasi-monochromatic gravity waves were evident on 16 of the 19 days of observations. They were characterized by typical vertical wavelength of 5 to 15 km and intrinsic periods centered at about 9 hours. The propagation direction of the gravity waves, determined by the gravity wave dispersion relation, was mostly equatorward. The vertical wave number spectra of the horizontal components of the mesoscale wind fluctuations are explained well by saturated gravity wave theory. The frequency spectrum of vertical wind component has a slope of + 1/3, while the oblique spectra have a slope of -5/3 up to 4 x 10(-3) (c/s); these agree fairly well with model gravity wave spectra. Doppler shift effects on the frequency spectra are recognized at higher frequencies. Upward flux was determined of horizontal momentum flux induced by waves with periods from 10 min to 8 hours, and westward and northward body forces of 5.1 and 4.0 m/s/day, were estimated respectively.

Tsuda, T.↗

NASA NTRS1984

Techniques for the study of gravity waves and turbulence (keynote paper), part 4

Probably one of the most important achievements mesosphere stratosphere troposphere (MST) radars can make toward increasing the understanding of the dynamics of the atmosphere is to determine the exact relationship between the generation of turbulence and the sources of high shear or convectively unstable flows. An important theoretical tool, the gravity-wave breaking through which one can begin to understand spontaneous generation of turbulence model is discussed. In this model, large amplitude gravity waves produce local regions where the Richardson number (N sup 2/U sub Z sup 2) is less than 1/4 thus giving rise to turbulent flows. Thus the appearance of turbulent layers can often be interpreted as a breaking-gravity-wave signature. Even though the techniques for studying gravity waves and turbulence may be quite different (and historically have resulted in somewhat separate bodies of literature), it is clear from the wave-breaking model that the phenomena are intimately linked. The techniques for measurements of gravity wave flow fields and turbulent regions by MST radar should show cognizance of some of the theoretical questions raised by the wave-breaking model.

Schoeberl, M. R.↗

NASA NTRS1994

Heating of the nighttime D region by very low frequency transmitters

VLF signals propagating in the Earth-ionosphere waveguide are used to probe the heated nighttime D region over three U.S. Navy very low frequency (VLF,3-30 kHz) transmitters. Ionospheric cooling and heating are observed when a transmitter turns off and on in the course of normal operations. Heating by the 24.0-kHz NAA transmitter in Cutler, Maine, (1000 kW radiated power) was observed by this method in 41 of 52 off/on episodes during December 1992, increasing the amplitude and retarding the phase of the 21.4-kHz NSS probe wave propagating from Annapolis, Maryland, to Gander, Newfoundland, by as much as 0.84 dB and 5.3 deg, respectively. In 6 of these 41 episodes, the amplitude of the 28.5-kHz NAU probe wave propagating from Puerto Rico to Gander was also perturbed by as much as 0.29 dB. The latter observations were unexpected due to the greater than 770 km distance between NAA and the NAU-Gander great circle path. Heating by the NSS (21.4 kHz, 265 kW) and NLK (24.8 kHz, 850 kW) transmitters was observed serendipitously in data from earlier measurements of the amplitudes of VLF signals propagating in the Earth-ionosphere waveguide. A three-dimensional model of wave absorption and electron heating in a magnetized, weakly ionized plasma is used to calculate the extent nad shape of the collision frequency (i.e., electron temperature) enhancement above a VLF transmitter. The enhancements are annular, with a geomagnetic north-south asymmetry and a radius at the outer half-maximum of the collision frequency enhancement of about 150 km. Heating by the NAA transmitter is predicted to increase the nighttime D region electron temperature by as much as a factor of 3. The calculated changes in the D region conductivity are used in a three-dimensional model of propagation in the Earth-ionosphere wavelength to predict the effect of the heated patch on a subionospheric VLF probe wave. The range of predicted scattered field amplitudes is in general consistent with the observed signal perturbations. Discrepanices in the predictions are attributed to lack of knowledge of the D region electron density profile along the probe wave great circle paths.

Rodriguez, Juan V.↗

NASA NTRSDate not supplied

Machine Learning Global Simulation of Nonlocal Gravity Wave Propagation

Global climate models typically operate at a grid resolution of hundreds of kilometers and fail to resolve atmospheric mesoscale processes, e.g., clouds, precipitation, and gravity waves (GWs).Model representation of these processes and their sources is essential to the global circulation and planetary energy budget, but subgrid scale contributions from these processes are often only approximately represented in models using parameterizations. These parameterizations are subject to approximations and idealizations, which limit their capability and accuracy. The most drastic of these approximations is the “single-column approximation” which completely neglects the horizontal evolution of these processes, resulting in key biases in current climate models. With a focus on atmospheric GWs, we present the first-ever global simulation of atmospheric GW fluxes using machine learning (ML) models trained on the WINDSET dataset to emulate global GW emulation in the atmosphere, as an alternative to traditional single-column parameterizations. Using an Attention U-Net-based architecture trained on globally resolved GW momentum fluxes, we illustrate the importance and effectiveness of global nonlocality, when simulating GWs using data-driven schemes.

Aman Gupta↗

NASA NTRS1981

Characterization of metal-polymer boundaries using Stoneley and Leaky waves

Optical and acoustooptical techniques for bonded surface inspection and interface wave detection are discussed. Interface wave propagation models based on viscous, elastic, and viscoelastic mechanical properties are reviewed. Gross boundary anomalies such as large defects and localized disbond regions were measured using conventional high frequency pulse echo and pitch catch ultratechniques. Bond durabilty is estimated by comparing detected wave attenuation versus frequency as a function of prebond surface preparation with the experimental limit obtained for perfect welded contact. Sensitivity of interface acoustic field periodicity to both residual and dc applied stress at the boundary surface is observed as well as hysteresis due to viscoelastic polymeric behavior as the applied stress is cycled.

Claus, R. O.↗

NASA NTRS2011

A Theoretical Basis for the Scaling Law of Broadband Shock Noise Intensity in Supersonic Jets

A theoretical basis for the scaling of broadband shock noise intensity In supersonic jets was formulated considering linear shock-shear wave interaction. Modeling of broadband shock noise with the aid of shock-turbulence interaction with special reference to linear theories is briefly reviewed. An hypothesis has been postulated that the peak angle of incidence (closer to the critical angle) for the shear wave primarily governs the generation of sound in the interaction process with the noise generation contribution from off-peak incident angles being relatively unimportant. The proposed hypothesis satisfactorily explains the well-known scaling law for the broadband shock-associated noise in supersonic jets.

Kandula, Max↗

NASA NTRS1986

Production of flickering aurora and field-aligned electron flux by electromagnetic ion cyclotron waves

Recent observations have suggested that flickering aurora is produced by a modulation of the field-aligned component of the electron flux within an auroral arc. It is proposed that a portion of the field-aligned electrons are of ionospheric origin and that these electrons are accelerated and their flux modulated by electromagnetic ion cyclotron waves that occur below the main acceleration region on auroral arc field lines. A model of the electromagnetic ion cyclotron wave shows that the parallel phase velocity of the wave increase as the wave propagates toward the ionosphere. A test particle calculation shows that ionospheric electrons trapped or reflected by the wave are accelerated to energies of several keV and that their flux is modulated at the wave frequency. The relative amplitudes of the model wave electric fields are consistent with the observations of small-scale low-frequency ionospheric and magnetospheric electric fields near auroral arcs of approximately 10 mV/m and 100 mV/m, respectively. The large-amplitude ion cyclotron waves also produce a ponderomotive force and a self-consistent ambipolar electric field. Energy considerations show that the downward energy flux in the electromagnetic ion cyclotron wave can be several percent of the total downward auroral electron energy flux.

Temerin, M.↗

NASA NTRS1988

Satellite-observed characteristics of midwest severe thunderstorm anvils

The cloud top and anvil structure of severe thunderstorms observed by the GOES satellite are analyzed for five SESAME cases in 1979 and four non-SESAME cases in 1980-1982. The data is compared with previous models and hypotheses, paying particular attention to the V feature and thermal couplets in the IR observations. The characteristics of the cases are examined and related to the upper-level temperature and wind conditions. It is found that the warm points downwind of the cloud top are due to subsidence. The anaylsis suggests the presence of subsidence due to mountainlike waves. A model in which the close-in warm point is produced by both internal cloud air motions and stratospheric flow around and over the cloud top. It is suggested that the distant warm point is due to either a wave perturbation from air flowing over the cloud top, or air flowing horizonatlly around the elevated portion of the cloud top and anvil.

Heymsfield, Gerald M.↗

NASA NTRS1993

Mars Global Reference Atmospheric Model (Mars-GRAM): Release No. 2 - Overview and applications

The Mars Global Reference Atmospheric Model (Mars-GRAM), a science and engineering model for empirically parameterizing the temperature, pressure, density, and wind structure of the Martian atmosphere, is described with particular attention to the model's newest version, Mars-GRAM, Release No. 2 and to the improvements incorporated into the Release No. 2 model as compared with the Release No. 1 version. These improvements include (1) an addition of a new capability to simulate local-scale Martian dust storms and the growth and decay of these storms; (2) an addition of the Zurek and Haberle (1988) wave perturbation model, for simulating tidal perturbation effects; and (3) a new modular version of Mars-GRAM, for incorporation as a subroutine into other codes.

James, B.↗

NASA NTRS1981

Refraction of coastal ocean waves

Refraction of gravity waves in the coastal area off Cape Hatteras, NC as documented by synthetic aperture radar (SAR) imagery from Seasat orbit 974 (collected on September 3, 1978) is discussed. An analysis of optical Fourier transforms (OFTs) from more than 70 geographical positions yields estimates of wavelength and wave direction for each position. In addition, independent estimates of the same two quantities are calculated using two simple theoretical wave-refraction models. The OFT results are then compared with the theoretical results. A statistical analysis shows a significant degree of linear correlation between the data sets. This is considered to indicate that the Seasat SAR produces imagery whose clarity is sufficient to show the refraction of gravity waves in shallow water.

Shuchman, R. A.↗

NASA NTRS1973

Radiative instability of a cloudy planetary atmosphere.

A cloudy planetary atmosphere at rest is shown to be unstable to disturbances of large horizontal scale. The energy source for the instability is the change in radiative heat flux associated with vertical displacement near the emitting level. A simple model is described in which the net heating rate in the cloud is proportional to the vertical displacement of the cloud. The constant of proportionality may be either positive or negative. Disturbances may take the form of either quasi-steady geostrophic motions or amplified inertia-gravity waves. The model is applied to Jupiter's zonal winds and to motions near the Venus cloud tops, and provides a possible explanation for many important features of these two flows.

Gierasch, P. J.↗