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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.

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At least 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Propagation of weak shock waves through turbulence.

The effect of turbulence on the structure of weak shock waves is investigated. The equilibrium structure is shown to be governed by a balance between nonlinear steepening and the turbulent scattering of acoustic energy out of the main wave direction. The scattered energy appears as perturbations behind the shock front. For conditions typical of sonic booms in atmospheric turbulence the wave structure is governed by a Burgers equation similar to that describing viscous shocks, except that parameters related to the turbulence appear instead of the viscosity coefficient. The magnitude of the perturbations following a shock is estimated from first-order scattering applied to a thickened shock. Predictions of shock thicknesses and perturbations compare favourably with available experimental data.

Plotkin, K. J.↗
NASA NTRS1975

Propagation of transients in a random medium

The propagation of transient scalar waves in a three-dimensional random medium is considered. The analysis is based on the smoothing method. An integro-differential equation for the coherent (or average) wave is derived and solved for the case of a statistically homogeneous and isotropic medium and a delta-function source. This yields the coherent Green's function of the medium. It is found that the waveform of the coherent wave depends generally on the distance from the source measured in terms of a certain dimensionless parameter. Based on the magnitude of this parameter, three propagation zones, called the near zone, the far zone, and the intermediate zone, are defined.

Wenzel, A. R.↗
NASA NTRS1977

A contribution to computer analysis of coupled-cavity traveling wave tubes

A flexible accurate large-signal computer program has been developed for the design of coupled-cavity traveling wave tubes. The program is written for a TSS-360 time sharing system. The beam is described by a disk model and the slow wave structure by a sequence of cavities or cells. The computational approach is arranged so that each cavity may have different geometrical or electrical parameters than its neighbors. This allows the program user to simulate a tube of almost arbitrary complexity. Input and output couplers, severs, complicated velocity tapers, and other features peculiar to one or a few cavities may be modeled by a correct choice of input data. The beam-wave interaction is handled by a new approach in which the RF fields are expanded in solutions to the TM wave equation retaining all significant space harmonics. The program was used to perform a design study of the TWT developed for the CTS satellite. Good agreement was obtained between the predictions of the program and the measured performance of the flight tube. The internal check on power balance was satisfied within plus or minus 0.2 per cent of input beam power.

Connolly, D. J.↗
NASA NTRS1983

Intensity moments by path integral techniques for wave propagation through random media, with application to sound in the ocean

A theory is developed which describes intensity moments for wave propagation through random media. It is shown using the path integral technique that these moments are significantly different from those of a Rayleigh distribution in certain asymptotic regions. The path integral approach is extended to inhomogeneous, anisotropic media possessing a strong deterministic velocity profile. The behavior of the corrections to Rayleigh statistics is examined, and it is shown that the important characteristics can be attributed to a local micropath focusing function. The correction factor gamma is a micropath focusing parameter defined in terms of medium fluctuations. The value of gamma is calculated for three ocean acoustic experiments, using internal waves as the medium fluctuations. It is found that all three experiments show excellent agreement as to the relative values of the intensity moments. The full curved ray is found to yield results that are significantly different from the straight-line approximations. It is noted that these methods are applicable to a variety of experimental situations, including atmospheric optics and radio waves through plasmas.

Bernstein, D. R.↗
NASA NTRS1986

Middle Atmosphere Program. Handbook for MAP, volume 19

This MAP handbook is concerned with rocket techniques and instrumentation as they are currently employed in the middle atmosphere. It is composed of nine chapters, written by experts on rocket experiments. The emphasis is on measurement techniques rather than results, although results are incorporated wherever they provide examples which illustrate the measurement features. The chapters first cover techniques relating to measurements of neutral dynamics and chemistry, then measurements of the various intermittent and excessive radiation sources which effect the middle atmospheric environment, and finally measurements of the plasma environment including electric fields. The weighting toward plasma related parameters is not accidental, but reflects both the historical headstart given by early development of radio wave and probe techniques to measure electron density, and by the relatively limited number of techniques available for neutral atmospheric measurements.

Goldberg, R. A.↗
NASA NTRS1990

Generation of low-frequency waves at Comet Halley

The extent to which the properties of the waves observed at comet Halley during the Giotto encounter can be understood within the context of linearized Vlasov theory is investigated. In the region that is magnetically connected to the comet, fluctuations in the plasma frame of reference are detected near 4 and 10 mHz, close to the water cyclotron frequency, as well as at 20-60 mHz, which is well above the water cyclotron frequency. Using a variety of approximations for the ion distribution function, it is shown that waves having properties similar to those observed can then be generated with appropriate choices of plasma parameters. In the region that is magnetically disconnected from the comet, distinct peaks are observed in the magnetic power spectrum at 7, 21, 29, and 35 mHz, with a hint of a peak at 14 mHz.

Goldstein, M. L.↗
NASA NTRS1991

Particle motion in the tail current sheet

Theory of particle motion in current sheets is reviewed. For small, approximately constant normal magnetic field, Bz, particles oscillate about the current sheet and 'live' within the sheet for one-half gyroperiod based on Bz. This lifetime replaces the mean collision time in the Lorentzian conductivity and thus gives rise to the concept of an inertial (or gyro-) conductivity. A substorm model by Coroniti utilizes this conductivity to allow reconnection to proceed without anomalous processes, due to wave-particle interactions. Chaotic particle orbits may at times be important to the dynamics, depending on parameters such as particle energy, current sheet thickness, and field line curvature. A current sheet model with neutral line predicts a ridge structure and asymmetries in the distribution function. Ion distributions near the plasma sheet boundary layer, during the CDAW 6 interval, are consistent with the model predictions. In recent studies by Mitchell et al. and Williams et al., the major current carriers during the growth phase of a substorm were found to be adiabatic electrons not more than 1 keV, but just before a current disruption event, the tail current was mainly carried by energetic ions undergoing current sheet oscillation.

Speiser, T. W.↗
NASA NTRS1992

Properties of radar backscatter of forests measured with a multifrequency polarimetric SAR

Fully polarimetric airborne synthetic aperture radar (AIRSAR) data, collected in Germany during the MAC Europe campaign, are calibrated using software packages developed at the Joint Research Center (JRC) in Italy for both L- and C-bands. During the period of the overflight dates, extensive ground truth was collected in order to describe the physical and statistical parameters of the canopy, the understory, and the soil. These parameters are compiled and converted into electromagnetic parameters suitable for input to the new polarimetric three-layer canopy model developed at the Wave Scattering Research Center (WSRC) at the University of Texas at Arlington. Comparisons between the theoretical predictions from the model and the calibrated data are carried out. Initial results reveal that the trend of the average phase difference can be predicted by the model, and that the backscattering ratio *shh/ svv is sensitive to the distribution of the primary branches.

Amar, F.↗
NASA NTRS1993

An investigation of conducted and radiated emissions from a hollow-cathode plasma contactor

An investigation conducted on the electrical interference induced by the operation of a hollow-cathode plasma contractor in a ground-based facility is described. The types of electrical interference, or noise, which are important to Space Station Freedom designers are classified as either conducted or radiated emissions. The procedures required to perform conducted and radiated emission measurements on a plasma contactor are examined. The experimental data obtained are typically examined in the frequency domain (i.e. amplitudes of the noise fluctuations versus frequency). Results presented indicate the conducted emissions, which are the current fluctuations from the contactor into the space station wiring, are affected by operating parameters such as expellant flow rate and discharge current. The radiated emissions, which are the electromagnetic waves induced and emitted by the contactor, appear to be influenced by the contactor emission current. Other experimental results suggest possible sources which are responsible for the observed noise. For example, the influence of the plasma environment downstream from the contactor on noise emission levels is described. In addition, a brief discussion is given on the correlation between conducted and radiated emissions and the mechanisms through which both are influenced by the plasma downstream of the contactor.

Buchholtz, Brett W.↗
NASA NTRS1994

Flow properties of the solar wind obtained from white light data and a two-fluid model

The flow properties of the solar wind from 1 R(sub s) to 1 AU were obtained using a two fluid model constrained by density and scale height temperatures derived from white light observations, as well as knowledge of the electron temperature in coronal holes. The observations were obtained with the white light coronographs on SPARTAN 201-1 and at Mauna Loa (Hawaii), in a north polar coronal hole from 1.16 to 5.5 R(sub s) on 11 Apr. 1993. By specifying the density, temperature, Alfven wave velocity amplitude and heating function at the coronal base, it was found that the model parameters fit well the constraints of the empirical density profiles and temperatures. The optimal range of the input parameters was found to yield a higher proton temperature than electron temperature in the inner corona. The results indicate that no preferential heating of the protons at larger distances is needed to produce higher proton than electron temperatures at 1 AU, as observed in the high speed solar wind.

Habbal, Shadia Rifai↗
NASA NTRS1994

Identification of low-frequency fluctuations in the terrestrial magnetosheath

On the basis of magnetohydrodynamic (MHD) theory we develop a scheme for distinguishing among the four low-frequency modes which may propagate in a high-beta anisotropic plasma such as the magnetosheath: the fast and slow magnetosonic, the Alfven, and mirror modes. We use four parameters: the ratio of transverse to compressional powers in the magnetic field, the ratio of the wave powers in the thermal pressure and in the magnetic field, the ratio of the perturbations in the thermal and magnetic pressures, and the ratio of the wave powers in the velocity and in the magnetic field. In the test case of an Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) magnetosheath pass near the Sun-Earth line downstream of a quasi-perpendicular shock, the four modes can be clearly distinguished both spatially and spectrally. Near the bow shock, the waves are Alfvenic in a large frequency range, 1 to 100 mHz. In the middle and inner magnetosheath, the waves below 10 mHz are Alfvenic. The fast mode waves occur in the higher-frequency end of the enhanced spectrum, 80 mHz for the middle magnetosheath and 55 mHz for the inner sheath. The wave enhancement in the intermediate frequencies is slow modes in the inner sheath and mirror modes in the middle sheath. This confirms the earlier report of the existence of the slow mode waves near the magnetopause. These slow waves provide evidence that the magnetopause is an active source of the waves in the sheath. We also show that the measured frequency of a wave is close to an invariant if the magnetosheath flow is in a steady state. Therefore changes in the frequencies of enhanced waves indicate emergence, or damping, or mode conversion of the waves.

Song, P.↗
NASA NTRS1992

Similarity rules in testing of microgravity fluid behaviors

The dynamical behavior of fluids in a full scale prototype and various 10% subscale containers with identical values of similarity parameters have been investigated. It shows that the Bond number can be used to simulate the wave characteristics of slosh wave excitation, whereas the Weber number can be used to simulate the wave amplitude. It is shown that a dynamical capillar number can be used to simulate the induced perturbation of the fluid stress distribution exerted on the wall.

Hung, R. J.↗
NASA NTRS1993

Characterization of Asymmetric Coplanar Waveguide Discontinuities

A general technique to characterize asymmetric coplanar waveguide (CPW) discontinuities with air bridges where both the fundamental coplanar and slotline modes may be excited together is presented. First, the CPW discontinuity without air bridges is analyzed using the space-domain integral equation (SDIE) approach. Second, the parameters (phase, amplitude, and wavelength) of the coplanar and slotline modes are extracted from an amplitude modulated-like standing wave existing in the CPW feeding lines. Then a 2n x 2n generalized scattering matrix of the n-port discontinuity without air bridges is derived which includes the occurring mode conversion. Finally, this generalized scattering matrix is reduced to an n x n matrix by enforcing suitable conditions at the ports which correspond to the excited slotline mode. For the purpose of illustration, the method is applied to a shielded asymmetric short-end CPW shunt stub, the scattering parameters of which are compared with those of a symmetric one. Experiments are performed on both discontinuities and the results are in good agreement with theoretical data. The advantages of using air bridges in CPW circuits as opposed to bond wires are also discussed.

Dib, Nihad I.↗
NASA NTRS2001

Microshutter Arrays for the NGST NIRSpec

We are developing a programmable multiobject field selector for the NGST NIRSpec. This device is a microshutter array, a close-packed 2000 x 2000 array of 100 micrometer x 100 micrometer shutters fabricated with Micro-ElectroMechanical Systems (MEMS) technologies. The shutters are opened using a magnet scanned over the array. The shutters are held open by electrostatic potentials on adjacent vertical electrodes. Modeling of the magnetic actuation of the microshutters allowed optimization of the magnetic field configuration, the distance between magnet and shutters, and the force and torque produced by the magnet. The results of laboratory tests are consistent with our modeling. We have demonstrated actuation, latching and addressing. We are able to rotate shutters out of the plane up to 90 degree and more, to latch them electrostatically to the walls and release them selectively. During the last six month all critical elements of the array have been demonstrated. Fabrication processes were developed to deposit vertical electrodes on support grid side walls and insulated light-shields that block the gaps between shutters and the support grid. Physical optics analysis was performed to assess the expected optical performance of the microshutters. A preliminary Fourier optics analysis on the microshutter array performance has been carried out. More detailed analysis of wave propagation through 3D structures is a next step. An optical test station for verifying optical parameters of the shutter array at room temperature has been developed. It allows to measure parameters, such as transmission, scattering, contrast ratio, and diffraction of the array structure. Initial data have been acquired and are being analyzed and compared to theoretical predictions.

Moseley, Harvey↗
NASA NTRS2011

Report on Recent Upgrades to the Curved Duct Test Rig at NASA Langley Research Center

The Curved Duct Test Rig (CDTR) is an experimental facility that is designed to assess the acoustic and aerodynamic performance of aircraft engine nacelle liners in close to full scale. The test section is between 25% and 100% of the scale of aft bypass ducts of aircraft engines ranging in size from business jet to large commercial passenger jet. The CDTR has been relocated and now shares space with the Grazing Flow Impedance Tube in the Liner Technology Facility at NASA Langley Research Center. As a result of the relocation, research air is supplied to the CDTR from a 50,000 cfm centrifugal fan. This new air supply enables testing of acoustic liner samples at up to Mach 0.500. This paper documents experiments and analysis on a baseline liner sample, which the authors had analyzed and reported on prior to the move to the new facility. In the present paper, the experimental results are compared to those obtained previously in order to ensure continuity of the experimental capability. Experiments that take advantage of the facility s expanded capabilities are also reported. Data analysis features that enhance understanding of the physical properties of liner performance are introduced. The liner attenuation is shown to depend on the mode that is incident on the liner test section. The relevant parameter is the mode cut-on ratio, which determines the angle at which the sound wave is incident on the liner surface. The scattering of energy from the incident mode into higher order, less attenuated modes is demonstrated. The configuration of the acoustic treatment, in this case lined on one surface and hard wall on the opposite surface, is shown to affect the mode energy redistribution.

Gerhold, Carl H.↗
NASA NTRS2012

From Multi-Sensors Observations Towards Cross-Disciplinary Study of Pre-Earthquake Signals. What have We Learned from the Tohoku Earthquake?

The lessons we have learned from the Great Tohoku EQ (Japan, 2011) how this knowledge will affect our future observation and analysis is the main focus of this presentation.We present multi-sensors observations and multidisciplinary research in our investigation of phenomena preceding major earthquakes. These observations revealed the existence of atmospheric and ionospheric phenomena occurring prior to theM9.0 Tohoku earthquake of March 11, 2011, which indicates s new evidence of a distinct coupling between the lithosphere and atmosphere/ionosphere, as related to underlying tectonic activity. Similar results have been reported before the catastrophic events in Chile (M8.8, 2010), Italy (M6.3, 2009) and Sumatra (M9.3, 2004). For the Tohoku earthquake, our analysis shows a synergy between several independent observations characterizing the state of the lithosphere /atmosphere coupling several days before the onset of the earthquakes, namely: (i) Foreshock sequence change (rate, space and time); (ii) Outgoing Long wave Radiation (OLR) measured at the top of the atmosphere; and (iii) Anomalous variations of ionospheric parameters revealed by multi-sensors observations. We are presenting a cross-disciplinary analysis of the observed pre-earthquake anomalies and will discuss current research in the detection of these signals in Japan. We expect that our analysis will shed light on the underlying physics of pre-earthquake signals associated with some of the largest earthquake events

Ouzounov, D.↗
NASA NTRSDate not supplied

An Extension of the Truncated Ingard-Myers Impedance Boundary Condition for High Mach Number Grazing Flows

The Ingard-Myers impedance boundary condition is widely recognized to be prone to hydrodynamic Kelvin-Helmholtz-type instability, primarily due to its use of a vortex sheet to model the flow at the boundary. Recently, a stabilized approximation of the Ingard-Myers condition, called the Truncated Ingard-Myers Impedance Boundary Condition (TIMIBC), has been introduced. This approximation offers a good representation of the Ingard-Myers condition for grazing flows characterized by low to mid subsonic Mach numbers. This paper explores an extension of the TIMIBC, referred to as TIMIBC-ext, designed for liners in grazing flows with high subsonic Mach numbers. The TIMIBC-ext introduces a tunable parameter and demonstrates that by judiciously selecting the parameter value, the accuracy of the TIMIBC can be improved for either upstream or downstream propagating waves in grazing flows with high subsonic Mach numbers. Consequently, the TIMIBC-ext represents an enhancement to the TIMIBC when the location of the source relative to the liner (upstream or downstream) is known. Time domain implementations of the TIMIBC-ext are also provided, employing a multipole expansion model for the impedance function. A numerical example is provided to assess the performance of the TIMIBC-ext, comparing computational results with measurements from a recent set of measurements in the NASA Langley Grazing Flow Impedance Tube.

time domain acoustic scattering↗
DOE OSTI2019

Acoustic Source Parameters and Yield Estimation for SPE Phase I and Forensics Surface Events

Acoustic signals recorded for explosive events are sensitive to pressure changes in the source region and can provide valuable information about source characteristics. However, acoustic wave propagation in the atmosphere is significantly affected by weather conditions (e.g., temperature, pressure, and wind), and the atmospheric propagation effects should be considered properly to obtain accurate source information. Acoustic waveform inversion method has been increasing useful to determine quantitative source parameters by incorporating accurate Green’s function into the inversion and by effectively removing propagation path effects. This paper reviews recent progress in the acoustic waveform inversion and its application to explosion yield/emplacement (e.g., height-of-burst or depth-of-burial) estimation. The inversion method is applied to acoustic signals recorded during Source Physics Experiment Phase I and Forensics Surface Event conducted at the Nevada National Security Site (NNSS). Inverted acoustic source strengths are related to the explosion yield and emplacement, and their estimation uncertainties are explored by the comparison with the ground-truth data.

45 MILITARY TECHNOLOGY, WEAPONRY, AND NATIONAL DEF↗