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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 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

FIRST NEW SOLAR MODELS WITH OPAS OPACITY TABLES

Stellar seismology appears more and more as a powerful tool for a better determination of the fundamental properties of solar-type stars. However, the particular case of the Sun is still challenging. For about a decade now, the helioseismic sound-speed determination has continued to disagree with the standard solar model (SSM) prediction, questioning the reliability of this model. One of the sources of uncertainty could be in the treatment of the transport of radiation from the solar core to the surface. In this Letter, we use the new OPAS opacity tables, recently available for solar modeling, to address this issue. We discuss first the peculiarities of these tables, then we quantify their impact on the solar sound-speed and density profiles using the reduced OPAS tables taken on the grids of the OPAL ones. We use the two evolution codes, Modules for Experiments in Stellar Astrophysics and Code Liégeois d’Evolution Stellaire, that led to similar conclusions in the solar radiative zone. In comparison to commonly used OPAL opacity tables, the new solar models are computed for the most recent photospheric composition with OPAS tables and present improvements to the location of the base of the convective zone and to the description of the solar radiative zone in comparison to the helioseismic observations, even if the differences in the Rosseland mean opacity do not exceed 6%. We finally carry out a comparison to a solar model computed with the OP opacity tables.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

REFLECTION OF PROPAGATING SLOW MAGNETO-ACOUSTIC WAVES IN HOT CORONAL LOOPS: MULTI-INSTRUMENT OBSERVATIONS AND NUMERICAL MODELING

Slow MHD waves are important tools for understanding coronal structures and dynamics. In this paper, we report a number of observations from the X-Ray Telescope (XRT) on board HINODE and Solar Dynamic Observatory /Atmospheric Imaging Assembly (AIA) of reflecting longitudinal waves in hot coronal loops. To our knowledge, this is the first report of this kind as seen from the XRT and simultaneously with the AIA. The wave appears after a micro-flare occurs at one of the footpoints. We estimate the density and temperature of the loop plasma by performing differential emission measure (DEM) analysis on the AIA image sequence. The estimated speed of propagation is comparable to or lower than the local sound speed, suggesting it to be a propagating slow wave. The intensity perturbation amplitude, in every case, falls very rapidly as the perturbation moves along the loop and eventually vanishes after one or more reflections. To check the consistency of such reflection signatures with the obtained loop parameters, we perform a 2.5D MHD simulation, which uses the parameters obtained from our observation as inputs, and perform forward modeling to synthesize AIA 94 Å images. Analyzing the synthesized images, we obtain the same properties of the observables as for the real observation. From the analysis we conclude that a footpoint heating can generate a slow wave which then reflects back and forth in the coronal loop before fading. Our analysis of the simulated data shows that the main agent for this damping is anisotropic thermal conduction.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Improved Audio Reproduction System

Circuitry utilizing electrical feedback of instantaneous speaker coil velocity compensates for loudspeaker resonance, transient peaks and frequency drop-off so that sounds of widely varying frequencies and amplitudes can be reproduced accurately from high fidelity recordings of any variety.

Chang, C. S.↗
NASA NTRS1972

Aeroacoustic characteristics of jet flap type exhausts.

This work which is intended to shed more light on the acoustic and fluid dynamic characteristics of jet flap type exhaust flows, reports and discusses some typical experimental results obtained with cold model air jets in a reverberation chamber. Acoustic data on the total sound power output and the overall noise spectrum of various jet flap configurations are given and compared with data from conventional circular jets. In addition various flow data for the same jets resulting from hot wire measurements are reported and general conclusions are obtained regarding the expected noise source distribution. The results indicate the dependence of the acoustic and flow characteristics on the major nozzle parameters. For the jet flaps the total radiated sound power is found to follow a 3 to 6 power of the exhaust velocity while for the circular jets the 8 power law is confirmed. The noise produced in the secondary mixing region downstream of the flap trailing edge tends to exceed the noise issuing from the primary mixing region.

Gruschka, H. D.↗
NASA NTRS1975

Minimization of jet and core noise of a turbojet engine by swirling the exhaust flow

A noise abating improvement for jet engines has been investigated and demonstrated using a full scale turbojet engine which provided the means for significant reductions in jet and core noise with minimal thrust loss. This was accomplished by controlled solid body rotation (swirling) of the flow in the nozzle and jet exhaust. Relatively moderate angles of solid body rotation in the presence of temperature, density, pressure, and velocity gradients were utilized. Of great interest was the finding that significant reductions of sound pressure levels and sound power were obtained with only a small percentage of the total primary mass flow swirling at the exit of the nozzle. Therefore, tradeoffs between noise reduction and engine performance can be optimized to satisfy aircraft performance and federal noise standards with minimal wasted rotational kinetic energy.

Schwartz, I. R.↗
NASA NTRS1975

The low frequency sound from multipole sources in axisymmetric shear flows, with applications to jet noise

A closed-form solution for the sound radiation from multipole sources imbedded in an infinite cylindrical jet with an arbitrary velocity profile is obtained. It is valid in the limit where the wavelength is large compared with the jet radius. Simple formulae for the acoustic pressure field due to convected point sources are also obtained. The results show (in a simple way) how the mean flow affects the radiation pattern from the sources. For convected lateral quadrupoles it causes the exponent of the Doppler factor multiplying the far-field pressure signal to be increased from the value of 3 used by Lighthill to 5.

Goldstein, M. E.↗
NASA NTRS1976

Wave propagation along a magnetic tube

Motivated by the filamentary structure of solar magnetic fields, this paper considers the propagation of quasi-longitudinal waves along a slender magnetic tube in pressure equilibrium with the ambient medium. The velocity of such waves is expressed in terms of the Alven speed and the sound speed inside the tube. The divergence of a vertical magnetic tube in a stratified atmosphere introduces a critical frequency below which propagation is impossible.

Defouw, R. J.↗
NASA NTRS1976

Advanced atmospheric sounder and imaging radiometer /AASIR/ for STORMSAT

The principal mission of the three-axis stabilized STORMSAT spacecraft is to provide the necessary meteorological data for tracking, studying the detailed structure, and modeling mesoscale weather phenomena. In the area of mesoscale events, the following meteorological objectives are indicated: high-quality imagery, visible and infrared; wind velocity from cloud tracers (1 m/sec), atmospheric temperature profiles (1 K), and atmospheric humidity sounding. These objectives are reflected in the functional characteristics of the AASIR, which is a second generation meteorological sensor based on the Visible Infrared Spin-Scan Radiometer (VISSR) and the Atmospheric Sounder (VAS). The AASIR design and interface constraints with the STORMSAT spacecraft is discussed.

Chase, S. C.↗
NASA NTRS1977

Airframe noise of the DC-9

Airframe noise measurements are reported for the DC-9-31 aircraft flown at several speeds and with a number of flap, landing gear, and slat extension configurations. The data are corrected for atmospheric attenuation and spherical divergence, and are presented for an overhead position normalized to a 1-meter height. The sound pressure levels are found to vary approximately as the fifth power of flight velocity. Both lift and drag dipoles exist as a significant part of the airframe noise. The data are compared with airframe noise predictions using the drag element and the data analysis methods. Although some of the predictions are very good, further work is needed to refine these methods, particularly for the gear-down and flaps-down configurations.

Bauer, A. B.↗
NASA NTRS1978

Airframe noise of the DC-9-31

Airframe noise measurements are reported for the DC-9-31 aircraft flown at several speeds and with a number of flap, landing gear, and slat extension configurations. The data are corrected for wind effects, atmospheric attenuation, and spherical divergence, and are normalized to a 1 meter acoustic range. The sound pressure levels are found to vary approximately as the fifth power of flight velocity. Both lift and drag dipoles exist as a significant part of the airframe noise. The sideline data imply that a significant side-force dipole exists only for the flap- and gear-down configurations; for others, the data imply the existence of only the lift and drag dipoles. The data are compared with airframe noise predictions using the drag element and the data analysis methods. Although some of the predictions are good, further work is needed to refine the methods, particularly for the flap- and gear-down configurations.

Bauer, A. B.↗
NASA NTRS1979

On sound radiation from the trailing edge of an isolated airfoil in a uniform flow

An experimental investigation has been conducted to determine the physical process of the noise production from the trailing edge of an isolated two-dimensional airfoil embedded in a low turbulence uniform mean flow. The Reynolds number of the airfoil based on chord was greater than 1,000,000 and that the boundary layer was fully turbulent at the trailing edge. Smoke injection technique and spark shadowgraphy were used to study the structural features of the boundary layer. Two point joint statistical analyses were made on the surface pressure, the radiated sound and the relationship between the two quantities. Operating conditions included two free stream velocities and two angles of attack for both a naturally developed and an artificially tripped boundary layer. Flow visualizations revealed the existence of large scale coherent motions in the outer region of the boundary layer. The observed coherent motions had scales of the order of boundary layer thickness and a convection velocity near the free stream velocity. The production process of the airfoil trailing edge noise was determined to be the convection of the large scale coherent eddies over the trailing edge of the airfoil. The noise field so generated was found to be rather coherent and dipole-like.

Yu, J. C.↗
NASA NTRS1980

The near field interaction between a sound pulse and a jet shear layer

The in-flow and near field of a jet which is excited by an axial mass source located in the potential core is simulated numerically, taking due account of the jet spreading, and experimentally, using a shock-tube driver. Comparison is made for both excited and unexcited jets. It is shown that many of the features observed experimentally are consequences of the instability of the mean flow profile and not of turbulence. This instability is shown to be a significant amplifier of low frequency sound. The terms responsible for this amplification are those describing action between the fluctuating velocities and the gradient of the mean flow, which were identified by Ribner as the shear noise terms. The results demonstrate that a primary component of the observed shear noise in jets is generated by the mean profile instability.

Bayliss, A.↗
NASA NTRS1987

Hydrodynamical constraints on cosmic-ray acceleration in relativistic shocks

A two-fluid hydrodynamical model governing the transport of cosmic rays in a relativistically moving background plasma is developed. The equations are used to discuss the time asymptotic structure of a relativistic, plane-parallel shock wave significantly modified by first-order Fermi acceleration of cosmic rays. The model allows for an anisotropic cosmic-ray pressure tensor with pressure components parallel and perpendicular to the shock normal. It is surmised that the well-known energy threshold problem for diffusive shock acceleration that downstream particles have a large enough velocity to overtake the shock and subsequently scatter in the upstream medium before returning to the downstream medium may be expressed hydrodynamically by the condition V less than V(cr), where V is the fluid velocity relative to the shock and V(cr) is the relativistic cosmic-ray sound speed. Astrophysical implications of the results are briefly discussed.

Webb, G. M.↗
NASA NTRS1988

Data report for the Siple Coast (Antarctica) project

This report presents data collected during three field seasons of glaciological studies in the Antarctica and describes the methods employed. The region investigated covers the mouths of Ice Streams B and C (the Siple Coast) and Crary Ice Rise on the Ross Ice Shelf. Measurements included in the report are as follows: surface velocity and deformation from repeated satellite geoceiver positions; surface topography from optical levelling; radar sounding of ice thickness; accumulation rates; near-surface densities and temperature profiles; and mapping from aerial photography.

Bindschadler, R. A.↗
NASA NTRS1938

Noise from propellers with symmetrical sections at zero blade angle, II

In a previous paper (Technical Note No. 605), a theory was developed that required an empirical relation to calculate sound pressures for the higher harmonics. Further investigation indicated that the modified theory agrees with experiment and that the empirical relation was due to an interference phenomenon peculiar to the test arrangement used. Comparison is made between the test results for a two-blade arrangement and the theory. The comparison is made for sound pressures in the plane of the revolving blades for varying values of tip velocity. Comparison is also made at constant tip velocity for all values of azimuth angle B. A further check is made between the theory and the experimental results for the fundamental of a four-blade arrangement with blades of the same dimensions as those used in the two-blade arrangement.

Deming, A F↗
NASA NTRS1995

Mirror Instability in the Solar Wind: The Theory Revisited

"Magnetic holes", localized depressions in the interplanetary magnetic field, have been identified in Ulysses data over a range of several AU and as far as 23 degrees south in latitude by Winterhalter et al., who concluded that these structures are most likely the remnants of structures caused by occasional mirror-mode instability in the solar wind. However, these authors, like a number of previous investigators, used the mirror stability criterion derived from the kinetic theory under very special assumptions. On the other hand, theoretical investigations using the fully self-consistent kinetic theory (Vlasov-Maxwell equations) have shown that the mirror stability criterion is more complicated when electrons and ions have different anisotropies, as is normally the case in the solar wind. Winterhalter et al used an instability criterion of the form R is greater than 1, where R is a function of the thermal anisotropy; the correct criterion (for bi-Maxwellian distributions) is R R is greater than 1 - x(exp 2), where x is a real quantity that depends on both the proton anisotropy and electron anisotropy. So nonzero x would modify the Winterhalter et al results in the direction of reinforcing their conclusions. We have revisited the instability criterion in its most general form, allowing for (a) non-Maxwellian velocity distributions, (b) multiple ion species, and (c) interparticle streaming. These results should give sound theoretical grounding for future observational studies related to the mirror instability, by Ulysses and other spacecraft.

Barnes, A.↗
NASA NTRS1990

Aeroacoustic flowmeter

The flowmeter is based on a measurement of phase difference between two points on the circumference of a pipe separated axially by an integral multiple of sound wavelength. Plane sound waves are generated aeroacoustically by a non-protruding ring cavity energized either directly by the flow or by a subsidiary flow of the same medium. The frequency of the aeroacoustic source varies with temperature and therefore the temperature can be obtained. In the case of steam flow, temperature can be measured independently and therefore from the measured frequency (or speed of sound), the quality of wet steam can be measured. The flowmeter is linear in velocity and no calibrations are required.

Shakkottai, Parthasarathy↗
DOE OSTI2020

A Vector Sensor-Based Acoustic Characterization System for Marine Renewable Energy

NoiseSpotter is a passive acoustic monitoring system that characterizes, classifies, and geo-locates anthropogenic and natural sounds in near real time. It was developed with the primary goal of supporting the evaluation of potential acoustic effects of offshore renewable energy projects. The system consists of a compact array of three acoustic vector sensors, which measures acoustic pressure and the three-dimensional particle velocity vector associated with the propagation of an acoustic wave, thereby inherently providing bearing information to an underwater source of sound. By utilizing an array of three vector sensors, the application of beamforming techniques can provide sound source localization, allowing for characterization of the acoustic signature of specific underwater acoustic sources. Here, performance characteristics of the system are presented, using data from controlled acoustic transmissions in a quiet environment and ambient noise measurements in an energetic tidal channel in the presence of non-acoustic flow noise. Data quality is demonstrated by the ability to reduce non-acoustic flow noise contamination, while system utility is shown by the ability to characterize and localize sources of sound in the underwater environment.

16 TIDAL AND WAVE POWER↗