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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Shocked Clouds in the Vela Supernova Remnant

Unusually strong high-excitation C I has been detected in eleven lines of sight through the Vela supernova remnant by means of UV absorption-line studies of IUE data. Most of these lines of sight lie near the western edge of the X-ray bright region of the supernova remnant in a spatially distinct band approximately 1deg by 4deg oriented approximately north/south. The high-excitation C I (denoted C I*) is interpreted as evidence of a complex of shocked dense clouds inside the supernova remnant, due to the high pressures indicated in this region. To further analyze the properties of this region of C I*, we present new HIRES-processed IRAS data of the entire Vela SNR. A temperature map calculated from the HIRES IRAS data, based on a two-component dust model, reveals the signature of hot dust at several locations in the SNR. The hot dust is anti-correlated spatially with X-ray emission as revealed by ROSAT, as would be expected for a dusty medium interacting with a shock wave. The regions of hot dust are strongly correlated with optical filaments, supporting a scenario of dense clouds interior to the SNR that have been shocked and are now cooling behind the supernova blast wave. With few exceptions, the lines of sight to the strong C I* pass through regions of hot dust and optical filaments. Possible mechanisms for the production of the anomalously large columns of C I and C I* are discussed. Dense clouds on the back western hemisphere of the remnant may explain the relatively low X-ray emission in the western portion of the Vela supernova remnant due to the slower forward shock velocity in regions where the shock has encountered the dense clouds. An alternate explanation for the presence of neutral, excited state, and ionized species along the same line of sight may be a magnetic precusor that heats and compresses the gas ahead of the shock.

Nichols, Joy S.↗
NASA NTRS1981

Measurements of a solar flare-generated shock wave at 13.1 R/0/

The first measurements of the structure of wind speed, electron density, and electron density fluctuations are reported for a shock wave propagating through the acceleration region of the solar wind. Radio scattering observations, consisting of spectral broadening, mean phase and amplitude scintillations, were made on August 18, 1979, 13.1 solar radii east of the sun near the ecliptic plane, using the 2.3 and 8.4 GHz radio signals of Voyager 1. The results show a shock wave speed of about 3,500 km/sec; which, when compared with average transit time speed to 1 AU, shows that substantial deceleration took place with outward propagation from the sun. This result is consistent with a blast wave.

Woo, R.↗
NASA NTRS1988

Shocks produced by impulsively driven reconnection

Shock waves produced by impulsively driven reconnection are investigated by carrying out numerical experiments using two-dimensional magnetohydrodynamics. The results of the numerical experiments imply that there are three different categories of shocks associated with impulsively driven reconnection: (1) fast-mode, blast waves which rapidly propagate away from the reconnection site; (2) slow-mode, Petschek shocks which are attached to the reconnection site; and (3) fast-mode, termination shocks which terminate the plasma jets flowing out from the reconnection site.

Forbes, T. G.↗
NASA NTRS1961

Calculation of Flow Fields from Bow-Wave Profiles for the Downstream Region of Blunt-Nosed Circular Cylinders in Axial Hypersonic Flight

A method by which known bow-wave profiles may be analyzed to give the flow fields around blunt-nosed cylinders in axial hypersonic flow is presented. In the method, the assumption is made that the pressure distribution curve in a transverse plane is similar to that given by blast- wave theory. Numerical analysis based on the one-dimensional energy and continuity equations then leads to distributions of all the flow variables in the cross section, for either a perfect gas or a real gas. The entire flow field need not be solved. Attention can be confined to any desired station. The critical question is the validity of the above assumption. It is tested for the case of a hemisphere cylinder in flight at 20,000 ft/sec. The flow is analyzed for three stations along the cylindrical afterbody, and found to compare very closely with the results of an exact (inviscid) solution. The assumed form of the pressure distribution occurs at stations as close as 1.2 diameters to the body nose. However, it is suggested that the assumption may not apply this far forward in general, particularly when bodies of nonsmooth contour are considered.

Seiff, Alvin↗
DOE OSTI2022

Numerical Analysis of Blast Effects and Mitigation in the Far-Field from Small Explosions

Requirements for explosive safety are often given in terms of a “K-Factor”, correlating incident blast effects with the distance and TNT equivalent weight of an explosion. Traditionally, this is conducted using empirical correlations to experimental measurements (e.g., the Kingery–Bulmash equations). In the far-field, empirical verification of incident overpressure and impulse magnitudes can be difficult; extrapolations from data give expected values at reasonable standoff distances that sometimes are too small to measure on available equipment but are larger than some regulations require. The present paper describes the results of numerical hydrocode analysis to verify the expected incident overpressure and impulse from small hemispherical ground charges of TNT at these relatively large distances. Furthermore, the dynamic effect of incident blast waves on lightweight, modular mitigation barriers is studied to gauge their effectiveness at providing safety standard compliance.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1973

Pressure waves generated by steady flames.

Analysis of pressure waves that can be generated by clouds of explosive gas mixtures in a free atmosphere which is initially at a uniform state. The treatment is restricted only to the final stage of constant flame velocity when the flowfield is self-similar. By the introduction of reduced blast-wave parameters as phase-plane coordinates, the problem is resolved into the determination of the appropriate integral curves on this plane. Results, including space profiles of gasdynamic parameters, have been computed for a specific case of a hydrocarbon-air mixture characterized by a specific heat ratio of 1.3, sound speed at NTP of 345 m/sec, and volumetric expansion ratio corresponding to constant pressure deflagration of 7. Maximum overpressure ratios that can be generated by such flames in point-and line-symmetrical waves range from .00053, for the lower bound in the burning speed, up to 6 for the deflagration, while, for the average speeds of 5 to 10 m/sec, they are at a level of 0.05 to 0.10.

Kuhl, A. L.↗
NASA NTRS2008

CAWSES November 7-8, 2004, Superstorm: Complex Solar and Interplanetary Features in the Post-Solar Maximum Phase

The complex interplanetary structures during 7 to 8 Nov 2004 are analyzed to identify their properties as well as resultant geomagnetic effects and the solar origins. Three fast forward shocks, three directional discontinuities and two reverse waves were detected and analyzed in detail. The three fast forward shocks 'pump' up the interplanetary magnetic field from a value of approx.4 nT to ~44 nT. However, the fields after the shocks were northward, and magnetic storms did not result. The three ram pressure increases were associated with major sudden impulses (SI + s) at Earth. A magnetic cloud followed the third forward shock and the southward Bz associated with the latter was responsible for the superstorm. Two reverse waves were detected, one at the edge and one near the center of the magnetic cloud (MC). It is suspected that these 'waves' were once reverse shocks which were becoming evanescent when they propagated into the low plasma beta MC. The second reverse wave caused a decrease in the southward component of the IMF and initiated the storm recovery phase. It is determined that flares located at large longitudinal distances from the subsolar point were the most likely causes of the first two shocks without associated magnetic clouds. It is thus unlikely that the shocks were 'blast waves' or that magnetic reconnection eroded away the two associated MCs. This interplanetary/solar event is an example of the extremely complex magnetic storms which can occur in the post-solar maximum phase.

solar flares↗
NASA NTRS1994

Grain destruction in shocks in the interstellar medium

Destruction of interstellar dust occurs predominantly in supernova shock waves in the warm neutral/ionized medium (density approximately = 0.25/cu cm, temperature approximately = 10(exp 4) K). Recent theoretical developments and laboratory data for sputtering processes and grain-grain collisional vaporization allows us to better evaluate the grain destruction rate in interstellar shocks in the warm medium. We find that, independent of composition, grain denstruction in supernova blast waves is dominated by nonthermal sputtering for shock velocities greater than 50 km/s and less than or equal to 150 km/s and thermal sputtering at higher shock velocities. We use a detailed scheme for the vaporization of grains colliding at high velocities (v(sub s) greater than or equal to 20 km/s) and show that the grain-grain collision destruction process is only dominant for shock velocities of less than or equal to 50-80 km/s and is less important than previously assumed. Nevertheless, the grain-grain destruction rates are of order 30%-90% of the sputtering rates at v(sub s) greater than 100 km/s and less than 200 km/s and are important in vaporizing the cores of grains. Detailed results for grain destruction as a function of grain size and composition are presented. We also present results for silicon carbide, iron, ice, and porous test particles. For carbonaceous grains we find that the fractional destruction is less than or equal to 0.29, and for silicate it is less than or equal to 0.45, for v(sub s) less than or equal to 200 km/s. We have calculated grain lifetimes, using the three-phase model of the interstellar medium, and find lifetimes of 4 x 10(exp 8) yr for carbonaceous grains and 2.2 x 10(exp 8) yr for silicate grains. Given that the typical stardust injection timescale of 2.5 x 10(exp 9) yr, we conclude that efficient mechanisms for grain growth in the interstellar medium must exist in order that a significant fraction of the refractory elements be incorporated in dust, as observed. Therefore, although our improved model has less vaporization of dust due to grain-grain collisions, sputtering still destroys dust efficiently and grain mantle growth in the interstellar medium is required, a conclusion reached in previous models of grain destruction in the interstellar medium. Carbonaceous mantles on silicate grains can protect the silicate cores from sputtering destruction in interstellar shock waves, provided that the protective mantles can efficiently reform in the interstellar medium. Also, if the grains are porous the postshock grain velocities are lower than for solid particles, and grain destruction is reduced. Porosity and mantling may increase the grain lifetime by factors of approximately 3 and approximately 3-4, respectively. The fraction of interstellar silicon in silicate stardust is therefore less than or equal to 0.25, but more silicon might be depleted in the form of a grain mantle.

Jones, A. P.↗
NASA NTRS1991

Pressure vessel burst test program. II

The current status is disucssed of a program to study the characteristics of blast waves and fragmentation generated by ruptured gas-filled pressure vessels. Current methods for assessing vessel safety and burst parameters are briefly reviewed, and pneumatic burst testing operations and testing results are examined. A comparison is made with current methods for burst assessment. It is tentatively concluded that, at close distances, vessel burst overpressures are less than those of high-explosive (HE) blasts with equivalent energy and are greater than HE far from the vessel. The impulse appears to be the same for both vessel bursts and equivalent energy HE blasts. The functional relationship between shock velocity and overpressure ratio appears to be the same for vessel bursts as for HE blasts. The initial shock overpressure appears to be much less than vessel pressure and may be found using the one-dimensional shock tube equation.

Cain, Maurice R.↗
NASA NTRS1992

Particle acceleration in solar flares - Observations

Contrary to our historical understanding, the energetic particles in most major solar proton events do not come from the flare itself. The particle abundances, ionization states, time evolution, and longitude distributions all indicate that the particles are accelerated from the ambient plasma by a shock wave driven by a coronal mass ejection in these events. In contrast, the particles that do come from impulsive solar flares are unique in character. These particles are electron rich, have He-3/He-4 enhancements of up to 10,000, and enhancements in heavy elements such as Fe/C by factors of 10. The high ionization state of Fe, +20 indicates that the material has been heated to temperatures of about 2 x 10 exp 7 K. It is generally believed that preferential heating by selective absorption of plasma waves is combined with stochastic acceleration in these events. Recent studies of the broad gamma-ray lines emitted by energetic particles within the flare loops indicate that they are also Fe-rich, He-3 rich and proton-poor like the particles seen at 1 AU. In large impulsive events, particles from the impulsive phase may be reaccelerated by a coronal blast-wave shock.

Reames, Donald V.↗
NASA NTRS1978

Surface waves generated by shallow underwater explosions

Surface water waves generated by surface and near surface point explosions are calculated. Taking the impulse distribution imparted at the water surface by the explosion as the overriding mechanism for transferring energy of the explosive to surface wave motion, the linearized theory of Kranzer and Keller is used to obtain the wave displacement in the far field. The impulse distribution is obtained by integrating the pressure wave over an appropriate time interval on a horizontal surface just beneath the undisturbed water surface. For surface explosions, a modified form of the similarity method first used by Collins and Holt is used to obtain the flow field. In the case of submerged explosions, the flow field is estimated by making necessary modifications to Sedov's similarity solution to account for the venting that accompanies the interaction of the leading (blast) wave with the ocean surface. Surface waves generated by a charge at six depths of placement (0.15 m, 0.30 m, 0.61 m, 0.91 m, 1.37 m, 3.05 m) are considered in addition to surface explosions. The results seem to support the existence of an upper critical depth phenomenon (of the type already established for chemical explosions) for point (nuclear) explosions.

Falade, A.↗
NASA NTRS1974

Hypersonic flows generated by parabolic and paraboloidal shock waves

A computer algorithm has been developed to determine the blunt-body flowfields supporting symmetric parabolic and paraboloidal shock waves at infinite free-stream Mach number. Solutions are expressed in an analytic form as high-order power series, in the coordinate normal to the shock, whose coefficients can be determined exactly. Analytic continuation is provided by the use of Pade approximations. Test cases provide solutions of very high accuracy. In the axisymmetric case for gamma equals 715 the solution has been found far downstream, where it agrees with the modified blast-wave results. For plane flow, on the other hand, a limit line appears within the shock layer, a short distance past the sonic line, suggesting the presence of an imbedded shock. Local solutions in the downstream limit are discussed.

Schwartz, L. W.↗
NASA NTRS1970

Galactic astronomy; Proceedings of the Second Summer Institute for Astronomy and Astrophysics, State University of New York, Stony Brook, N.Y., June 19-July 17, 1968. Volumes 1 & 2.

Basic problems on the structure of our galaxy are presented. The essentially spiral structure of the galaxies is examined by a theory of the spiral shape, and by a study of the spiral structure with the 21-cm line of neutral hydrogen. Irregular spirals are discussed, and some problems that arise as a result of large deviations from the regular spiral structure are treated. The density wave theory of spiral structure is compared with the observed spiral structure of the galaxies. A section on the star migration technique, which is used to test the density wave theory of galactic spirals, is included. The interstellar medium is described in terms of such phenomena as supernova explosions and cosmic rays, both of which contribute to the energy balance of the medium. Other topics that are germaine to a description of the interstellar medium are the UV spectrum, kinematics of interstellar clouds, and ionization equilibrium. The implications of spiral galactic shock pattern on star formation along spiral arms are investigated, and by means of a blast wave model, it is shown that a hydrodynamic shock of galactic scale exists in galaxy M82.

Chiu, H.-Y.↗
NASA NTRS1981

A cosmic-ray-mediated shock in the solar system

It is pointed out that the flare-induced blast wave of Aug. 4, 1972, the most violent disturbance in the solar wind on record, produced cosmic rays with an efficiency of about 50%. Such a high efficiency is predicted by the self-regulating production model of cosmic-ray origin in shocks. Most interplanetary shocks, according to simple theoretical analysis, are not strong enough to produce cosmic rays efficiently. However, if shock strength is the key parameter governing efficiency, as present interplanetary data suggest, then shocks from supernova blasts, quasar outbursts, and other violent astrophysical phenomena should be extremely efficient sources of cosmic rays.

Eichler, D.↗
NASA NTRS1972

Association between interplanetary shock waves and delayed solar particle events.

In studying the propagation of energetic charged particles from a large solar flare, we can no longer regard the interplanetary medium as remaining in a steady state; disturbances in the flow of solar wind from these flares may have great effects on the observed fluxes of charged particles. Delayed particle events, also known as 'energetic storm particle' events, may exhibit an increase by an order of magnitude or greater in the flux of protons above 10 MeV over a period of the order of 6 hours. These events are seen in association with the passage of an interplanetary shock past the earth. It is proposed that the particles are accelerated locally at the time of the passage of the interplanetary blast wave.

Datlowe, D.↗
DOE OSTI2022

Observation of breakdown wave mechanism in avalanche ionization produced atmospheric plasma generated by a picosecond CO 2 laser

Understanding the formation and long-timescale evolution of atmospheric plasmas produced by ultrashort, long-wavelength infrared (LWIR) pulses is an important but partially understood problem. Of particular interest are plasmas produced in air with a peak laser intensity ~10 12 W/cm 2 , the so-called clamping intensity observed in LWIR atmospheric guiding experiments where tunneling and multi-photon ionization operative at near-IR or shorter wavelengths are inoperative. We find that avalanche breakdown on the surface of aerosol (dust) particles can act to seed the breakdown of air observed above the 200 GW/cm 2 threshold when a train of 3 ps 10.6 μm laser pulses separated by 18 ps is used. The breakdown first appears at the best focus but propagates backward toward the focusing optic as the plasma density approaches critical density and makes forward propagation impossible. The velocity of the backward propagating breakdown can be as high as 10 9 cm/s, an order of magnitude greater than measured with ns pulse-produced breakdown, and can be explained rather well by the so-called breakdown wave mechanism. Transverse plasma expansion with a similar velocity is assisted by UV photoionization and is observed as a secondary longitudinal breakdown mechanism in roughly 10% of the shots. When a cm-size, TW power beam is propagated, interception of aerosol particles is guaranteed and several (40 cm -3 ) breakdown sites appear, each initially producing a near-critical density plasma. On a 10 ns–1 μs timescale, shockwaves from each site expand radially and coalesce to produce a large hot gas channel. The radial velocity of the expansion agrees well with the prediction of the blast wave theory developed for ultrafast atmospheric detonations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2019

A Year in the Life of GW 170817: The Rise and Fall of a Structured Jet from a Binary Neutron Star Merger

We present the results of our year-long afterglow monitoring of GW 170817, the first binary neutron star merger detected by Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo. New observations with the Australian Telescope Compact Array and the Chandra X-ray Telescope were used to constrain its late-time behaviour. The broad-band emission, from radio to X-rays, is well-described by a simple power-law spectrum with index β ∼ 0.585 at all epochs. After an initial shallow rise ∝t0.9, the afterglow displayed a smooth turnover, reaching a peak X-ray luminosity of LX ≈ 5 × 1039 erg s−1 at 160 d, and has now entered a phase of rapid decline, approximately ∝t−2. The latest temporal trend challenges most models of choked jet/cocoon systems, and is instead consistent with the emergence of a relativistic structured jet seen at an angle of ≈22◦ from its axis. Within such model, the properties of the explosion (such as its blast wave energy EK ≈ 2 × 1050 erg, jet width θc ≈ 4◦, and ambient density n ≈ 3 × 10−3 cm−3) fit well within the range of properties of cosmological short gamma-ray bursts.

Acceleration of particles↗