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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2006

Shock, Post-Shock Annealing, and Post-Annealing Shock in Ureilites

The thermal and shock histories of ureilites can be divided into four periods: 1) formation, 2) initial shock, 3) post-shock annealing, and 4) post-annealing shock. Period 1 occurred approx.4.55 Ga ago when ureilites formed by melting chondritic material. Impact events during period 2 caused silicate darkening, undulose to mosaic extinction in olivines, and the formation of diamond, lonsdaleite, and chaoite from indigenous carbonaceous material. Alkali-rich fine-grained silicates may have been introduced by impact injection into ureilites during this period. About 57% of the ureilites were unchanged after period 2. During period 3 events, impact-induced annealing caused previously mosaicized olivine grains to become aggregates of small unstrained crystals. Some ureilites experienced reduction as FeO at the edges of olivine grains reacted with C from the matrix. Annealing may also be responsible for coarsening of graphite in a few ureilites, forming euhedral-appearing, idioblastic crystals. Orthopyroxene in Meteorite Hills (MET) 78008 may have formed from pigeonite by annealing during this period. The Rb-Sr internal isochron age of approx.4.0 Ga for MET 78008 probably dates the annealing event. At this late date, impacts are the only viable heat source. About 36% of ureilites experienced period 3 events, but remained unchanged afterwards. During period 4, approx.7% of the ureilites were shocked again, as is evident in the polymict breccia, Elephant Moraine (EET) 83309. This rock contains annealed mosaicized olivine aggregates composed of small individual olivine crystals that exhibit undulose extinction. Ureilites may have formed by impact-melting chondritic material on a primitive body with heterogeneous O isotopes. Plagioclase was preferentially lost from the system due to its low impedance to shock compression. Brief melting and rapid burial minimized the escape of planetary-type noble gases from the ureilitic melts. Incomplete separation of metal from silicates during impact melting left ureilites with relatively high concentrations of trace siderophile elements.

Rubin, Alan E.↗
NASA NTRS1984

Shock experiments on maskelynite-bearing anorthosite

A series of shock recovery experiments over 9.9-60.4 GPa have been carried out on naturally shocked anorthosite from the Mistastin impact structure in Labrador consisting primarily of diaplectic plagioclase glass or maskelynite, An(50), and pyroxene. Petrographic observations of the experimental products indicate that the component minerals and diaplectic glasses generally retained their initial character throughout, the only exception being the increase in fracturing which occurred in the 9.9 GPa shot. Reshocking at pressures higher than the initial shock tends to lower the refractive index of maskelynite. The increase in refractive index of maskelynite reshocked to pressures lower than the initial pressure is interpreted as due to shock densification of the diaplectic glass above the Hugoniot elastic limit and below the mixed phase regime. The data suggest that the low-high-low density transition of maskelynite occurs about 8 GPa below that of the crystal of corresponding composition.

Lambert, P.↗
NASA NTRS1972

Prediction and measurement of heat transfer rates for the shock-induced unsteady laminar boundary layer on a flat plate

The unsteady laminar boundary layer induced by the flow-initiating shock wave passing over a flat plate mounted in a shock tube was theoretically and experimentally studied in terms of heat transfer rates to the plate for shock speeds ranging from 1.695 to 7.34 km/sec. The theory presented by Cook and Chapman for the shock-induced unsteady boundary layer on a plate is reviewed with emphasis on unsteady heat transfer. A method of measuring time-dependent heat-transfer rates using thin-film heat-flux gages and an associated data reduction technique are outlined in detail. Particular consideration is given to heat-flux measurement in short-duration ionized shocktube flows. Experimental unsteady plate heat transfer rates obtained in both air and nitrogen using thin-film heat-flux gages generally agree well with theoretical predictions. The experimental results indicate that the theory continues to predict the unsteady boundary layer behavior after the shock wave leaves the trailing edge of the plate even though the theory is strictly applicable only for the time interval in which the shock remains on the plate.

Cook, W. J.↗
NASA NTRS1986

The size distributions of fragments ejected at a given velocity from impact craters

The mass distribution of fragments that are ejected at a given velocity for impact craters is modeled to allow extrapolation of laboratory, field, and numerical results to large scale planetary events. The model is semi-empirical in nature and is derived from: (1) numerical calculations of cratering and the resultant mass versus ejection velocity, (2) observed ejecta blanket particle size distributions, (3) an empirical relationship between maximum ejecta fragment size and crater diameter, (4) measurements and theory of maximum ejecta size versus ejecta velocity, and (5) an assumption on the functional form for the distribution of fragments ejected at a given velocity. This model implies that or planetary impacts into competent rock, the distribution of fragments ejected at a given velocity is broad, e.g., 68% of the mass of the ejecta at a given velocity contains fragments having a mass less than 0.1 times a mass of the largest fragment moving at that velocity. The broad distribution suggests that in impact processes, additional comminution of ejecta occurs after the upward initial shock has passed in the process of the ejecta velocity vector rotating from an initially downward orientation. This additional comminution produces the broader size distribution in impact ejecta as compared to that obtained in simple brittle failure experiments.

Okeefe, J. D.↗
NASA NTRS1987

The size distributions of fragments ejected at a given velocity from impact craters

The mass distribution of fragments that are ejected at a given velocity for impact craters is modeled to allow extrapolation of laboratory, field, and numerical results to large scale planetary events. The model is semi-empirical in nature and is derived from: (1) numerical calculations of cratering and the resultant mass versus ejection velocity, (2) observed ejecta blanket particle size distributions, (3) an empirical relationship between maximum ejecta fragment size and crater diameter, (4) measurements and theory of maximum ejecta size versus ejecta velocity, and (5) an assumption on the functional form for the distribution of fragments ejected at a given velocity. This model implies that for planetary impacts into competent rock, the distribution of fragments ejected at a given velocity is broad, e.g., 68 percent of the mass of the ejecta at a given velocity contains fragments having a mass less than 0.1 times a mass of the largest fragment moving at that velocity. The broad distribution suggests that in impact processes, additional comminution of ejecta occurs after the upward initial shock has passed in the process of the ejecta velocity vector rotating from an initially downward orientation. This additional comminution produces the broader size distribution in impact ejecta as compared to that obtained in simple brittle failure experiments.

O'Keefe, John D.↗
NASA NTRS1979

Molecule formation and infrared emission in fast interstellar shocks. I Physical processes

The paper analyzes the structure of fast shocks incident upon interstellar gas of ambient density from 10 to the 7th per cu cm, while focusing on the problems of formation and destruction of molecules and infrared emission in the cooling, neutral post shock gas. It is noted that such fast shocks initially dissociate almost all preexisting molecules. Discussion covers the physical processes which determine the post shock structure between 10 to the 4 and 10 to the 2 K. It is shown that the chemistry of important molecular coolants H2, CO, OH, and H2O, as well as HD and CH, is reduced to a relatively small set of gas phase and grain surface reactions. Also, the chemistry follows the slow conversion of atomic hydrogen into H2, which primarily occurs on grain surfaces. The dependence of this H2 formation rate on grain and gas temperatures is examined and the survival of grains behind fast shocks is discussed. Post shock heating and cooling rates are calculated and an appropriate, analytic, universal cooling function is developed for molecules other than hydrogen which includes opacities from both the dust and the lines.

Hollenbach, D.↗
NASA NTRS1986

Configurations of corotating shocks in the outer heliosphere

Configurations of corotating shocks in the outer heliosphere are computed on the assumption that the shocks move at constant speeds between collisions. The basic physical process is the interaction of a forward shock with a reverse shock. The points at which this interaction takes place are determined algebraically in terms of the initial shock positions and speeds. Between the points at which the shocks interact, the shocks have the form of Archimedian spirals. Shock configurations are determined for the case of two corotating shock pairs originating at 2.5 AU and for a single shock pair at 2.5 AU. In both cases the heliosphere is divided into several distinct regions distinguished by the number of times the particles have passed through a shock. Since the shock strengths decrease when a forward shock interacts with a reverse shock, corotating shocks are expected to be weaker at larger distances from the sun.

Burlaga, L. F.↗
NASA NTRS1991

An approximate viscous shock layer technique for calculating chemically reacting hypersonic flows about blunt-nosed bodies

An approximate axisymmetric method was developed which can reliably calculate fully viscous hypersonic flows over blunt nosed bodies. By substituting Maslen's second order pressure expression for the normal momentum equation, a simplified form of the viscous shock layer (VSL) equations is obtained. This approach can solve both the subsonic and supersonic regions of the shock layer without a starting solution for the shock shape. The approach is applicable to perfect gas, equilibrium, and nonequilibrium flowfields. Since the method is fully viscous, the problems associated with a boundary layer solution with an inviscid layer solution are avoided. This procedure is significantly faster than the parabolized Navier-Stokes (PNS) or VSL solvers and would be useful in a preliminary design environment. Problems associated with a previously developed approximate VSL technique are addressed before extending the method to nonequilibrium calculations. Perfect gas (laminar and turbulent), equilibrium, and nonequilibrium solutions were generated for airflows over several analytic body shapes. Surface heat transfer, skin friction, and pressure predictions are comparable to VSL results. In addition, computed heating rates are in good agreement with experimental data. The present technique generates its own shock shape as part of its solution, and therefore could be used to provide more accurate initial shock shapes for higher order procedures which require starting solutions.

Cheatwood, F. Mcneil↗
NASA NTRS1991

An approximate viscous shock layer approach to calculating hypersonic flows about blunt-nosed bodies

An approximate axisymmetric method has been developed which can reliably calculate fully viscous hypersonic flows over blunt-nosed bodies. By substituting Maslen's second order pressure expression for the normal momentum equation, a simplified form of the viscous shock layer (VSL) equations is obtained. This approach can solve both the subsonic and supersonic regions of the shock layer without a starting solution for the shock shape. Since the method is fully viscous, the problems associated with coupling a boundary-layer solution with an inviscid-layer solution are avoided. This procedure is significantly faster than the parabolized Navier-Stokes (PNS) or VSL solvers and would be useful in a preliminary design environment. Problems associated with a previously developed approximate VSL technique are addressed. Surface heat transfer and pressure predictions are comparable to both VSL results and experimental data. The present technique generates its own shock shape as part of its solution, and therefore could be used to provide more accurate initial shock shapes for higher-order procedures which require starting solutions.

Cheatwood, F. MCN.↗
NASA NTRS1992

An approximate viscous shock layer technique for calculating nonequilibrium hypersonic flows about blunt-nosed bodies

An approximate axisymmetric method has been developed which can reliably calculate nonequilibrium fully viscous hypersonic flows over blunt-nosed bodies. By substituting Maslen's second-order pressure expression for the normal momentum equation, a simplified form of the viscous shock layer (VSL) equations is obtained. This approach can solve both the subsonic and supersonic regions of the shock layer without a starting solution for the shock shape. This procedure is significantly faster than the parabolized Navier-Stokes and VSL solvers and would be useful in a preliminary design environment. Solutions have been generated for air flows over several analytic body shapes. Surface heat transfer and pressure predictions are comparable to VSL results. Computed heating rates are in good agreement with experimental data. The present technique generates its own shock shape as part of its solution, and therefore could be used to provide more accurate initial shock shapes for higher-order procedures which require starting solutions.

Cheatwood, F. M.↗
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 NTRS1993

Molecular line emission models of Herbig-Haro objects. II - HCO(+) emission

We present time-dependent models of the chemistry and temperature of interstellar molecular gas clumps that are exposed to the radiation from propagating stellar-jet shocks. The X-ray, EUV, and FUV radiation from the shock initiates ion chemistry and also heats the gas in the clumps. Using representative parameters, we show that, on the shock transit time between the clumps, the abundances of the ionized molecular species that are produced in the clumps can exceed the values determined from steady state models by several orders of magnitude. Collisional excitation by the heated gas can lead to measurable line emission from several ionized species; as in previous investigations of X-ray-irradiated molecular gas, we find that electron impacts contribute significantly to this process. We apply these results to the interpretation of the HCO(+) line emission that has already been detected in several Herbig-Haro objects. We demonstrate that this picture provides a natural explanation of the fact that the line intensity typically peaks ahead of the associated shock, as well as of the reported low line-center velocities and narrow line widths. We tabulate several diagnostic line intensities of HCO(+) and other molecular species that may be used to infer the physical conditions in the emitting gas.

Wolfire, Mark G.↗
NASA NTRS1990

MHD study of temporal and spatial evolution of simulated interplanetary shocks in the ecliptic plane within 1 AU

The present parametric study of interplanetary shock propagation to 1 AU uses a two- and-one-half-dimensional MHD time-dependent model whose input conditions encompass initial shock velocity, driving-pulse duration, and pulse width at the near-sun position of 18 solar radii. It is found that, for input pulses with modest angular width and temporal duration, the propagation of the resulting interplanetary fast-forward shock waves primarily depends on the net input energy. While the properties of the reverse shocks are also a function of input energy, they depend on the specific values of the input pulse shock velocity, width, and duration.

Smith, Zdenka↗
NASA NTRS1997

Initialization and Predictability of a Coupled ENSO Forecast Model

The skill of a coupled ocean-atmosphere model in predicting ENSO has recently been improved using a new initialization procedure in which initial conditions are obtained from the coupled model, nudged toward observations of wind stress. The previous procedure involved direct insertion of wind stress observations, ignoring model feedback from ocean to atmosphere. The success of the new scheme is attributed to its explicit consideration of ocean-atmosphere coupling and the associated reduction of "initialization shock" and random noise. The so-called spring predictability barrier is eliminated, suggesting that such a barrier is not intrinsic to the real climate system. Initial attempts to generalize the nudging procedure to include SST were not successful; possible explanations are offered. In all experiments forecast skill is found to be much higher for the 1980s than for the 1970s and 1990s, suggesting decadal variations in predictability.

Chen, Dake↗
NASA NTRS1993

A piston-actuated shock-tube, with laser-Schlieren diagnostics

The essential construction features of a piston actuated shock tube are described, and its advantages relative to the conventional use of diaphragm ruptures for shock initiation are listed. Typical operational parameters are presented to illustrate the levels of reproducibility achieved. Tests with He and N2 drivers into about 99 percent Ar covered shock speeds from 1.539 +/- 0.002-0.8143 +/- 0.002 mm/microsec, corresponding to 2390-847 K incident shock temperatures. Application of this tube for recording postshock front density gradients of the endoergic dissociation of ethane and the exoergic condensation of iron atoms via the laser-Schlieren technique is described.

Hurst, S. M.↗
NASA NTRS2024

A Diversity of Temperature and Pressure Conditions Recorded by Zircon within Suevite from Ries Crater, Germany

The temperature and pressure conditions experienced by rocks during an impact event can be constrained using petrologic and microstructural analysis and is crucial to providing ground truth to the impact cratering process. Suevite is a polymict, impact melt-bearing breccia, specific to Ries crater in Germany. There are competing models for suevite formation and emplacement, such as clastic flows pushed out of the crater rim or ejecta plume fallback. Knowledge of the temperature and pressure pathways recorded by grains within the suevite can help distinguish between these and other models. The accessory phase zircon (ZrSiO4) and its high-pressure polymorph reidite are particularly useful in such circumstances as they are highly refractory minerals that can record the high-temperature and/or high-pressure conditions of an impact event. Here we present evidence for a wide array of temperature and pressure conditions recorded in zircon grains within a single thin section of suevite. Zircons in this study range from unshocked to highly shocked (>53 GPa), and record temperatures more than 1673 °C. These findings confirm previous studies concluding that suevites contain material exposed to very diverse pressure and temperature conditions during initial shock compression and excavation but do not, as a whole, experience extreme temperatures (>1673 °C) or pressures (>30 GPa).

mineralogy↗
NASA NTRS1976

Annular arc accelerator shock tube

An annular arc accelerator shock tube employs a cold gas driver to flow a stream of gas from an expansion section through a high voltage electrode section to a test section, thus driving a shock wave in front of it. A glow discharge detects the shock wave and actuates a trigger generator which in turn fires spark-gap switches to discharge a bank of capacitors across a centered cathode and an annular anode in tandem electrode sections. The initial shock wave passes through the anode section from the cathode section thereby depositing energy into the flow gas without the necessity of any diaphragm opening in the gas flow from the expansion section through the electrode sections.

Leibowitz, L. P.↗
NASA NTRS1986

Shock-induced volatile loss from a carbonaceous chondrite: Implications for planetary accretion

Solid recovery impact-induced volatile loss experiments on the Murchison C2M meteorite indicate that for an impact of a given velocity, H2O and total volatiles are driven from the sample in the same proportion as present initially. The primitive surface volatile budget of a planet growing by accretion would have the same bulk elemental composition as the volatiles in the incident planetesimals. Incipient devolatilization of Murchison occurs at an initial shock pressure of about 11 GPa and complete devolatilization occurs at a pressure of about 30 GPa. For the Earth, incipient and complete devolatilization of accreting planetesimals would occur when the planet reached approximately 12% and 27%, respectively, of its present-day radius. Impact-induced devolatilization would profoundly affect the volatile distribution within the accreting planet. Prior to metallic core formation and internal differentiation the growing planet would have a very small core with the same volatile content as the incident material, a volatile depleted mantle, and an extremely volatile rich surface. In the case of the Earth, 99.4 wt% of the total incident volatile material would end up on or near the planetary surface.

Tyburczy, J. A.↗