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rss_1.0 Recursos de colección

Caltech Authors (93,234 recursos)
Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 3,884

1. Shock compression and release of polycrystalline magnesium oxide - Duffy, Thomas S.; Ahrens, Thomas J.
Release wave profile and Hugoniot equation of state data for low‐porosity polycrystalline MgO are reported to 36 and 133 GPa, respectively. At low pressure, the Hugoniot lies above single‐crystal data, reflecting material strength of 3.1±0.8 GPa. Unloading wave velocities to 27 GPa are consistent with extrapolated compressional sound velocities. The unloading profiles can be fit using a viscoplastic model, but there are significant differences between waveforms for different experimental geometries.

2. Free surface velocity profiles in molybdenum shock compressed at 1400 °C - Duffy, Thomas S.; Ahrens, Thomas J.
The equation of state, constitutive properties and unloading wave velocities of molybdenum have been determined from free surface velocity profiles on samples shock compressed from a 1400 °C initial state. The equation of state of 1400 °C molybdenum agrees with previous streak camera measurements and the combined equation of state between 12 and 96 GPa is: U_S =4.78 (0.02)+1.42 (0.02)u_p . Unloading wave velocities measured between 12 and 81 GPa range from 6.30 to 7.91 km/s and are 4–8% below extrapolated 25 °C compressional velocities. The yield strength, Y, was found to be 0.79–0.94 GPa, compared with values of 1.3–1.6 GPa from ambient‐temperature...

3. Oblique impact jetting of geological materials - Yang, Wenbo; Ahrens, Thomas J.
To understand jetting of earth materials, gabbro slabs (5 mm thick) were accelerated to 1.5–2 km/s and impacted inclined gabbro (5–10 mm thick), novaculite (10 mm thick) and porous sandstone (12 mm thick) targets at angles of 30°–60°. The ejecta were collected using a catcher box filled with styrofoam and the particles were extracted using chloroform. The mass of the ejected particles per unit area (∼50 mg/cm^2) remains almost independent of impact velocity, inclination angle, thickness of the target and sample mineralogy and density. Based on this result, we predict that a 500 m diameter asteroid will produce only ∼10^7...

4. Stress wave attenuation in shock damaged rock - Luo, Cangli; Ahrens, Thomas J.
The attenuation of ultrasonic stress waves in samples of gabbroic rock subjected to shock loading in the 11 Gpa range were studied. We determined the damage deficits, D_p, and attenuation coefficients, α_p, for the samples with different damage deficits under dynamic strains of 2×10^(−7) and at frequencies around 2 MHz using ultrasonic pulse-echo method. A fit to the data yields the P-wave spatial attenuation coefficient versus damage deficit: α_p=40.9D_p−30.5D^2_p (db/cm). Basing on the O’Connell-Budiansky theory, the relation between attenuation coefficient and crack density is given. The predictions of α_p from Walsh’s theory agrees well with the experiment results for the samples...

5. Application of shock compression science to Earth and planetary physics - Ahrens, Thomas J.
After the development of shock compression methods for obtaining pressure-density Hugoniot curves, it became clear that these could be applied to both determining the equations-of-state and investigation of polymorphic phase changes in silicate minerals of planetary mantles and crusts, as well as, the iron alloys of the metallic cores of terrestrial planets. These data, when taken with seismological models of the Earth, yield constraints on the composition of the Earth’s mantle and core. Shock data for molten silicates provide a basis for understanding the initial layering of a cooling terrestrial magma ocean. Application of shock-wave data is critical to delineating...

6. Characteristic of dynamic tensile fracture in augite-peridotite - He, Hongliang; Jin, Xiaogang; Jing, Fuqian; Ahrens, Thomas J.
Planar impact experiments were carried out to induce controlled dynamic tensile fracture in augite-peridotite. Samples, backed with PMMA buffer and windows, were impacted with PMMA impactor at velocities of 30 to 160 m/s. This resulted in maximum tensile stresses were in the range of ∼50 to 290 MPa. Spall strength was determined to be ∼58.1 MPa from a particle velocity profile measurement. The spall strength/HEL ratios for augite-peridotite and several other rocks were discussed based on the Griffith’s yield criterion and the experimental measurements.

7. Shock Temperature of Stainless Steel and a High Pressure - High Temperature Constraint on Thermal Diffusivity of Al_2O_3 - Gallagher, Kathleen G.; Bass, Jay D.; Ahrens, Thomas J.; Fitzner, M.; Abelson, J. R.
Time dependent shock temperatures were measured for stainless steel (SS) films in contact with transparent anvils. The anvil/window material was the same as the driver material so that there would be symmetric heat flow from the sample. Inferred Hugoniot temperatures, T_h , of 5800–7500 K at 232–321 GPa are consistent with previous measurements in SS. Temperatures at the film‐anvil interface (T_i ), which are more directly measured than T_h , indicate that T_i did not decrease measurably during the approximately 250 ns that the shock wave was in Al_2O_3 or LiF anvils. Thus an upper bound is obtained for the...

8. Theory of shock magnetization of asteroids Gaspra and Ida - Chen, George Q.; Ahrens, Thomas J.; Hide, Raymond
The observed magnetism of asteroids such as Gaspra and Ida (and other small bodies in the solar system including the Moon and meteorites) may have resulted from an impact-induced shock wave producing a thermodynamic state in which iron-nickel alloy, dispersed in a silicate matrix, is driven from the usual low-temperature, low-pressure, α, kaemacite, phase to the paramagnetic, ε (hcp), phase. The magnetization was acquired upon rarefaction and reentry into the ferromagnetic, α, structure. The degree of re-magnetization depends on the strength of the ambient field, which may have been associated with a solar-system-wide magnetic field. A transient field induced by...

9. Free-Surface light emission from shocked Teflon - Gallagher, Kathleen G.; Yang, Wenbo; Ahrens, Thomas J.
Shock initiated light emission experiments were performed on Teflon shock loaded to pressures up to ∼17 GPa. Radiances up to 600×10^6W⋅m^(−2)/(ster ⋅nm), were measured over a range of 390 to 820 nm. We have measured the spectra of light emitted upon reflection of the shock at the free surface and observed it to be distinctly non‐thermal in nature. The lights appears to result from bond destruction such as observed in shock recovery experiments on Teflon and in quasistatic experiments conducted on other polymers.

10. Shock compression and isentropic release of rhyolite - Yang, W.; Chen, G.; Anderson, W. W.; Ahrens, Thomas J.
A series of shock compression experiments have been conducted on rhyolite at pressure ranging from 6 to 33 GPa. A velocity interferometer (VISAR) was employed to monitor the particle velocity of an aluminum reflector with a diffused surface bonded to the rhyolite sample. In the forward ballistic experiments, a slow rise shock wave front is observed at 6 GPa. While in the forward experiments their release waves are smeared, in a reverse ballistic experiment, the particle velocity variation at the shock wave plateau and the isentropic release wave arrival have been clearly observed. Using Swegle’s mixed phase model, we simulated...

11. Physics of intact capture of cometary coma dust samples - Anderson, W. W.; Ahrens, T. J.
The physics of hypervelocity impact into foams are of interest because of application to comet dust capture during flyby encounters. Particles much larger than the foam cells behave as if the foam were a continuum, so that standard equations of fluid mechanics describe the effects of drag and ablation. Calculations based on these arguments accurately reproduce experimental results.

12. Tectonic Evolution of the Death Valley Region - Wernicke, Brian
Progress in understanding the evolution of continents hinges on seamlessly applying techniques of modern structural geology to the largest possible regions of the crust. In most areas, meaningful practice of regional structural geology is limited by a lack of correspondence between highly strained crust and well-defined regional strain markers, that is, large-scale geologic features whose initial geometry can be reasonably inferred, and their kinematic evolution constrained, through structural, stratigraphic, isotopic, paleomagnetic, and geodetic study. A ~100,000-km^2 segment of the U.S. Cordilleran orogen, encompassing the celebrated landscapes of Death Valley National Park and five nearby parks that are among the most visited in the U.S., was severely deformed in late Cenozoic time. In addition to...

13. Magnitude and Timing of Extreme Continental Extension, Central Death Valley Region, California - Niemi, N. A.; Wernicke, B. P.; Brady, R. J.; Saleeby, J. B.; Dunne, G. C.
New geochronologic, stratigraphic, and sedimentologic data indicate extreme late Cenozoic extension across the central Death Valley region (fig. 9). ^(40)Ar/^(39)Ar geochronology of sanidine from tuffs intercalated with steeply tilted sediments along the eastern margin of the central Death Valley region, including sections near Chicago Pass and at Eagle Mountain, indicates deposition from approximately 15 to 11.7 Ma (fig. 10). Clasts of marble, orthoquartzite, fusilinid limestone, and leucogabbro are prominent at both locations. The only known source in the Death Valley region for this clast assemblage is in the southern Cotton wood Mountains, more than 100 km away on the western flank of the Death Valley region. U/Pb geochronology of baddeleyite confirms that leucogabbro...

14. Conditions for Identifiability in Sparse Spatial Spectrum Sensing - Pal, Piya; Vaidyanathan, P. P.
Spatial Spectrum estimation is a key technique used in a wide variety of problems arising in signal processing and communication, particularly those employing multiple antennas. In many scenarios such as direction finding using antenna arrays, it is crucial to estimate which directions in space contribute to active sources (indicated by a non zero power). It has been recently shown that if the sources from different directions are statistically uncorrelated, it is possible to identify as many as O(M2) active sources using only M physical antennas. A sparse representation for the spatial spectrum was further exploited to reconstruct the spectrum using...

15. Turbulent shear layer mixing with fast chemical reactions - Dimotakis, Paul E.
A model is proposed for calculating molecular mixing and chemical reactions in fully developed turbulent shear layers, in the limit of infinitely fast chemical kinetics and negligible heat release. The model is based on the assumption that the topology of the interface between the two entrained reactants in the layer, as well as the strain field associated with it, can be described by the similarity laws of the Kolmogorov cascade. The calculation estimates the integrated volume fraction across the layer occupied by the chemical product, as a function of the stoichiometric mixture ratio of the reactants carried by the free streams,...

16. On the instability of inviscid, compressible free shear layers - Zhuang, Mei; Kubota, Toshi; Dimotakis, Paul E.
The linear spatial instability of inviscid compressible laminar mixing of two parallel streams, comprised of the same gas, has been investigated with respect to two-dimensional wave disturbances. The effects of the velocity ratio, temperature ratio, and the temperature profile across the shear layer have been examined. A nearly universal dependence of the normalized maximum amplification rate on the convective Mach number is found, with the normalized maximum amplification rate decreasing significantly with increasing convective Mach number in the subsonic region. These results are in accord with those of recent growth rate experiments in compressible turbulent free shear layers and other similar recent calculations.

17. On Mixing and Structure of the Concentration Field of Turbulent Jets - Dowling, David R.; Dimotakis, Paul E.
This work is an investigation of the mixing of the nozzle fluid of a round turbulent jet with the entrained reservoir fluid, using laser-Rayleigh scattering methods. Our measurements, at a Reynolds number of 5000, cover the axial range from 20 to 80 jet exit diameters and resolve the full range of temporal & spatial concentration scales. The measured mean & rms values of the concentration, and the mean scalar dissipation rate, when estimated from the time derivative of concentration, are consistent with jet similarity laws. Concentration fluctuation power spectra are found to be self-similar along rays emanating from the jet virtual origin. The probability density functions for the concentration, the time derivative of concentration, and the...

18. The Role of Cerium Sites in the Scintillation Mechanism of LSO - Naud, J. D.; Tombrello, T. A.; Melcher, C. L.; Schweitzer, J. S.
The crystal structure of Lu_2(SiO_4)O:Ce (“LSO”) has two trivalent cation sites which may be occupied by Ce^(3+) to form luminescence centers. Previous investigations revealed the existence of two distinct sets of Ce^(3+) excitation and emission spectra and suggested that the differences in the spectra are due to differences in the crystal fields at the two lattice sites that shift the 5d levels of Ce^(3+). In the present report, we re-examine this issue and present new evidence which suggests a different interpretation. In particular, spectra measured at 13 K suggest that both lattice sites give rise to indistinguishable excitation and emission...

19. Mixing and Reaction at Low Heat Release in the Non-Homogeneous Shear Layer - Frieler, C. E.; Dimotakis, P. E.
The effects of freestream density ratio on the mixing and combustion in a high Reynolds number, subsonic, gas-phase, non-buoyant, two-dimensional turbulent mixing layer, have been investigated. Measurements of temperature rise (heat release) have been made which enable us to examine the effect of freestream density ratio on several aspects of the mixed fluid state within the turbulent combustion region. In experiments with very high and very low stoichiometric mixture ratios ("flip" experiments), the heat release from an exothermic reaction serves as a quantitative label for the lean reactant freestream fluid that becomes molecularly mixed. Properly normalized, the sum of the mean temperature rise profiles of the two flip experiments represent the probability of fluid molecularly mixed...

20. Inviscid instability characteristics of free shear layers with non-uniform density - Koochesfahani, M. M.; Frieler, C. E.
The linear spatial instability of two-dimensional two-stream plane mixing layers has been studied extensively in the past. In the case of uniform density, Michalke (1965) investigated the single-stream shear layer and Monkewitz & Huerre (1982) considered the effect of the velocity ratio. Maslowe & Kelly (1971) studied the stratified (non-uniform density) shear layers and showed that density variations can be destabilizing. In all these studies, the mean velocity profile has been assumed to be monotonically increasing from the value on the low-speed stream to that on the high-speed stream and usually the hyperbolic tangent form is used. It should be noted, however,...

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