Vibrational entropy of L12 Cu3Au measured by inelastic neutron scattering

The phonon density of states of elemental Au, Cu, and Cu3Au with L12 chemical order were measured by inelastic neutron scattering and used to calculate the vibrational entropy of formation of the ordered compound from the elemental metals. A vibrational entropy of formation of (0.06±0.03) kB/atom at 300 K was obtained, with the vibrational entropy of the ordered alloy being larger than that of the elemental metals. The phonon DOS of the disordered Cu3Au was simulated by adding the phonon DOS curves of fcc Cu, L12 Cu3Au, and fcc Au to match the numbers of first-nearest-neighbor pairs in a disordered alloy. The vibrational entropy obtained with this simulated DOS disagrees with calorimetric data and theoretical estimates, indicating that the phonon DOS of disordered Cu3Au depends on chemical order at spatial lengths larger than is set by first-nearest-neighbor pairs

Phonon entropy of alloying and ordering of Cu-Au

Inelastic neutron scattering spectra were measured with a time-of-flight spectrometer on six disordered Cu-Au alloys at 300 K. The neutron-weighted phonon density of states was obtained from a conventional analysis of these spectra. Several methods were developed to account for this neutron weighting and obtain the phonon entropy of the disordered alloys. The phonon entropies of formation of disordered fcc Cu-Au alloys obtained in this way were generally mutually consistent, and were also consistent with predictions from a cluster approximation obtained from ab-initio calculations by Ozolin[underaccent cedilla [below] s-breve, Wolverton, and Zunger. We estimate a phonon entropy of disordering of 0.15±0.05kB/atom in Cu3Au at 300 K. A resonance mode associated with the motions of the heavy Au atoms in the Cu-rich alloys was observed at 9 meV. An analysis of the resonance mode provided a check on the partial phonon entropy of Au atoms

Temperature dependence of the phonon entropy of vanadium

The phonon density-of-states (DOS) of elemental vanadium was measured at elevated temperatures by inelastic neutron scattering. The phonon softening predicted by thermal expansion against the bulk modulus is much larger than the measured shifts in phonon energies. We conclude that the phonon anharmonicities associated with thermal expansion are largely canceled by effects from phonon-phonon scattering. Prior measurements of the heat capacity and calculations of the electronic entropy of vanadium are assessed, and consistency requires an explicit temperature dependence of the phonon DOS. Using data from the literature, similar results are found for chromium, niobium, titanium, and zirconium

CFD Modelling of Bore Erosion in Two-Stage Light Gas Guns

A well-validated quasi-one-dimensional computational fluid dynamics (CFD) code for the analysis of the internal ballistics of two-stage light gas guns is modified to explicitly calculate the ablation of steel from the gun bore and the incorporation of the ablated wall material into the hydrogen working cas. The modified code is used to model 45 shots made with the NASA Ames 0.5 inch light gas gun over an extremely wide variety of gun operating conditions. Good agreement is found between the experimental and theoretical piston velocities (maximum errors of +/-2% to +/-6%) and maximum powder pressures (maximum errors of +/-10% with good igniters). Overall, the agreement between the experimental and numerically calculated gun erosion values (within a factor of 2) was judged to be reasonably good, considering the complexity of the processes modelled. Experimental muzzle velocities agree very well (maximum errors of 0.5-0.7 km/sec) with theoretical muzzle velocities calculated with loading of the hydrogen gas with the ablated barrel wall material. Comparison of results for pump tube volumes of 100%, 60% and 40% of an initial benchmark value show that, at the higher muzzle velocities, operation at 40% pump tube volume produces much lower hydrogen loading and gun erosion and substantially lower maximum pressures in the gun. Large muzzle velocity gains (2.4-5.4 km/sec) are predicted upon driving the gun harder (that is, upon using, higher powder loads and/or lower hydrogen fill pressures) when hydrogen loading is neglected; much smaller muzzle velocity gains (1.1-2.2 km/sec) are predicted when hydrogen loading is taken into account. These smaller predicted velocity gains agree well with those achieved in practice. CFD snapshots of the hydrogen mass fraction, density and pressure of the in-bore medium are presented for a very erosive shot

Optimization Study of the Ames 0.5 Two-Stage Light Gas Gun

There is a need for more faithful simulation of space debris impacts on various space vehicles. Space debris impact velocities can range up to 14 km/sec and conventional two-stage light gas guns with moderately heavy saboted projectiles are limited to launch velocities of 7-8 km/sec. Any increases obtained in the launch velocities will result in more faithful simulations of debris impacts. It would also be valuable to reduce the maximum gun and projectile base pressures and the gun barrel erosion rate. In this paper, the results of a computational fluid dynamics (CFD) study designed to optimize the performance of the NASA Ames 0.5' gun by systematically varying seven gun operating parameters are reported. Particularly beneficial effects were predicted to occur if (1) the piston mass was decreased together with the powder mass and the hydrogen fill pressure and (2) the pump tube length was decreased. The optimum set of changes in gun operating conditions were predicted to produce an increase in muzzle velocity of 0.7-1.0 km/sec, simultaneously with a substantial decrease in gun erosion. Preliminary experimental data have validated the code predictions. Velocities of up to 8.2 km/sec with a 0.475 cm diameter saboted aluminum sphere have been obtained, along with large reductions in gun erosion rates

Impinging jet separators for liquid metal magnetohydrodynamic power cycles

In many liquid metal MHD power, cycles, it is necessary to separate the phases of a high-speed liquid-gas flow. The usual method is to impinge the jet at a glancing angle against a solid surface. These surface separators achieve good separation of the two phases at a cost of a large velocity loss due to friction at the separator surface. This report deals with attempts to greatly reduce the friction loss by impinging two jets against each other. In the crude impinging jet separators tested to date, friction losses were greatly reduced, but the separation of the two phases was found to be much poorer than that achievable with surface separators. Analyses are presented which show many lines of attack (mainly changes in separator geometry) which should yield much better separation for impinging jet separators)

Meteor Entry Characterization in the Electric Arc Shock Tube

This poster summarizes potential test opportunities in Ames' EAST facility that would benefit studies of meteor and asteroid entry into Earth's atmosphere. The Electric Arc Shock Tube (EAST) facility produces high speed (up to Mach 50) shockwaves at prescribed velocities, densities and atmospheric compositions

Techniques for Surface-Temperature Measurements and Transition Detection on Projectiles at Hypersonic Velocities--Status Report No. 2

The latest developments in a research effort to advance techniques for measuring surface temperatures and heat fluxes and determining transition locations on projectiles in hypersonic free flight in a ballistic range are described. Spherical and hemispherical titanium projectiles were launched at muzzle velocities of 4.6-5.8 km/sec into air and nitrogen at pressures of 95-380 Torr. Hemisphere models with diameters of 2.22 cm had maximum pitch and yaw angles of 5.5-8 degrees and 4.7-7 degrees, depending on whether they were launched using an evacuated launch tube or not. Hemisphere models with diameters of 2.86 cm had maximum pitch and yaw angles of 2.0-2.5 degrees. Three intensified-charge-coupled-device (ICCD) cameras with wavelength sensitivity ranges of 480-870 nm (as well as one infrared camera with a wavelength sensitivity range of 3 to 5 microns), were used to obtain images of the projectiles in flight. Helium plumes were used to remove the radiating gas cap around the projectiles at the locations where ICCD camera images were taken. ICCD and infrared (IR) camera images of titanium hemisphere projectiles at velocities of 4.0-4.4 km/sec are presented as well as preliminary temperature data for these projectiles. Comparisons were made of normalized temperature data for shots at approx.190 Torr in air and nitrogen and with and without the launch tube evacuated. Shots into nitrogen had temperatures ~6% lower than those into air. Evacuation of the launch tube was also found to lower the projectile temperatures by approx.6%

New higher-order Godunov code for modelling performance of two-stage light gas guns

A new quasi-one-dimensional Godunov code for modeling two-stage light gas guns is described. The code is third-order accurate in space and second-order accurate in time. A very accurate Riemann solver is used. Friction and heat transfer to the tube wall for gases and dense media are modeled and a simple nonequilibrium turbulence model is used for gas flows. The code also models gunpowder burn in the first-stage breech. Realistic equations of state (EOS) are used for all media. The code was validated against exact solutions of Riemann's shock-tube problem, impact of dense media slabs at velocities up to 20 km/sec, flow through a supersonic convergent-divergent nozzle and burning of gunpowder in a closed bomb. Excellent validation results were obtained. The code was then used to predict the performance of two light gas guns (1.5 in. and 0.28 in.) in service at the Ames Research Center. The code predictions were compared with measured pressure histories in the powder chamber and pump tube and with measured piston and projectile velocities. Very good agreement between computational fluid dynamics (CFD) predictions and measurements was obtained. Actual powder-burn rates in the gun were found to be considerably higher (60-90 percent) than predicted by the manufacturer and the behavior of the piston upon yielding appears to differ greatly from that suggested by low-strain rate tests

Vibrational and electronic entropy of β-cerium and γ-cerium measured by inelastic neutron scattering

Time-of-flight (TOF) inelastic neutron-scattering spectra were measured on β-cerium (double hcp) and γ-cerium (fcc) near the phase-transition temperature. Phonon densities of states (DOS) and crystal-field levels were extracted from the TOF spectra. A softening of the phonon DOS occurs in the transition from β- to γ-cerium, accounting for an increase in vibrational entropy of ΔSvibγ-β=(0.09±0.05)kB/atom. The entropy calculated from the crystal-field levels and a fit to calorimetry data from the literature were significantly larger in β-cerium than in γ-cerium below room temperature, but the difference was found to be negligible at the experimental phase-transition temperature. A contribution to the specific heat from Kondo spin fluctuations was consistent with the quasielastic magnetic scattering, but the difference between phases was negligible. To be consistent with the latent heat of the β-γ transition, the increase in vibrational entropy at the phase transition may be accompanied by a decrease in electronic entropy not associated with the crystal-field splitting or spin fluctuations. At least three sources of entropy need to be considered for the β-γ transition in cerium