Currently there is a substantial lack of data for interactions of shock waves with particle fields having volume fractions residing between the dilute and granular regimes. To close this gap, a novel multiphase shock tube has been constructed to drive a planar shock wave into a dense gas-solid field of particles. A nearly spatially isotropic field of particles is generated in the test section by a gravity-fed method that results in a spanwise curtain of spherical 100-micron particles having a volume fraction of about 20%. Interactions with incident shock Mach numbers of 1.66, 1.92, and 2.02 are reported. High-speed schlieren imaging simultaneous with high-frequency wall pressure measurements are used to reveal the complex wave structure associated with the interaction. Following incident shock impingement, transmitted and reflected shocks are observed, which lead to differences in particle drag across the streamwise dimension of the curtain. Shortly thereafter, the particle field begins to propagate downstream and spread. For all three Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced about 30-40% by the presence of the particle field.

An experimental apparatus has been developed to determine thermal accommodation coefficients for a variety of gas-surface combinations. Results are obtained primarily through measurement of the pressure dependence of the conductive heat flux between parallel plates separated by a gas-filled gap. Measured heat-flux data are used in a formula based on Direct Simulation Monte Carlo (DSMC) simulations to determine the coefficients. The assembly also features a complementary capability for measuring the variation in gas density between the plates using electron-beam fluorescence. Surface materials examined include 304 stainless steel, gold, aluminum, platinum, silicon, silicon nitride, and polysilicon. Effects of gas composition, surface roughness, and surface contamination have been investigated with this system; the behavior of gas mixtures has also been explored. Without special cleaning procedures, thermal accommodation coefficients for most materials and surface finishes were determined to be near 0.95, 0.85, and 0.45 for argon, nitrogen, and helium, respectively. Surface cleaning by in situ argon-plasma treatment reduced coefficient values by up to 0.10 for helium and by ∼0.05 for nitrogen and argon. Results for both single-species and gas-mixture experiments compare favorably to DSMC simulations.

The mesoscopic scale response of low-density pressings of granular sugar (sucrose) to shock loading has been examined in gas-gun impact experiments using both VISAR and a line-imaging, optically recording velocity interferometer system in combination with large-volume-element, high-resolution, three-dimensional numerical simulations of these tests. Time-resolved and spatially resolved measurements of material motion in waves transmitted by these pressings have been made as a function of impact velocity, sample thickness, and sample particle size distribution. Observed wave profiles exhibit a precursor regime arising from elastic stress wave propagation and a dispersive compaction wave with superimposed localized particle velocity fluctuations of varying amplitude. Material motion associated with dynamic stress bridging leads compaction wave arrival by ∼2μs at the lowest impact velocity (0.25kms−1) employed in this study and <200ns at the higher values (0.7–0.8kms−1). Over the same range, the compaction wave becomes markedly less dispersive with wave ramp durations declining from ∼500to∼50ns. For impact velocities near 0.5kms−1 and samples varying in thickness from 2.27to8.03mm, a roughly steady wave behavior is obtained at the thinner end of the range; however, evidence of subtle wave evolution is apparent over this thickness range. Pressings of sieved sugar with different particle size distributions exhibit distinguishable differences in stress bridging and compaction wave behavior. These pressings somewhat limit stochastic behavior and provide favorable conditions for the development of quasiperiodic fluctuations in particle velocity, particularly in impacts generating incomplete compaction. The experimental results are consistent with the exceedingly complex wave field behavior evident in the numerical simulations and provide useful benchmark wave profiles (at the sample boundary) for validation of material models used in these calculations.

A previously-developed experimental facility has been used to determine gas-surface thermal accommodation coefficients from the pressure dependence of the heat flux between parallel plates of similar material but different surface finish. Heat flux between the plates is inferred from measurements of temperature drop between the plate surface and an adjacent temperaturecontrolled water bath. Thermal accommodation measurements were determined from the pressure dependence of the heat flux for a fixed plate separation. Measurements of argon and nitrogen in contact with standard machined (lathed) or polished 304 stainless steel plates are indistinguishable within experimental uncertainty. Thus, the accommodation coefficient of 304 stainless steel with nitrogen and argon is estimated to be 0.80 and 0.87 , respectively, independent of the surface roughness within the range likely to be encountered in engineering practice. Measurements of the accommodation of helium showed a slight variation with 304 stainless steel surface roughness: 0.36 for a standard machine finish and 0.40 for a polished finish. Planned tests with carbon-nanotube-coated plates will be performed when 304 stainless-steel blanks have been successfully coated.0.02 ± 0.02 ± 0.02 ± 0.02 ± 4

No abstract

No abstract

In order to provide real-time data for validation of three dimensional numerical simulations of heterogeneous materials subjected to impact loading, an optically recording velocity interferometer system (ORVIS) has been adapted to a line-imaging instrument capable of generating precise mesoscopic scale measurements of spatially resolved velocity variations during dynamic deformation. Combining independently variable target magnification and interferometer fringe spacing, this instrument can probe a velocity field along line segments up to 15 mm in length. In high magnification operation, spatial resolution better than 10 µm can be achieved. For events appropriate to short recording times, streak camera recording can provide temporal resolution better than 0.2 ns. A robust method for extracting spatially resolved velocity-time profiles from streak camera image data has been developed and incorporated into a computer program that utilizes a standard VISAR analysis platform. The use of line-imaging ORVIS to obtain measurements of the mesoscopic scale dynamic response of shocked samples has been demonstrated on several different classes of heterogeneous materials. Studies have focused on pressed, granular sugar as a simulant material for the widely used explosive HMX. For low-density (65 percent theoretical maximum density) pressings of sugar, material response has been investigated as a function of both impact velocity and changes in particle size distribution. The experimental results provide a consistent picture of the dispersive nature of the wave transmitted through these samples and reveal both transverse and longitudinal wave structures on mesoscopic scales. This observed behavior is consistent with the highly structured mesoscopic response predicted by 3-D simulations. Preliminary line-imaging ORVIS measurements on HMX as well as other heterogeneous materials such as foam and glassreinforced polyester are also discussed.4

In order to provide detailed response data for validation of three dimensional numerical simulations of heterogeneous materials subjected to impact loading, an optically recording velocity interferometer system (ORVIS) has been adapted to a lime-imaging instrument capable of generating precise mesoscopic scale measurements of spatially resolved velocity variations during dynamic deformation. While the use of this diagnostic has been demonstrated on several different classes of heterogeneous materials, studies have focused on pressed granular sugar as a simulant material for the widely used explosive HMX. Tests on this material address mesoscopic scale thermomechanical effects in the absence of complications due to rapid reactions. For low-density (65 percent theoretical maximum density) pressings of sugar, material response has been investigated as a fimction of both impact velocity and changes in particle size distribution. The amplitude and Iiequency of observed transverse and longitudinal wave structures are generally consistent with the highly structured mesoscopic response seen in the 3-D simulations. In parallel with instrumental developments, several approaches for detailed analysis of the spatially resolved velocity-time data are being evaluated. The utility of Fast Fourier Transform (ITT) filtering for reduction of certain types of ORVIS image data has been demonstrated.