Jaime N. Castaneda

Investigation of the mesoscopic scale response of low-density pressings of granular sugar under impact

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 wa...

An Experimental Assembly for Precise Measurement of Thermal Accommodation Coefficients.

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.

Measurements of thermal accommodation coefficients.

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 temperature-controlled 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 {+-} 0.02 and 0.87 {+-} 0.02, 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 {+-} 0.02 for a standard machine finish and 0.40 {+-} 0.02 for a polished finish. Planned tests with carbon-nanotube-coated plates will be performed when 304 stainless-steel blanks have been successfully coated.

Measurement of Gas‐Surface Accommodation

Thermal accommodation coefficients have been determined for a variety of gas‐surface combinations using an experimental apparatus developed to measure both the pressure dependence of the conductive heat flux and the variation of gas density between parallel plates separated by a gas‐filled gap. Effects of gas composition, surface roughness and surface contamination have been examined with this system, and the behavior of gas mixtures has also been explored. Results are discussed in comparison with previous parallel‐plate experimental studies as well as numerical simulations.

Investigation of Dispersive Waves in Low‐Density Sugar and HMX Using Line‐Imaging Velocity Interferometry

A line‐imaging optically recording velocity interferometer system (ORVIS) has been used in gas‐gun impact experiments to compare the mesoscopic scale response of low‐density (65% theoretical maximum density) pressings of the explosive HMX to that of an inert simulant (granulated sugar). Dispersive waves transmitted through 2.27‐ to 6.16‐mm‐thick beds of the porous sugar typically include mesoscale fluctuations that occur on length scales consistent with those seen in 3‐D numerical simulations. Conditions that approximate steady wave behavior occur at a sample thickness ⩾4 mm. Transmitted wave profiles in HMX include complex effects of chemical reaction. For coarse‐grain HMX samples, reaction expands vigorously over a narrow range (0.4–0.47 km‐s−1) of impact velocity. Localized regions of reaction growth are evident in the spatially resolved velocity‐time data.

Interaction of a planar shock with a dense field of particles.

Understanding the particle-particle and shock-particle interactions that occur in dense gas-solid flows is limited by a lack of knowledge of the underlying phenomena. Gas-solid flows are characterized by the particle volume fraction φp of the flow [1]. For particle volume fractions less than about 0.1%, flow is considered dilute and the effects of particle collisions are negligible [2]. For packed particles, where the φp is greater than about 50%, the flow regime is said to be granular. The dilute and granular regimes have been well studied, but conversely, a substantial knowledge gap exists for dense gas-solid flows, which have intermediate particle volume fractions of about 0.1 to 50%. This regime exists at microsecond time scales during blast-induced dispersal of material when the shocked particles are closely spaced. Very little experimental data exist for the interaction of a shock wave with a dense gas-solid field of particles, with rare exceptions such as Rogue et al. [3]. Though they provided useful observations of particle trajectories and pressures following the impingement of a shock on a granular bed of particles, much remains unknown regarding the interactions that are involved in the dense gas-solid flow regime. Without data specifically acquired for such flows, simulations of energetic material detonation during the early-time expansion will continue to suffer from limited physical fidelity. To fill the gap in data for shock-particle interactions with initial volume fractions residing between the dilute and granular limits, a multiphase shock tube was recently constructed [4]. The unique facility uses a gravity-fed seeding method to generate a dense, spatially isotropic field of 100-micron diameter particles into which a planar shock is driven. High-speed schlieren imaging and pressure data are used to provide insight into the flow and particle behavior in the dense gas-solid regime.

Interaction of a Planar Shock with a Dense Field of Particles in a Multiphase Shock Tube

A novel multiphase shock tube has been constructed to test the interaction of a planar shock wave with a dense gas-solid field of particles. The particle field is generated by a gravity-fed method that results in a spanwise curtain of 100-micron particles producing a volume fraction of about 15%. Interactions with incident shock Mach numbers of 1.67 and 1.95 are reported. High-speed schlieren imaging is used to reveal the complex wave structure associated with the interaction. After the impingement of the incident shock, transmitted and reflected shocks are observed, which lead to differences in flow properties across the streamwise dimension of the curtain. Tens of microseconds after the onset of the interaction, the particle field begins to propagate downstream, and disperse. The spread of the particle field, as a function of its position, is seen to be nearly identical for both Mach numbers. Immediately downstream of the curtain, the peak pressures associated with the Mach 1.67 and 1.95 interactions are about 35% and 45% greater than tests without particles, respectively. For both Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced by about 30-40% by the presence of the particle field.

Development of a Multiphase Shock Tube for Energetic Materials Characterization

A novel multiphase shock tube to study particle dynamics in gas-solid flows has been constructed and tested. Currently, there is a gap in data for flows having particle volume fractions between the dusty and granular regimes. The primary purpose of this new facility is to fill that gap by providing high quality data of shock-particle interactions in flows having dense gas particle volume fractions. Towards this end, the facility aims to drive a shock into a spatially isotropic field, or curtain, of particles. Through bench-top experimentation, a method emerged for achieving this challenging task that involves the use of a gravity-fed contoured particle seeder. The seeding method is capable of producing fields of spatially isotropic particles having volume fractions of about 1 to 35%. The use of the seeder in combination with the shock tube allows for the testing of the impingement of a planar shock on a dense field of particles. The first experiments in the multiphase shock tube have been conducted and the facility is now operational.

ADL ORVIS: An air-delay-leg, line-imaging optically recording velocity interferometer system

An interferometry system that enables acquisition of spatially resolved velocity-time profiles with very high velocity sensitivity has been designed and applied to two diverse, instructive experimental problems: (1) measurement of low-amplitude reverberations in laser-driven flyer plates and (2) measurement of ramp-wave profiles in symmetric impact studies of fused silica. The delay leg in this version of a line-imaging optically recording velocity interferometer system (ORVIS) consists of a long air path that includes relay optics to transmit the optical signal through the interferometer cavity. Target image quality from the delay path at the image recombination plane is preserved by means of a compact and flexible optical design utilizing two parabolic reflectors (serving as the relay optics) in a folded path. With an instrument tuned to a velocity per fringe constant of 22.4 m s(-1) fringe(-1), differences of 1-2 m s(-1) across the probe line segment can be readily distinguished. Measurements that capture small spatial variations in flyer velocity are presented and briefly discussed. In the fused silica impact experiments, the ramp-wave profile observed by this air-delay instrument compares favorably to the profile recorded simultaneously by a conventional line-imaging ORVIS.

Joint Temperature and Soot-Volume-Fraction Measurements in Turbulent Meter-Scale Pool Fires

The development of a combined dual-pump coherent anti-Stokes Raman scattering (CARS) and laser-induced incandescence (LII) instrument for the spatially resolved measurement of subgrid-scale temperature/soot data in liquid-fueled pool fires is discussed. Temperature pdfs obtained from the N2 Qbranch CARS signal at the center of a 2-m-diameter toluene/methanol pool fire are summarized. A more detailed discussion of the recent development of a water-jacketed, fiber-optically coupled LII probe for in-fire soot-volume-fraction imaging is presented. Tomographically resolved laser-light-extinction characterization of the soot field in a fuel-rich premixed ethylene-air flame used for calibration of the LII technique is reported, and the performance of the LII-imaging system in the calibration flame is discussed. Twodimensional LII images, which are representative of the spatially resolved, instantaneous soot-volumefraction distributions in a 2-m-diameter toluene/methanol pool fire are provided, and a histogram of the LII signal that is representative of the pdf of the soot-volume-fraction fluctuations at the center of the fire are extracted from these in-fire imaging results. These data demonstrate the potential of the CARS and LII instruments to determine temperature and soot volume fraction in a sooting fire with high temporal and spatial resolution.