F. J. Cherne

Ejecta source model based on the nonlinear Richtmyer-Meshkov instability

We describe a simple algebraic model for the particulate spray that is ejected from a shocked metal surface based on the nonlinear evolution of the Richtmyer-Meshkov instability (RMI). The RMI is a shock-driven hydrodynamic instability at a material interface in which the dense and tenuous fluids penetrate each other as spikes and bubbles, respectively. In our model, the ejecta areal density is determined by the product of the post-shock metal density and the saturated bubble amplitude, which depends on both the amplitude and wavelength of the initial surface imperfections of the metal. The maximum ejecta velocity is determined by the ever-growing spikes, which are accelerated relative to the RMI growth rate by the spatial harmonics that sharpen them. The model is formulated to fit new hydrodynamics and molecular dynamics simulations of the RMI and validated by existing ejecta experiments over a wide range of material properties, shock strengths, and surface perturbations. The results are also contrasted ...

On shock driven jetting of liquid from non-sinusoidal surfaces into a vacuum

Previous work employed Richtmyer-Meshkov theory to describe the development of spikes and bubbles from shocked sinusoidal surfaces. Here, we discuss the effects of machining different two-dimensional shaped grooves in copper and examine the resulting flow of the material after being shocked into liquid on release. For these simulations, a high performance molecular dynamics code, SPaSM, was used with machined grooves of kh0 = 1 and kh0 = 1/8, where 2h0 is the peak-to-valley height of the perturbation with wavelength λ, and k = 2π/λ. The surface morphologies studied include a Chevron, a Fly-Cut, a Square-Wave, and a Gaussian. We describe extensions to an existing ejecta source model that better captures the mass ejected from these surfaces. We also investigate the same profiles at length scales of order 1 cm for an idealized fluid equation of state using the FLASH continuum hydrodynamics code. Our findings indicate that the resulting mass can be scaled by the missing area of a sinusoidal curve with an effe...

Second shock ejecta measurements with an explosively driven two-shockwave drive

We develop and apply an explosively driven two-shockwave tool in material damage experiments on Sn. The two shockwave tool allows the variation of the first shockwave amplitude over range 18.5 to 26.4 GPa, with a time interval variation between the first and second shock of 5 to 7 μs. Simulations imply that the second shock amplitude can be varied as well and we briefly describe how to achieve such a variation. Our interest is to measure ejecta masses from twice shocked metals. In our application of the two-shockwave tool, we observed second shock ejected areal masses of about 4 ± 1 mg/cm2, a value we attribute to unstable Richtmyer-Meshkov impulse phenomena. We also observed an additional mass ejection process caused by the abrupt recompression of the local spallation or cavitation of the twice shocked Sn.

Jet formation in cerium metal to examine material strength

Examining the evolution of material properties at extreme conditions advances our understanding of numerous high-pressure phenomena from natural events like meteorite impacts to general solid mechanics and fluid flow behavior. Recent advances in synchrotron diagnostics coupled with dynamic compression platforms have introduced new possibilities for examining in-situ, spatially resolved material response with nanosecond time resolution. In this work, we examined jet formation from a Richtmyer-Meshkov instability in cerium initially shocked into a transient, high-pressure phase, and then released to a low-pressure, higher-temperature state. Ceriums rich phase diagram allows us to study the yield stress following a shock induced solid-solid phase transition. X-ray imaging was used to obtain images of jet formation and evolution with 2–3 μm spatial resolution. From these images, an analytic method was used to estimate the post-shock yield stress, and these results were compared to continuum calculations that...

The study of high-speed surface dynamics using a pulsed proton beam

We present experimental results supporting physics based ejecta model development, where we assume ejecta form as a special limiting case of a Richtmyer-Meshkov (RM) instability with Atwood number A = -1. We present and use data to test established RM spike and bubble growth rate theory through application of modern laser Doppler velocimetry techniques applied in a novel manner to coincidentally measure bubble and spike velocities from shocked metals. We also explore the link of ejecta formation from a solid material to its plastic flow stress at high-strain rates (

A Source Model for Ejecta

107/s) and high strains (700%).

Explosively driven two-shockwave tools with applications

We present a Richtmyer–Meshkov instability based model for the ejected mass per unit area produced when a shockwave impinges upon a free surface with specified surface roughness amplitude and wavelength. The model is valid for sinusoidal and non-sinusoidal surface profiles and for multiple shocks, and is readily implemented in hydrodynamic computer codes. We compare the model with microscopic and macroscopic direct numerical simulations and with an extensive Sn experimental data set. The model works well for all surface profiles and captures both the time evolution and total mass for all shapes considered.

Dynamic Compression of Iron Single Crystals

We present the development of an explosively driven physics tool to generate two mostly uniaxial shockwaves. The tool is being used to extend single shockwave ejecta models to account for a second shockwave a few microseconds later. We explore techniques to vary the amplitude of both the first and second shockwaves, and we apply the tool experimentally at the Los Alamos National Laboratory Proton Radiography (pRad)facility. The tools have been applied to Sn with perturbations of wavelength λ = 550 μm, and various amplitudes that give wavenumber amplitude products of kh {3/4,1/2,1/4,1/8}, where h is the perturbation amplitude, and k = 2π/λ is the wavenumber. The pRad data suggest the development of a second shock ejecta model based on unstable Richtmyer-Meshkov physics.

Richtmyer-Meshkov instability examinedwith large-scalemolecular dynamics simulations

Iron undergoes a polymorphic phase transformation from alpha phase (bcc) to the epsilon phase (hcp) when compressed to stresses exceeding 130 kbar. Because the epsilon phase is denser than the alpha phase, a single shock wave is unstable and breaks up into an elastic wave, a plastic wave, and a phase transition wave known as the P2 wave. Although there exists a large body of continuum data related to the phase transition of shocked polycrystalline iron, data for single crystal iron is lacking. Such data are required for a more complete understanding of the response of iron subjected to dynamic loading conditions. The objective of the current work was to obtain wave profiles for iron single crystals, oriented along the [100], [110], and [210] directions, subjected to quasi‐isentropic loading using the Sandia Z‐machine and shock wave loading through plate impact (LA‐UR‐05‐6624).

Measurement of the sound velocities behind the shock wave front in tin

We have performed a series of large-scale classical molecular dynamics simulations with nearly 54 million atoms to examine the development of the Richtmyer-Meshkov (RM) instability. The calculations utilize an embedded atom method potential for copper, and were performed at shock pressures between 82 GPa and 401 GPa, which is both above and below the melt transition. A sinusoidal profile with a 257 nm wavelength and varying amplitudes was created on the free surface to study how the spikes and the bubbles grow as a function of amplitude and shock strength. For conditions where the copper is melted, we observe the growth of the RM instability into bubbles and spikes similar to fluid simulations. At conditions below the melt transition, certain amplitudes showed a series of accelerations/decelerations in the growth of the spike until a complete arrest of the spike growth occurred due to the underlying strength of the material.