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Nguyen, J. H.

Publications and source records attributed to Nguyen, J. H..

4 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Suppression of Richtmyer-Meshkov Instability via Special Pairs of Shocks and Phase Transitions

The classical Richtmyer-Meshkov instability (RMI) is a hydrodynamic instability characterizing the evolution of an interface following shock loading. In contrast to other hydrodynamic instabilities such as Rayleigh-Taylor, it is known for being unconditionally unstable: regardless of the direction of shock passage, any deviations from a flat interface will be amplified. In this article, we show that for negative Atwood numbers, there exist special sequences of shocks which result in a nearly perfectly suppressed instability growth. Here, we demonstrate this principle computationally and experimentally with stepped fliers and phase transition materials. A fascinating immediate corollary is that in specific instances, a phase-transitioning material may self-suppress RMI.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Bounds on the Rate-Dependent Plastic Flow of Tantalum up to 75 GPa

In this study we report on the continued development of thermodynamics-based analysis of shock waves propagation with the objective of extracting information related to materials strength at high strain rates and pressures. Building on previous results reported for peak stresses of 10 GPa and 25 GPa, we present a series of three-step gas-gun shock experiments designed to explore the pressure and strain rate dependence of plastic flow in polycrystalline tantalum. These experiments at nominal peak stresses of 50 GPa and 75 GPa show the irreversible deformation before pullback to be almost entirely confined to the shock loading, with negligible plastic relaxation on the post-shock plateau. We also add a reverse-ballistics shot at 25 GPa, which was designed to reveal the pullback response with negligible interference from free-surface effects. General thermodynamic considerations allow us to place bounds on the plastic behavior even for parts of the curve that change far too rapidly for the velocimetric time resolution of (conservatively) ~ 5 ns. To analyze the data, we found it necessary to substantially improve the interpolation/extrapolation scheme in order to improve its robustness, flexibility and range of applicability. We describe the new scheme based on splines, as well as an extension of free-surface corrections to the post-shock rarefaction waves. Reanalysis with the new scheme produces results essentially within the error bars previously reported, showing that the known systematic errors associated with free-surface effects are relatively inconsequential for determining thermodynamic paths.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Stastically determined experimental uncertainties in photon Doppler velocimetry (PDV) measurements

Shock-ramp experiments place high demands on the precision, accuracy, and temporal resolution of transformed velocimetry signals. To minimize uncertainty after total propagation of a shockramp experiment, ideally multiple PDV traces are used. However, each trace has some uncertainty that is a function of the PDV system and analysis of the velocimetry data. In order to determine a reasonable experimental estimate of minimum uncertainty, we calculate observed uncertainties in as-fielded PDV systems using multiple channels to sample the velocity on each shot. We also calculate the uncertainty’s sensitivity to choice of analysis methods.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗