Nicholas Hawker

Modeling asymmetric cavity collapse with plasma equations of state

We explore the effect that equation of state (EOS) thermodynamics has on shock-driven cavity-collapse processes. We account for full, multidimensional, unsteady hydrodynamics and incorporate a range of relevant EOSs (polytropic, QEOS-type, and SESAME). In doing so, we show that simplified analytic EOSs, like ideal gas, capture certain critical parameters of the collapse such as velocity of the main transverse jet and pressure at jet strike, while also providing a good representation of overall trends. However, more sophisticated EOSs yield different and more relevant estimates of temperature and density, especially for higher incident shock strengths. We model incident shocks ranging from 0.1 to 1000 GPa, the latter being of interest in investigating the warm dense matter regime for which experimental and theoretical EOS data are difficult to obtain. At certain shock strengths, there is a factor of two difference in predicted density between QEOS-type and SESAME EOS, indicating cavity collapse as an experimental method for exploring EOS in this range.

Experimental characterisation of light emission during shock-driven cavity collapse

The authors describe experimental work examining the collapse of a cavity by a strong shockwave. A millimeter size cavity is cast in Phytagel, which is then impacted by a metallic projectile accelerated by a compressed gas gun, reaching velocities up to 500 m/s. The impact generates a strong shockwave that propagates into the gel at greater than sonic velocity. Schlieren images are presented that illustrate both this process and the subsequent cavity collapse at a sub-microsecond timescale. As the shockwave reaches the cavity, it is shown to cause a rapid asymmetric collapse process characterized by the formation of a high-speed transverse jet. The pressure of the shockwave is found to be 100+ MPa as measured via a custom-built fiber-optic probe hydrophone. Previous work examining shock-driven cavity collapse observed luminescence, postulated to be due to the high-speed impact of the transverse jet on the far bubble wall; this experimental observation is replicated. Further, the light emission is characte...

Characterizing shock waves in hydrogel using high speed imaging and a fiber-optic probe hydrophone

The impact of a stainless steel disk-shaped projectile launched by a single-stage light gas gun is used to generate planar shock waves with amplitudes on the order of 102MPa in a hydrogel target material. These shock waves are characterized using ultra-high-speed imaging as well as a fiber-optic probe hydrophone. Although the hydrogel equation of state (EOS) is unknown, the combination of these measurements with conservation of mass and momentum allows us to calculate pressure. It is also shown that although the hydrogel behaves similarly to water, the use of a water EOS underpredicts pressure amplitudes in the hydrogel by ∼10% at the shock front. Further, the water EOS predicts pressures approximately 2% higher than those determined by conservation laws for a given value of the shock velocity. Shot to shot repeatability is controlled to within 10%, with the shock speed and pressure increasing as a function of the velocity of the projectile at impact. Thus the projectile velocity may be used as an adequat...

Simulation of warm dense matter in intense bubble collapse

Previous work by the authors includes computational study of shock-bubble interaction. This work demonstrated strong compression and heating within the bubble, with gas reaching densities of order of magnitude 1 g/cc and temperatures of 10 eV. These conditions correspond to the warm dense matter regime. This paper addresses limitations of previous work through utilisation of various equations of state (EOS) appropriate for the modelling of dense plasma. This is achieved through the design and implementation of a generic interface based on tabulated EOS data. Any EOS may be utilised through the framework, requiring only knowledge of pressure and energy as functions of density and temperature. The solutions to various issues such as table interpolation, tabulated change of variables, arbitrary calculation of entropy and calculation of thermodynamic derivatives are presented. In addition, the trade-offs between CPU time, memory requirement and computational accuracy are discussed. Validation work is presente...

Single and multiple bubble shock‐induced collapse: A computational study.

We present computational simulations of shock‐bubble interaction, leading to violent bubble collapse. Two‐dimensional, axisymmetric and 3‐D simulations of single and multiple bubbles, at various arrangements, are discussed. A front‐tracking methodology (FronTier) is used to model accurately the liquid‐gas surface, together with shock‐capturing compressible methodologies for computing the shock propagation. A wide range of shock intensities is simulated (from 1 GPa, up to 100 GPa), and pressure, density, and temperature profiles are analyzed. The results obtained reveal a particularly complex behavior, even in the single bubble case, highly nonspherical, with a multitude of shock and contact structures emerging and interacting. Well‐known phenomena, such as jet formation, are reproduced and parameterized. The subnanosecond and near‐nanometer resolutions of the simulations reveal spatially and temporally discrete secondary collapse events and allow for the evaluation of conditions relevant to numerous proce...