We report an experimental and numerical investigation of the fragmentation mechanisms of micrometer-sized metal droplet irradiated by ultrashort laser pulses. The results of the experiment show that the fast one-side heating of such a droplet may lead to either symmetric or asymmetric expansion followed by different fragmentation scenarios. To unveil the underlying processes leading to fragmentation we perform simulation of liquid-tin droplet expansion produced by the initial conditions similar to those in experiment using the smoothed particle hydrodynamics (SPH) method. Simulation demonstrates that a thin heated surface layer generates a ultrashort shock wave propagating from the frontal side to rear side of the droplet. Convergence of such shock wave followed by a rarefaction tale to the droplet center results in the cavitation of material inside the central region by the strong tensile stress. Reflection of the shock wave from the rear side of droplet produces another region of highly stretched material where the spallation may occur producing a thin spallation layer moving with a velocity higher than expansion of the central shell after cavitation. It is shown both experimentally and numerically that the threshold laser intensity necessary for the spallation is higher than the threshold required to induce cavitation in the central region of droplet. Thus, the regime of asymmetrical expansion is realized if the laser intensity exceeds the spallation threshold. The transverse and longitudinal expansion velocities obtained in SPH simulations of different regimes of expansion are agreed well with our experimental data. * grigorev@phystech.edu †

The benchmark exercise discussed in this paper was conducted within the OECD/NEA project HYMERES. The specific experiment in the PANDA facility chosen for the present benchmark addresses the stratification erosion induced by a vertical steam jet, which originates from the exit of a circular pipe located below the bottom of the helium-rich layer. The mixing is somewhat slowed down by a small circular plate above the jet source. The exercise consisted of a blind phase, and an open phase. Two sets of blind simulations were requested: one set obtained using a "common model", and a second set produced by a "best estimate" model. For the "common model", a list of recommendations was given, whereas for the "best estimate" model, each participant was free to choose the modelling approach. The submitted results for the erosion times were in a large band, and especially the large differences in the results with the "common model" were not expected. The results of the best estimate simulations showed that the combination of mesh and modelling approach can lead to a wide spread of results. The most important difficulty in interpreting the results and finding the reason of the large deviations was the lack of information on the velocity field downstream of the obstruction. Therefore, for the open phase extended data from auxiliary, "zero" tests (for similar conditions but without helium layer) were provided to the participants to permit a more basic validation of their models, using a "multi-step approach". The step-by-step validation permitted some progress with respect to some of the items identified in the blind benchmark.However, large discrepancies with data in the final analyses of the test are observed, which cannot be easily attributed to specific model deficiencies or insufficient detail of the mesh. These results raised some questions in relation to best practice guidelines for the use of CFD codes for containment analysis and indicated needs for further CFD-grade experiments.

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Ceramic materials have a long-term industrial demand due to their high mechanical hardness and chemical and temperature resistance. They are brittle and tend to lose strength under heavy loads which complicates the development of a comprehensive material model for simulation of engineering prototypes containing ceramic parts. We developed an improved failure model of ceramics based on the well-known Johnson–Holmquist approach. This model redefines the damage rate equation using a consistent definition of the total plastic strain in the failed material. It reduces the number of free model parameters and enables the plastic strain to be explicitly accumulated during the failure process. The corresponding non-iterative algorithm utilizing this explicit failure model is developed. It is successfully validated by simulation of the wave profiles obtained in plate-impact experiments with boron carbide using the contact smoothed particle hydrodynamic method.