Modern parallel architectures require applications to generate massive parallelism so as to feed their large number of cores and their wide vector units. We revisit the extensively studied classical Molecular Dynamics N-body problem in the light of these hardware constraints. We use Adaptive Mesh Refinement techniques to store particles in memory, and to optimize the force computation loop using multi-threading and vectorization-friendly data structures. Our design is guided by the need for load balancing and adaptivity raised by highly dynamic particle sets, as typically observed in simulations of strong shocks resulting in material micro-jetting. We analyze performance results on several simulation scenarios, over nodes equipped by Intel Xeon Phi Knights Landing (KNL) or Intel Xeon Skylake (SKL) processors. Performance obtained with our OpenMP implementation outperforms state-of-the-art implementations (LAMMPS) on both steady and micro-jetting particles simulations. In the latter case, our implementation is 4.7 times faster on KNL, and 2 times faster on SKL.

We perform very large scale molecular dynamics (MD) simulations to investigate the ejection process from shock-loaded tin surfaces in regimes where the metal first undergoes solid to solid phase transitions and then melts on release. In these conditions, a classical two-wave structure propagates within the metal. When it interacts with the surface, our MD simulations reveal very different behaviors. If the surface geometry is perfectly flat or contains almost flat perturbations (sinusoidal type), a solid cap made of crystallites forms at the free surface, over a thickness of a few tens of nanometers. This surface cap melts more slowly than the bulk, and as a result, the ejection process is greatly slowed down. If the surface geometry contains V-shape geometrical perturbations, the oblique interaction of the incident shock wave with the planar interface of the defect leads to a sharp increase of temperature at the defect's bottom. At this place, the metal undergoes a solid to liquid phase change over the entire length of the groove, and this promotes the ejection of matter in the form of sheets of liquid metal. However, this phase change is not spatially uniform, and the sheets keep in memory this process by exhibiting a non-uniform leading edge and large ripples. These ripples grow over time, which ends up causing the fragmentation of the sheets as they develop. In this case, the fragmentation is non-uniform, and it differs from the rather uniform fragmentation process observed when the metal directly melts upon receiving the shock.

The high velocity impact of a drop on a surface causes the formation and, afterward, the breakup of a jet. The understanding of the jet breakup requires at first a detailed description of the processes preceding its formation and the determination of its initial characteristics. In this paper, we first describe the initial impact processes, revisiting the corresponding 2D shock theory; the resulting model is general and independent of the choice of the equation of state. In particular, we define in a simple way the criteria for the existence of a shock or a jet solution. The theoretical predictions are then compared with a numerical simulation using liquid tin as the material for the drop and a rigid material for the target. The theoretical shock solution fits very well the results obtained with an Eulerian hydrocode. Molecular dynamics simulations were used to simulate the growth and breakup of the jet. The theoretical jet orientation and velocity are consistent with the simulation. Finally, the rather complex velocity profile of the jet is associated with the specific phenomena related to the history of the drop.

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