Emmanuel N. Millán

Nucleation of plasticity in nanoparticle collisions.

While at small collision velocities collisions of nanoparticles (NPs) are elastic, they become plastic at higher velocities. We study the elastic-plastic threshold and the onset of plasticity using molecular dynamics simulation for a Lennard-Jones material. The reasons behind the R^{-2/3} increase of the threshold velocity for small NP radii R found recently are discussed. At the threshold, NP orientation strongly influences the generation of plasticity, and averaging over many orientations is required to predict the critical velocity for dislocation generation. The onset of plasticity is governed by the generation of isolated stacking faults and nanotwins spanning the entire NP. At higher velocities, the fraction of defects becomes proportional to the total number of atoms in the NP.

Performance analysis of Cellular Automata HPC implementations

We present a free open source code for Cellular Automata (CA) using MPI.Weak and strong scaling tests are carried out in 3 different architectures.Performance of our code compares well with performance of other mature HPC codes.Hardware counters are used to help identifying performance issues. Display Omitted Cellular Automata (CA) are of interest in several research areas and there are many available serial implementations of CA. However, there are relatively few studies analyzing in detail High Performance Computing (HPC) implementations of CA which allow research on large systems. Here, we present a parallel implementation of a CA with distributed memory based on MPI. As a first step to insure fast performance, we study several possible serial implementations of the CA. The simulations are performed in three infrastructures, comparing two different microarchitectures. The parallel code is tested with both Strong and Weak scaling, and we obtain parallel efficiencies of ~ 75%-85%, for 64 cores, comparable to efficiencies for other mature parallel codes in similar architectures. We report communication time and multiple hardware counters, which reveal that performance losses are related to cache references with misses, branches and memory access.

Dust-aggregate impact into granular matter: A systematic study of the influence of projectile velocity and size on crater formation and grain ejection

Context. Dust impact into granular materials leads to crater formation and material ejection. Aims. The impact of dust aggregates, composed of a number N p of grains, into a granular bed consisting of the same grains is studied as a function of impact velocity v and projectile size N p . No gravitational effects are included. Methods. Granular-mechanics simulations are used to study the outcome of dust-aggregate impacts. The granular bed and the aggregates are composed of silica grains and have filling factor 0.36. Results. Both the crater volume and the ejection yield increase sublinearly with total impact energy. No crater rims are formed. Crater shapes change from hemispheric to elongated when increasing either projectile size or velocity. The crater walls are compacted by the impact within a zone of a size comparable to the crater radius. Ejecta are produced at the edges of the impact; only a small fraction of the ejecta stem from the projectile. The energy distribution of the ejecta follows at high energies a 1/ E 2 decay reminiscent of sputtering from atomic targets. The maximum of the distribution is shifted to higher energies for larger projectiles; this is caused by the increasing depth from which ejected grains originate. Conclusions. Due to the dissipative nature of intergranular collisions and the porosity of the target, crater morphology and ejecta yield deviate characteristically from impacts into atomic materials.