Francisco Muñoz

Comparison between computational fluid dynamics, fluid–structure interaction and computational structural dynamics predictions of flow-induced wall mechanics in an anatomically realistic cerebral aneurysm model

Haemodynamically induced stress plays an important role in the progression and rupture of cerebral aneurysms. The current work describes computational fluid dynamics (CFD), fluid–structure interaction (FSI) and computational structural dynamics (CSD) simulations in an anatomically realistic model of a carotid artery with two saccular cerebral aneurysms in the ophthalmic region. The model was obtained from three-dimensional (3D) rotational angiographic imaging data. CFD and FSI were studied under a physiologically representative waveform of inflow. The arterial wall was assumed elastic or hyperelastic, as a 3D solid or as a shell depending on the type of modelling used. The flow was assumed to be laminar, non-Newtonian and incompressible. The CFD, FSI and CSD models were solved with the finite elements package ADINA. Predictions of velocity field and wall shear stress (WSS) on the aneurysms made using CFD and FSI were compared. The CSD model of the aneurysms using complete geometry was compared with isolated aneurysm models. Additionally, the effects of hypertensive pressure on CSD aneurysm models are also reported. The vortex structure, WSS, effective stress, strain and displacement of the aneurysm walls showed differences, depending on the type of modelling used.

Au13-nAgn clusters: a remarkably simple trend.

The planar to three dimensional transition of Au13-nAgn clusters is investigated. To do so the low lying energy configurations for all possible concentrations (n values) are evaluated. Many thousands of possible conformations are examined. They are generated using the procedure developed by Rogan et al. in combination with the semi-empirical Gupta potential. A large fraction of these (the low lying energy ones) are minimized by means of Density Functional Theory (DFT) calculations. We employ the Tao, Perdew, Staroverov, and Scuseria (TPSS) meta-GGA functional and the Perdew, Burke and Ernzerhof (PBE) GGA functional, and compare their results. The effect of spin-orbit coupling is studied as well as the s-d hybridization. As usual in this context the results are functional-dependent. However, both functionals lead to agreement as far as trends are concerned, yielding just two relevant motifs, but their results differ quantitatively.

Prediction and control of spin polarization in a Weyl semimetallic phase of BiSb

Sobhit Singh, A. C. Garcia-Castro, 3 Irais Valencia-Jaime, 2 Francisco Muñoz, 5 and Aldo H. Romero Department of Physics and Astronomy, West Virginia University, Morgantown, WV-26505-6315, USA Centro de Investigación y Estudios Avanzados del IPN, MX-76230, Querétaro, México Physique Théorique des Matériaux, Université de Liège, B-4000 Sart-Tilman, Belgium Departamento de F́ısica, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile Centro para el Desarrollo de la Nanosciencia y la Nanotechnoloǵıa CEDENNA, Santiago, Chile

A new isomer of C20 and a way to a new C240

Here we show that the dynamic simulation of a molecular collision can give insight into new molecular species. In this way, a new stable isomer of C(20) (IV) has been found. It is planar with pentagonal form. This isomer is high in energy compared to the three most stable previously known isomers of C(20): cage (I), bowl (II) and ring (III). Most interestingly, we show that using this new isomer it is possible to construct a macrobucky C(240) (V) that is also stable. The electronic structure of this new isomer of C(240) is very different from properties of the C(240) fullerene. Contrary to the C(240) fullerene, in the new isomer the π electrons are localized.

Bilayer graphene under pressure: Electron-hole Symmetry Breaking, Valley Hall Effect, and Landau Levels

Financiamiento Basal para Centros Cientificos y Tecnologicos de Excelencia FB 0807 Fondecyt 1150806 American Physical Society ANPCyT PICTs 2013-1045 Bicentenario 2010-1060 CONICET PIP 11220110100832 SeCyT-UNC 06/C415 ICTP associateship program Simons Foundation

Magnetic phases at the molecular scale: the case of cylindrical Co nanoparticles

The magnetic phases of cobalt nanocylinders at the molecular scale have been studied by means of density functional theory together with micromagnetism. Diameters of the objects are under 1 nm. The magnetic phases resulting from first-principle calculations are far from obvious and quite different from both semiclassical results and extrapolations from what is measured for larger objects. These differences reinforce the importance of the quantum mechanical approach for small nanoscopic particles. One of the main results reported here is precisely the unexpected order in the last filled orbitals, which produce objects with alternating magnetic properties as the length of the cylinder increases. The resulting anisotropy is not obvious. The vortex phase is washed out due to the aspect ratio of the systems and the strength of the exchange constants for Co. Nevertheless, we do a pedagogical experiment by turning gradually down the exchange constants to investigate the kind of vortex states which are hidden underneath the ferromagnetic phases.

COMPARISON BETWEEN CFD, CFD-FSI AND SOLID MECHANICS IN PATIENT-SPECIFIC CEREBRAL ANEURYSMS

Cerebral aneurysms are pathological dilatations that frequently occur in the principal arteries of the circle of Willis. They are usually located near bifurcations and on arteries with high curvatures. The stresses induced by the hemodynamics control the aneurysm growth and rupture. The oscillatory and low wall shear stress (WSS) on aneurysm surface changes endothelial cell remodelling and the pulsatile blood pressure produces aneurysm enlargement. To study patient-specific cerebral aneurysms different approach can be taken, as in-vivo, in vitro and numerical simulations in simplified models. Computational Fluid dynamics (CFD) has been performed in 34 models with rigid walls, [Valencia, 2007]. CFD including fluid-structure interaction (FSI) has been reported in [Torii, 2007]. The mechanical study of isolated aneurysm models including hyperelastic and anisotropic wall reported maximum effective stress around 1 MPa, [Ma, 2007], Considering that breaking stress of cerebral aneurysms ranged from 073 to 1.9 MPa has been measured by [MacDonald, 2000], can be concluded the high relevance of the mechanical load in the aneurysm rupture risk. In this work, we compare results obtained with CFD with CFD-FSI; mechanical models that include aneurysm and artery with isolated aneurysm models. In the mechanical models results with elastic and hyperelastic artery walls are also reported.