First-principles calculations of phase transition, elasticity, and thermodynamic properties for TiZr alloy

tructural transformation, pressure dependent elasticity behaviors, phonon, and thermodynamic properties of the equiatomic TiZr alloy are investigated by using first-principles density-functional theory. Our calculated lattice parameters and equation of state for $\alpha$ and $\omega$ phases as well as the phase transition sequence of $\alpha$$\mathtt{\rightarrow}$$\omega$$\mathtt{\rightarrow}$$\beta$ are consistent well with experiments. Elastic constants of $\alpha$ and $\omega$ phases indicate that they are mechanically stable. For cubic $\beta$ phase, however, it is mechanically unstable at zero pressure and the critical pressure for its mechanical stability is predicted to equal to 2.19 GPa. We find that the moduli, elastic sound velocities, and Debye temperature all increase with pressure for three phases of TiZr alloy. The relatively large $B/G$ values illustrate that the TiZr alloy is rather ductile and its ductility is more predominant than that of element Zr, especially in $\beta$ phase. Elastic wave velocities and Debye temperature have abrupt increase behaviors upon the $\alpha$$\mathtt{\rightarrow}$$\omega$ transition at around 10 GPa and exhibit abrupt decrease feature upon the $\omega$$\mathtt{\rightarrow}$$\beta$ transition at higher pressure. Through Mulliken population analysis, we illustrate that the increase of the \emph{d}-band occupancy will stabilize the cubic $\beta$ phase. Phonon dispersions for three phases of TiZr alloy are firstly presented and the $\beta$ phase phonons clearly indicate its dynamically unstable nature under ambient condition. Thermodynamics of Gibbs free energy, entropy, and heat capacity are obtained by quasiharmonic approximation and Debye model.Comment: 9 pages, 10 figure

Constraining the Skyrme effective interactions and the neutron skin thickness of nuclei using isospin diffusion data from heavy ion collisions

Recent analysis of the isospin diffusion data from heavy-ion collisions based on an isospin- and momentum-dependent transport model with in-medium nucleon-nucleon cross sections has led to the extraction of a value of $L=88\pm 25$ MeV for the slope of the nuclear symmetry energy at saturation density. This imposes stringent constraints on both the parameters in the Skyrme effective interactions and the neutron skin thickness of heavy nuclei. Among the 21 sets of Skyrme interactions commonly used in nuclear structure studies, the 4 sets SIV, SV, G$_\sigma$, and R$_\sigma$ are found to give $L$ values that are consistent with the extracted one. Further study on the correlations between the thickness of the neutron skin in finite nuclei and the nuclear matter symmetry energy in the Skyrme Hartree-Fock approach leads to predicted thickness of the neutron skin of $0.22\pm 0.04$ fm for $^{208}$Pb, $0.29\pm 0.04$ fm for $^{132}$Sn, and $0.22\pm 0.04$ fm for $^{124}$Sn.Comment: 10 pages, 4 figures, 1 Table, Talk given at 1) International Conference on Nuclear Structure Physics, Shanghai, 12-17 June, 2006; 2) 11th China National Nuclear Structure Physics Conference, Changchun, Jilin, 13-18 July, 200

An immersed boundary method for the fluid--structure--thermal interaction in rarefied gas flow

An immersed boundary method for the fluid--structure--thermal interaction in rarefied gas flow is presented. In this method, the slip model is incorporated with the penalty immersed boundary method to address the velocity and temperature jump conditions at the fluid--structure interface in rarefied gas flow within slip regime. In this method, the compressible flow governed by Navier-Stokes equations are solved by using high-order finite difference method; the elastic solid is solved by using finite element method; the fluid and solid are solved independently and the fluid--structure--thermal interaction are achieved by using a penalty method in a partitioned way. Several validations are conducted including Poiseuille flow in a 2D pipe, flow around a 2D NACA airfoil, moving square cylinder in a 2D pipe, flow around a sphere and moving sphere in quiescent flow. The numerical results from present method show good agreement with the previous published data obtained by other methods, and it confirms the the good ability of the proposed method in handling fluid--structure--thermal interaction for both weakly compressible and highly compressible rarefied gas flow. To overcome the incapability of Navier-Stokes equations at high local Knudsen numbers in supersonic flow, an artificial viscosity is introduced to ease the sharp transition at the shock wave front. Inspired by Martian exploration, the application of proposed method to study the aerodynamics of flapping wing in rarefied gas flow is conducted in both 2D and 3D domains, to obtain some insights for the flapping-wing aerial vehicles operating in Martian environment

Optimal View Angle in Collective Dynamics of Self-propelled Agents

We study a system of self-propelled agents in which each agent has a part of omnidirectional or panoramic view of its sensor disc, the field of vision of the agent in this case is only a sector of a disc bounded by two radii and the included arc. The inclination of these two radii is characterized as the view angle. Contrary to our intuition, we find that, the non-omnidirectional-view for swarm agents with periodic boundary conditions in noiseless Vicsek model can accelerate the transient process of the emergence of the ordered state. One consequent implication is that, there are generally superfluous communications in the Vicsek Model, which may even obstruct the possible fast swarm emergence. This phenomenon may invoke further efforts and attentions to explore the underlying mechanism of the emergence in self-propelled agents.Comment: 4 pages, 6 figure