A TVD Scheme using Roe's Flux and the Ambient Boundary Condition

Details of a second-order accurate TVD scheme using the Roe's Riemann solver is described for the three-dimensional Euler equation. The differential equations are discretized using a finite volume formulation. The ambient boundary condition proposed by us is also explained

The Osher Upwind Scheme and its Application to Cosmic Gas Dynamics

Details of the second order Osher scheme for the multi-dimensional Euler equation are presented. The adopted method to attain the second order of accuracy differs from the existing second order formulation by Osher. The present method is easy to be implemented and can be applied to other first order upwind schemes. Two types of numerical integration forms are coded. One is written in the integral form (cell method), and the other is the usual finite-difference form. Both forms work well and can capture strong shocks without any auxiliary artificial damping. The integration form strictly satisfies the flux conservation even on geometrical singular coordinate lines, which inevitably appear in three dimensional calculations with bodies embedded. Hydrodynamic calculations of the interaction between a stellar wind and an accretion flow are performed to demonstrate the ability of the present method

Numerical and Experimental Studies on Choked Underexpanded Jets

Axisymmetric underexpanded supersonic jets are investigated numerically and experimentally. A time-dependent technique of solution is applied to solve the Euler equations for a compressible ideal gas. The characteristics of the Mach disk obtained by the numerical calculations are compared with the experiments, and a good agreement is obtained. It is shown that the numerical results are very sensitive to the choice of the boundary conditions imposed on the artificially introduced numerical boundaries. The boundary condition giving the best results is found to be the ambient gas condition. It is shown that the global jet structure with a nearly regular shock pattern, wich is stable and steady itself, is destabilized by the vortex rings (Kelvin-Helmholtz roll-up) on the jet boundary. These vortices produce shocks inside the jet, which are convected downstream with the eddies. This strongly suggests that a time-independent or a time-converged solution cannot be expected without making a suitable time-averaging of the time-dependent solutions

Spin-orbit coupling inactivity of Co2+^{2+} ion in geometrically frustrated magnet GeCo2_2O4_4

We report single-crystal neutron diffraction studies on a spinel antiferromagnet GeCo$_2$O$_4$, which exhibits magnetic order with a trigonal propagation vector and tetragonal lattice expansion ($c/a\simeq1.001$) below $T_{\rm N}=21$ K. For this inconsistency between spin and lattice in symmetry, magnetic Bragg reflections with a tetragonal propagation vector were discovered below $T_{\rm N}$. We discuss spin and orbital states of Co$^{2+}$ ion underlying the new magnetic component.Comment: 3 pages 2 figures, submitted to ICFCM proceeding (Journal of Physics: Conference Series, 2011

Numerical Simulations of Axisymmetric Flows in Astrophysics and their Visualization by a Video Movie

Numerical simulations of axisymmetric flow in four different astrophysical situations are performed, and their results are visualized by using a video movie. The situations considered are : 1) an accretion flow on to a gravitating compact object allowing maximum accrection ; 2) a jet formation in a flow past a gravitating rigid sphere ; 3) an interaction between a supersonic wind from a central object and a uniform supersonic incident flow ; 4) a wind bubble formed by spherical supersonic wind. The video tape of VHS/NTSC format is available from the author