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

An Idea on Normal Flow Discharge of a Small Sized River Draining a Low Laying Area - In Case of the Shinbori River, A Tributary of the Nagara River in Gifu City

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Two-dimensional hydrodynamic simulation of an accretion flow with radiative cooling in a close binary system

Two-dimensional numerical simulations of an accretion flow in a close binary system are performed by solving the Euler equations with radiative transfer. In the present study, the specific heat ratio is assumed to be constant while radiative cooling effect is included as a non-adiabatic process. The cooling effect of the disc is considered by discharging energy in the vertical directions from the top and bottom surfaces of the disc. We use the flux-limited diffusion approximation to calculate the radiative heat flux values. Our calculations show that a disc structure appears and the spiral shocks are formed on the disc. These features are similar to that observed in the case of an adiabatic gas with a lower specific heat ratio, $\gamma=1.01$. It is found that when radiative cooling effect is accounted for, the mass of the disc becomes larger than that assuming $\gamma=5/3$, and smaller than that assuming $\gamma=1.01$. It is concluded that employing an adiabatic gas with a lower specific heat ratio is almost a valid assumption for simulating accretion disc with radiative cooling effect.Comment: 9 pages, 5 figures. Accepted for NMRA

Three-Dimensional Simulations of Standing Accretion Shock Instability in Core-Collapse Supernovae

We have studied non-axisymmetric standing accretion shock instability, or SASI, by 3D hydrodynamical simulations. This is an extention of our previous study on axisymmetric SASI. We have prepared a spherically symmetric and steady accretion flow through a standing shock wave onto a proto-neutron star, taking into account a realistic equation of state and neutrino heating and cooling. This unperturbed model is supposed to represent approximately the typical post-bounce phase of core-collapse supernovae. We then have added a small perturbation (~1%) to the radial velocity and computed the ensuing evolutions. Not only axisymmetric but non-axisymmetric perturbations have been also imposed. We have applied mode analysis to the non-spherical deformation of the shock surface, using the spherical harmonics. We have found that (1) the growth rates of SASI are degenerate with respect to the azimuthal index m of the spherical harmonics Y_l^m, just as expected for a spherically symmetric background, (2) nonlinear mode couplings produce only m=0 modes for the axisymmetric perturbations, whereas m=!0 modes are also generated in the non-axisymmetric cases according to the selection rule for the quadratic couplings, (3) the nonlinear saturation level of each mode is lower in general for 3D than for 2D because a larger number of modes are contributing to turbulence in 3D, (4) low l modes are dominant in the nonlinear phase, (5) the equi-partition is nearly established among different m modes in the nonlinear phase, (6) the spectra with respect to l obey power laws with a slope slightly steeper for 3D, and (7) although these features are common to the models with and without a shock revival at the end of simulation, the dominance of low l modes is more remarkable in the models with a shock revival.Comment: 37 pages, 16 figures, and 1 table, submitted to Ap

Noble gases and K-Ar ages of five Rumuruti chondrites Yamato(Y)-75302, Y-791827, Y-793575, Y-82002, and Asuka-881988

Noble gases and K concentrations have been determined for aliquot samples prepared from the five Antarctic R-chondrites Yamato (Y)-75302,Y-791827,Y-793575,Y-82002,and Asuka (A)-881988. K-Ar ages of about 4.2Ga were obtained for Y-75302,Y-791827,Y-793575 and A-881988,while Y-82002 showed a slightly younger age of 3.9Ga. The Y-75302,Y-791827 and Y-82002 are enriched in solar light noble gases. Four meteorites Y-75302,Y-791827,Y-82002 and A-881988 have cosmic-ray exposure ages of about 20Ma, while the age of Y-793575 is about 7Ma. Based on the noble gas compositions and K-Ar ages, Y-75302 and Y-791827 are probably paired and Y-82002 may belong to this pair. Relatively high and variable ^Xe/^Xe ratios between 2-3.7 as well as enrichments of heavier Xe isotopes were observed in all R-chondrites

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