[en] Tensor equations are derived from a nonlinear spinor equation in Minkowski space

[en] Short communication

[en] A general description of some modern numerical techniques for the simulation of astrophysical fluid flow is presented. The methods are introduced with a thorough discussion of the especially simple case of advection. Attention is focused on the piecewise-parabolic method (PPM). A description of the SLIC method for treating multifluid problems is also given. The discussion is illustrated by a number of advection and hydrodynamics test problems. Finally, a study of Kelvin-Helmholtz instability of supersonic jets using PPM with SLIC fluid interfaces is presented

[en] A hydrodynamic model of the vacuum arc thruster and its plume is described. Primarily an effect of the magnetic field on the plume expansion and plasma generation is considered. Two particular examples are investigated, namely the magnetically enhanced co-axial vacuum arc thruster (MVAT) and the vacuum arc thruster with ring electrodes (RVAT). It is found that the magnetic field significantly decreases the plasma plume radial expansion under typical conditions. Predicted plasma density profiles in the plume of the MVAT are compared with experimental profiles, and generally a good agreement is found. In the case of the RVAT the influence of the magnetic field leads to plasma jet deceleration, which explains the non-monotonic dependence of the ion current density, on an axial magnetic field observed experimentally

[en] The described analysis of JACEE events is, of course, extremely rough and based on some arbitrary assumptions. But it demonstrates possibilities offered by observations of such events. At the same time it is evident that a better theoretical basis is needed. While the solution of hydrodynamical equations with initial conditions fixed in the Landau fashion gives the Gausian rapidity distribution with the flow in which approximately v ≅ x/t, in the papers it was proposed to consider the strict solution of the equations with v = x/t. It corresponds to different initial conditions and gives the rapidity plateau. This scaling solution was accepted as a starting point in many subsequent works. However, this solution needs to be properly adjusted to the external vacuum boundary. The problem was investigated and it was shown that the tail regions behave as particle-like formations, or as pistons, their masses changing during the expansion due to exchange of momentum with the central part. This shows how important it is to better understand the interaction of leading fireballs with cental quasiplateau

[en] Burgers close-quote equation, involving very high Reynolds numbers, is numerically solved by using a new approach based on the distributed approximating functional for representing spatial derivatives of the velocity field. For moderately large Reynolds numbers, this simple approach can provide very high accuracy while using a small number of grid points. In the case where the Reynolds number ≥10⁵, the exact solution is not available and a discrepancy exists in the literature. Our results clarify the behavior of the solution under these conditions. copyright 1997 American Institute of Physics

[en] We develop weak shock theory in a form which would allow us to utilize output from a hydrodynamic code (e.g. KOVEC) as either an initial or boundary condition and then follow the wave evolution to much greater distances than the codes themselves can attain

[en] Equations are derived which describe the time-dependence of a viscous, tilted accretion disc subject to external torques. A full hydrodynamical treatment is used and the equations linearized in terms of a small tilt. It is shown that previous attempts at deriving these equations are inadequate due to the inconsistency of some of the assumptions made. The behaviour of a tilted viscous disc is not simple to describe. Viscous forces tend to flatten the disc, but they also produce torques within the disc which induce local precession of disc elements. The main complication is that the horizontal motions induced in the disc by the tilt are resonantly driven so that their magnitude and phase are governed by the viscosity. This leads to the disc being smoothed on a time-scale faster than the usual viscous one. In order to obtain a simple diffusion equation governing the tilt angle ß, it is necessary in a Keplerian disc that the viscosity parameter α be greater than the opening angle of the disc (H/R). If this condition is not satisfied consideration must be given to the proper dynamical modes of the disc, and the simple approach to the problem adopted here is inadequate. (author)

[en] The disruption of stars by supermassive black holes has been linked to more than a dozen flares in the cores of galaxies out to redshift z ∼ 0.4. Modeling these flares properly requires a prediction of the rate of mass return to the black hole after a disruption. Through hydrodynamical simulation, we show that aside from the full disruption of a solar mass star at the exact limit where the star is destroyed, the common assumptions used to estimate M-dot (t), the rate of mass return to the black hole, are largely invalid. While the analytical approximation to tidal disruption predicts that the least-centrally concentrated stars and the deepest encounters should have more quickly-peaked flares, we find that the most-centrally concentrated stars have the quickest-peaking flares, and the trend between the time of peak and the impact parameter for deeply penetrating encounters reverses beyond the critical distance at which the star is completely destroyed. We also show that the most-centrally concentrated stars produced a characteristic drop in M-dot (t) shortly after peak when a star is only partially disrupted, with the power law index n being as extreme as –4 in the months immediately following the peak of a flare. Additionally, we find that n asymptotes to ≅ – 2.2 for both low- and high-mass stars for approximately half of all stellar disruptions. Both of these results are significantly steeper than the typically assumed n = –5/3. As these precipitous decay rates are only seen for events in which a stellar core survives the disruption, they can be used to determine if an observed tidal disruption flare produced a surviving remnant. We provide fitting formulae for four fundamental quantities of tidal disruption as functions of the star's distance to the black hole at pericenter and its stellar structure: the total mass lost, the time of peak, the accretion rate at peak, and the power-law index shortly after peak. These results should be taken into consideration when flares arising from tidal disruptions are modeled.

[en] The transport properties of a turbulent plasma are investigated in the vicinity of a self-similar turbulent state (SSTS). This state plays the same role of reference as the local equilibrium for a collisional plasma. The stability of the SSTS against velocity gradients is proved. Under certain well-defined conditions a plasmadynamical regime is established, in which the initial conditions are forgotten and the macroscopic plasmadynamical quantities obey closed differential equations. The anomalous viscosity arising in this regime is investigated in detail. This quantity is time-dependent (a feature impressed by the self-similar character of the reference state). A number of additional typically turbulent terms appear in the equation of motion: they arise from interference between flow and collisions or magnetic field. (author)