Search results for "hydrodynamic"
showing 10 items of 530 documents
Ab initiosimulations of accretion disc instability
2003
We show that accretion disks, both in the subcritical and supercritical accretion rate regime, may exhibit significant amplitude luminosity oscillations. The luminosity time behavior has been obtained by performing a set of time-dependent 2D SPH simulations of accretion disks with different values of alpha and accretion rate. In this study, to avoid any influence of the initial disk configuration, we produced the disks injecting matter from an outer edge far from the central object. The period of oscillations is 2 - 50 s respectively for the two cases, and the variation amplitude of the disc luminosity is 10^38 - 10^39 erg/s. An explanation of this luminosity behavior is proposed in terms o…
Role of local absorption on the X-ray emission from MHD accretion shocks in classical T Tauri stars
2014
Accretion processes onto classical T Tauri stars (CTTSs) are believed to generate shocks at the stellar surface due to the impact of supersonic downflowing plasma. Although current models of accretion streams provide a plausible global picture of this process, several aspects are still unclear. For example, the observed X-ray luminosity in accretion shocks is, in general, well below the predicted value. A possible explanation discussed in the literature is in terms of significant absorption of the emission due to the thick surrounding medium. Here we consider a 2D MHD model describing an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere. The m…
Shock-cloud interaction in the Vela SNR II. Hydrodynamic model
2006
In the framework of the study of the X-ray and optical emission in supernova remnants we focus on an isolated X-ray knot in the northern rim of the Vela SNR (Vela FilD), whose X-ray emission has been studied and discussed in Paper I. We aim at understanding the physical origin of the X-ray and optical emission in FilD, at understanding the role of the different physical processes at work, and at obtaining a key for the interpretation of future X-ray observations of SNRs. To this end we have pursued an accurate ``forward'' modeling of the interaction of the Vela SNR shock with an ISM cloud. We perform hydrodynamic simulations and we directly compare the observables synthesized from the simul…
A coupled map as a model of the dynamics of the magnetotail current sheet
2001
Abstract A magnetic field model of the magnetotail current sheet in the form of a coupled-map lattice (CML) is presented. It is continuously driven (“running”) and based on the MHD diffusion equation. Solar wind vBS data (solar wind speed multiplied by the southward component of IMF) are used for driving the model, and it is shown to exhibit perturbations (avalanches) with power-law scalings in their distributions of duration and size. Such distributions may indicate self-organized critical (SOC) behavior. Furthermore, it is shown that the power spectra of the model outputs are of bicolor power-law form with different slopes for high and low frequencies. Although the “running” model gives p…
Low compressibility accretion disc formation in close binaries: the role of physical viscosity
2006
Aims. Physical viscosity naturally hampers gas dynamics (rarefaction or compression). Such a role should support accretion disc development inside the primary gravitation potential well in a close binary system, even for low compressibility modelling. Therefore, from the astrophysical point of view, highly viscous accretion discs could exist even in the low compressibility regime showing strong thermal differences to high compressibility ones Methods. We performed simulations of stationary Smooth Particle Hydrodynamics (SPH) low compressibility accretion disc models for the same close binary system. Artificial viscosity operates in all models. The absence of physical viscosity and a superso…
A three-dimensional smoothed particle hydrodynamics simulation of the active phase of SS Cyg-type discs and its implications for the mass transfer bu…
1993
We perform a smoothed particle hydrodynamics (SPH) three-dimensional simulation of the outburst phase of the accretion disc of a typical SS Cyg-like dwarf nova in the framework of the mass transfer burst model (MTBM), where we assume that the active phase is triggered by a sudden increase in the accretion rate due to some instability in the secondary's atmosphere. The evolution of the accretion disc is followed for a single orbital period, starting from the initial quiescent disc configuration obtained by us in a previous paper. This is a suitable integration time for determining the geometrical and physical properties of the disc in the impulsive phase and is comparable with observed outbu…
Convective stability of hot matter in ultrarelativistic heavy-ion collisions
1992
Abstract The convective stability of strongly interacting matter undergoing hydrodynamic flow in ultrarelativistic heavy-ion collisions is studied in both the quark-gluon plasma and hadron gas phases. We find that this stability depends on the form of the initial conditions assumed for the hydrodynamic flow. In the case of initial conditions corresponding to partial transparency the flow of the quark-gluon plasma is stable whereas the flow of the hadron gas is convectively unstable. The timescale for hydrodynamic oscillations around the (stable or unstable) equilibrium state is found to be larger than the expected lifetime of the system, suggesting that the flow in either case is close to n…
Fully Developed Mixed Magnetohydrodynamic Convection in a Vertical Square Duct
2008
The fully developed flow of an electrically conducting, internally heated fluid in a vertical square duct under the influence of buoyancy and magnetohydrodynamic forces is studied. The flow being parallel, the governing equations are two-dimensional and linear; an analytical solution exists for temperature, while velocity and electric potential are computed by a finite difference technique under different electric boundary conditions, forced to natural convection intensity ratios and values of the magnetic induction. Limiting values of pressure gradient and mean velocity are determined for the flow to be unidirectional throughout the duct's section; recirculation occurs for intermediate val…
Cosmic magnetic fields with masclet: an application to galaxy clusters
2020
We describe and test a new version of the adaptive mesh refinement (AMR) cosmological code MASCLET. The new version of the code includes all the ingredients of its previous version plus a description of the evolution of the magnetic field under the approximation of the ideal magneto-hydrodynamics (MHD). To preserve the divergence-free condition of MHD, the original divergence cleaning algorithm of Dedner et al. (2002) is implemented. We present a set of well-known 1D and 2D tests, such as several shock-tube problems, the fast rotor and the Orszag-Tang vortex. The performance of the code in all the tests is excellent with estimated median relative errors of $\nabla \cdot {\bf B}$ in the 2D t…
A coupled-map model for the magnetotail current sheet
1999
A magnetic field model of the magnetotail current sheet in the form of a coupled-map lattice (CML) is presented. It is a continuously driven and based on the MHD diffusion equation. Solar wind vBs data (solar wind speed multiplied by the southward component of IMF) are used for driving the model, and it is shown to exhibit perturbations (avalanches) with power-law scalings in their distributions of duration and size. Such distributions may indicate self-organized critical (SOC) behavior. Furthermore, it is shown that the power spectra of the model outputs are of bicolor power-law form with different slopes for high and low frequencies. The model parameters determine the frequency of the bre…