共查询到20条相似文献，搜索用时 46 毫秒
1.
Similarity solutions, describing the flow of a perfect gas behind spherical shock waves, are investigated including the radiation heat flux. The shock is assumed to be propagating in a medium at rest. Shock radius varies exponentially with time and density is inversely proportional to fifth power of the shock radius immediately ahead of the shock front. 相似文献
2.
Self-similar flows of a perfect gas behind the cylindrical shock wave propagating exponentially in an atmosphere whose density varies inversely as the fourth power of shock radius are investigated. The effects of radiation flux has also been taken into consideration. The total energy of the wave is constant. 相似文献
3.
An approximate analytical solution for self-similar flow behind a spherically-symmetric magnetogasdynamic strong shock wave is investigated using the method of Laumbach and Probstein (1969). The total energy of flow is taken to be dependent on the shock radius obeying a power law. The shock is propagating into a perfect gas at rest with non-uniform density and magnetic field. 相似文献
4.
J. B. Singh 《Astrophysics and Space Science》1983,93(1):15-23
In this paper self-similar solutions have been investigated for the propagation of axisymmetric radiative gasdynamic shocks caused by an explosion into an inhomogeneous ideal gas permeated by a current free azimuthal magnetic field. The effects of radiation flux and magnetic field together have been seen in the region of interest on the other flow variables. The total energy of the flow between the inner expanding surface and the shock is taken to be dependent on shock radius obeying a power law. The radiative pressure and energy have been neglected. 相似文献
5.
Gravitational instability of an infinitely conducting hydromagnetic composite rotating plasma is considered to include simultaneously the finite Larmor radius effects and the frictional effects with neutrals. It is found that Jeans' criterion of instability holds good in the presence of rotation, finite Larmor radius and collisions with neutrals. The particular cases of the above effects on the waves propagated along and perpendicular to the magnetic field have been discussed. The effect of rotation is to decrease the Larmor radius by an amount depending upon the wave number of perturbation. 相似文献
6.
We investigate the conditions for the existence of an expanding virial shock in the gas falling within a spherical dark matter halo. The shock relies on pressure support by the shock-heated gas behind it. When the radiative cooling is efficient compared with the infall rate, the post-shock gas becomes unstable; it collapses inwards and cannot support the shock. We find for a monatomic gas that the shock is stable when the post-shock pressure and density obey . When expressed in terms of the pre-shock gas properties at radius r it reads as ρ r Λ( T )/ u 3 < 0.0126 , where ρ is the gas density, u is the infall velocity and Λ( T ) is the cooling function, with the post-shock temperature T ∝ u 2 . This result is confirmed by hydrodynamical simulations, using an accurate spheri-symmetric Lagrangian code. When the stability analysis is applied in cosmology, we find that a virial shock does not develop in most haloes that form before z ∼ 2 , and it never forms in haloes less massive than a few 1011 M⊙ . In such haloes, the infalling gas is not heated to the virial temperature until it hits the disc, thus avoiding the cooling-dominated quasi-static contraction phase. The direct collapse of the cold gas into the disc should have non-trivial effects on the star formation rate and on outflows. The soft X-ray produced by the shock-heated gas in the disc is expected to ionize the dense disc environment, and the subsequent recombination would result in a high flux of Lα emission. This may explain both the puzzling low flux of soft X-ray background and the Lα emitters observed at high redshift. 相似文献
7.
In the present paper self-similar solutions have been investigated for the propagation of piston driven, radiative gas-dynamic shocks into an inhomogeneous ideal gas permeated by a current free azimuthal magnetic field for spherical symmetry. The effects of radiation flux and magnetic field together have been seen in the region of interest on the other flow variables. The total energy of the flow between the piston and the shock is taken to be dependent on the shock radius obeying a power law. The radiative pressure and energy have been neglected. This problem is more general than the others done so far. The word piston implies some means to drive plasma radially onwards. 相似文献
8.
Adi Nusser 《Monthly notices of the Royal Astronomical Society》2007,375(3):1106-1110
We present general relativistic solutions for self-similar spherical perturbations in an expanding cosmological background of cold pressure-less gas. We focus on solutions having shock discontinuities propagating in the surrounding cold gas. The pressure, p , and energy density, μ, in the shock-heated matter are assumed to obey p = w μ , where w is a positive constant. Consistent solutions are found for shocks propagating from the symmetry centre of a region of a positive density excess over the background. In these solutions, shocks exist outside the radius marking the event horizon of the black hole which would be present in a shock-less collapse. For large jumps in the energy density at the shock, a black hole is avoided altogether and the solutions are regular at the centre. The shock-heated gas does not contain any sonic points, provided the motion of the cold gas ahead of the shock deviates significantly from the Hubble flow. For shocks propagating in the uniform background, sonic points always appear for small jumps in the energy density. We also discuss self-similar solutions without shocks in fluids with w < −1/3 . 相似文献
9.
K. C. Sharma 《Journal of Astrophysics and Astronomy》1988,9(1):1-5
Thermal-convective instability of a hydromagnetic, composite, rotating, inviscid and infinitely conducting plasma in a stellar
atmosphere has been studied in the presence of finite Larmor radius. It is found that the criterion for monotonic instability
holds good in the presence of the effects due to rotation and finite Larmor radius. 相似文献
10.
G. Narasimhulu Naidu M. P. Ranga Rao Hira Lal Yadav 《Astrophysics and Space Science》1983,89(1):77-87
Self-similar unsteady flows with zero temperature gradient behind strong spherical shocks propagating in non-uniform perfect gas at rest are investigated. The total energy of the flow is assumed to be varying with the shock radius obeying a power law. Approximate solutions in a closed analytical form are obtained using the integral method. Also these solutions are shown to be useful to calculate easily and quickly the shock temperature, X-ray surface brightness and luminosity which are important in astrophysical problems. It is found that these approximate solutions are in close agreement with numerical solutions. 相似文献
11.
The magneto-gravitational instability of an infinite, homogenous, and infinitely conducting plasma flowing through a porous medium is studied. The finite ion Larmor radius (FLR) effects and viscosity are also incorporated in the analysis. The prevalent magnetic field is assumed to be uniform and acting in the vertical direction. A general dispersion relation has been obtained from the relevant linearized perturbation equations of the problem. The wave propagation parallel and perpendicular to the direction of the magnetic field have been discussed. It is found that the condition of the instability is determined by the Jeans criterion for a self-gravitating, infinitely conducting, magnetized fluid through a porous medium. Furthermore, for transverse perturbation FLR is found to have stabilizing influence when the medium is considered inviscid. 相似文献
12.
We consider the self-similar problem of a supernova explosion in a radially inhomogeneous medium by taking into account the generation of accelerated relativistic particles. The initial density of the medium is assumed to decrease with distance from the explosion center as a power law, ρ 0 = A/r θ. We use a two-fluid approach in which the total pressure in the medium is the sum of the circumstellar gas pressure and the relativistic particle pressure. The relativistic particle pressure at the shock front is specified as an external parameter. This approach is applicable in the case where the diffusion coefficient of accelerated particles is small and the thickness of the shock front is much smaller than its radius. We have numerically solved a system of ordinary differential equations for the dimensionless quantities that describe the velocity and density behind the shock front as well as the nonrelativistic gas and relativistic particle pressures for various parameters of the inhomogeneity of the medium and various compression ratios of the medium at the shock front. We have established that the shock acceleration of cosmic rays affects most strongly the formation of a supernova shell (making it thinner) in a homogeneous circumstellar medium. A decrease in the circumstellar matter density with distance from the explosion center causes the effect of shock-accelerated relativistic particles on the supernova shell formation to weaken considerably. Inhomogeneity of the medium makes the shell thicker and less dense, while an increase in the compression ratio of the medium at the shock front causes the shell to become thinner and denser. As the relativistic particle density increases, the effect of circumstellar matter inhomogeneity on the shell formation becomes weaker. 相似文献
13.
We numerically model the formation and acceleration of a neutral gas shell as an ionization-shock front propagates in a spherical cloud by taking into account the photoionization and radiative heating of the gas, the spectral radiative transfer. We suggest and implement an approximation of the cooling function that allows calculations to be performed in a wide range of gas ionization fractions and temperatures. The total mass, average velocity, and thickness of the shell have been determined. The results are compared with approximate formulas known in the literature. Based on the parameters of the shell found, we estimate its acceleration, characteristic scales, and the growth times of unstable perturbations. We analyze the influence of the cloud particle density, cloud radius, stellar temperature, and radiation spectrum on the integrated characteristics of the neutral gas in the layer between the ionization and shock fronts. The distribution of matter in the shell and its thickness are shown to differ significantly from those used in approximate models. 相似文献
14.
In broad astrophysical contexts of large-scale gravitational collapses and outflows and as a basis for various further astrophysical
applications, we formulate and investigate a theoretical problem of self-similar magnetohydrodynamics (MHD) for a non-rotating
polytropic gas of quasi-spherical symmetry permeated by a completely random magnetic field. Within this framework, we derive
two coupled nonlinear MHD ordinary differential equations (ODEs), examine properties of the magnetosonic critical curve, obtain
various asymptotic and global semi-complete similarity MHD solutions, and qualify the applicability of our results. Unique
to a magnetized gas cloud, a novel asymptotic MHD solution for a collapsing core is established. Physically, the similarity
MHD inflow towards the central dense core proceeds in characteristic manners before the gas material eventually encounters
a strong radiating MHD shock upon impact onto the central compact object. Sufficiently far away from the central core region
enshrouded by such an MHD shock, we derive regular asymptotic behaviours. We study asymptotic solution behaviours in the vicinity
of the magnetosonic critical curve and determine smooth MHD eigensolutions across this curve. Numerically, we construct global
semi-complete similarity MHD solutions that cross the magnetosonic critical curve zero, one, and two times. For comparison,
counterpart solutions in the case of an isothermal unmagnetized and magnetized gas flows are demonstrated in the present MHD
framework at nearly isothermal and weakly magnetized conditions. For a polytropic index γ=1.25 or a strong magnetic field, different solution behaviours emerge. With a strong magnetic field, there exist semi-complete
similarity solutions crossing the magnetosonic critical curve only once, and the MHD counterpart of expansion-wave collapse
solution disappears. Also in the polytropic case of γ=1.25, we no longer observe the trend in the speed-density phase diagram of finding infinitely many matches to establish global
MHD solutions that cross the magnetosonic critical curve twice.
相似文献
15.
G. S. Bisnovatyi-Kogan 《Astrophysics》2004,47(3):404-411
An exact solution is obtained for the electromagnetic field surrounding an infinitely long, conducting cylinder with a periodic axial electric current. The solution simultaneously gives the field near the cylinder and in the transition to the wave zone. The flux of electromagnetic radiation emitted by the cylinder is calculated for oscillations with long wavelengths greatly exceeding the radius of the cylinder. This solution can be used to describe the electromagnetic field around narrowly collimated jets from active galactic nuclei and quasars. 相似文献
16.
Chisnell-Chester-Whitham method has been used to study the propagation of diverging hydromagnetic cylindrical shock through an infinitely electrically conducting self-gravitating gas having an initial density distribution 0= r-w where is the density at the axis of symmetry andw is a constant, simultaneously for the two cases, viz.: (i) when the shock is weak and (ii) when it is strong. The magnetic field is taken to be axial and initially of constant strength. Analytical relations for shock velocity and shock strength have been obtained. the expressions for the pressure, the density and the particle velocity immediately behind the shock have also been derived. 相似文献
17.
P. D. Ariel 《Astrophysics and Space Science》1988,141(1):141-149
The gravitational instability of an incompressible, infinitely conducting plasma layer of finite thickness surrounded a non-conducting matter has been investigated taking into account the effect of the finite Larmor radius. The magnetic field is assumed to be directed parallel to the interfaces. Only the perturbations transverse to the magnetic field are considered, though both the symmetric and asymmetric nature of the perturbations are taken into account. Using the normal mode technique, dispersion relations are obtained.It is found that the finite larmor radius has, in general, a stabilizing influence on the configuration. Even when the system is thoroughly unstable, it has been shown that there exists a critical value of the wave-number, such that the motion is stabilized for wave-numbers of perturbations exceeding this critical value. 相似文献
18.
Shi-Yang Jiang Fu-Yuan Zhao National Astronomical Observatory Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2007,7(2):325-328
We presume the re-brightening of SN 1006 in AD 1016 as recorded in the ancient Chinese literature to be true and the re-brightening was caused by the encounter either of photons or the shock wave from the SN outburst with the circumstellar thin envelope mate- rials. Based on these considerations, and combining the observational results on the optical proper motion of the N-W limb and the radio observations of the other parts of the supernova remnant G327.6 14.5, we re-determine the distance to SN 1006. For the photon-encounter model, the average radius of the envelope material would be 10ly; and for the shock wave- encounter model, the radius would be about 1 ly. We then set up four equations to solve for the distance of the SN, the initial shock speed, the expansion index for two different parts of the supernova remnant, and the real original radius of the thin envelope nebula. It is indicated that only the case of photon-encounter will lead to a reasonable result. We derived a distance of 5074ly (1.56kpc), an original shock expansion velocity of 0.071c, an expansion index of 0.72 for the N-W limb of the SNR, and 0.76 for the other parts . We deem that the SNR evolution is still in the stage of reverse shock. 相似文献
19.
Susan A. Lamb Richard A. Gerber Dinshaw S. Balsara 《Astrophysics and Space Science》1994,216(1-2):337-346
We present some results from three dimensional computer simulations of collisions between models of equal mass galaxies, one of which is a rotating, disk galaxy containing both gas and stars and the other is an elliptical containing stars only. We use fully self consistent models in which the halo mass is 2.5 times that of the disk. In the experiments we have varied the impact parameter between zero (head on) and 0.9R (whereR is the radius of the disk), for impacts perpendicular to the disk plane. The calculations were performed on a Cray 2 computer using a combined N-body/SPH program. The results show the development of complicated flows and shock structures in the direction perpendicular to the plane of the disk and the propagation outwards of a density wave in both the stars and the gas. The collisional nature of the gas results in a sharper ring than obtained for the star particles, and the development of high volume densities and shocks. 相似文献
20.
Relativistic shocks provide an efficient method for high-energy particle acceleration in many astrophysical sources. Multiple
shock systems are even more effective and of importance, for example, in the internal shock model of gamma-ray bursts. We
investigate the reacceleration of pre-existing energetic particles at such relativistic internal shocks by the first order
Fermi process of pitch angle scattering. We use a well established eigenfunction method to calculate the resulting spectra
for infinitely thin shocks. Implications for GRBs and relativistic jets are discussed.
Paul Dempsey would like to thank IRCSET for their financial support. 相似文献