共查询到20条相似文献，搜索用时 46 毫秒
1.
The paper considers the fully-developed slip flow in a vertical channel with radiative heat transfer and mass transfer in the presence of an externally applied magnetic field. The problem is modelled by the compressible Navier-Stokes equations, so that the gas is only slightly rarefied. Invoking the exact integral equation for radiation, the problem is reduced to a set of ordinary integro-differential equations. By realistic assumptions, the set is linearized and the temperature is reduced to a mixed Fredholm-Volterra integral equation which is solved by standard iterative procedure. Thereafter the concentration equation is solved by the WKB approximation while the velocity is obtained by the finite difference scheme. These solutions are discussed qualitatively. 相似文献
2.
G. S. Dubey U. S. Pandey J. P. Singh L. K. Singh S. N. Singh 《Journal of Astrophysics and Astronomy》1990,11(1):23-28
The equation governing the equilibrium of a centrally-symmetric, self-gravitating distribution of matter is obtained by assuming
that matter is described by magnetofluid and it is shown that the equation of stellar structure described by an ideal fluid
is recovered when magnetic fieldh is vanishing 相似文献
3.
M.J. Giles 《Planetary and Space Science》1979,27(11):1311-1320
4.
It has been proposed that the million-degree temperature of the corona is due to the combined effect of barely detectable
energy releases, called nanoflares, that occur throughout the solar atmosphere. Unfortunately, the nanoflare density and brightness
implied by this hypothesis means that conclusive verification is beyond present observational abilities. Nevertheless, we
investigate the plausibility of the nanoflare hypothesis by constructing a magnetohydrodynamic (MHD) model that can derive
the energy of a nanoflare from the nature of an ideal kink instability. The set of energy-releasing instabilities is captured
by an instability threshold for linear kink modes. Each point on the threshold is associated with a unique energy release;
thus we can predict a distribution of nanoflare energies. When the linear instability threshold is crossed, the instability
enters a nonlinear phase as it is driven by current sheet reconnection. As the ensuing flare erupts and declines, the field
transitions to a lower energy state, which is modelled by relaxation theory; i.e., helicity is conserved and the ratio of current to field becomes invariant within the loop. We apply the model so that all
the loops within an ensemble achieve instability followed by energy-releasing relaxation. The result is a nanoflare energy
distribution. Furthermore, we produce different distributions by varying the loop aspect ratio, the nature of the path to
instability taken by each loop and also the level of radial expansion that may accompany loop relaxation. The heating rate
obtained is just sufficient for coronal heating. In addition, we also show that kink instability cannot be associated with
a critical magnetic twist value for every point along the instability threshold. 相似文献
5.
A dynamical model of prominence loops is constructed on the basis of the theory of hydromagnetic buoyancy force. A prominence loop is regarded as a flux rope immersed in the solar atmosphere above a bipolar region of the photospheric magnetic field. The motion of a loop is partitioned into a translational motion, which accounts for the displacement of the centroidal axis of the loop, and an expansional motion, which accounts for the displacement of the periphery of the loop relative to the axis. The translational motion is driven by the hydromagnetic buoyancy force exerted by the surrounding medium of the solar atmosphere and the gravitational force exerted by the Sun. The expansional motion is driven by the pressure gradient that sustains the pressure difference between internal and external gas pressures and the self-induced Lorentz force that results from interactions among internal currents. The main constituent of the hydromagnetic buoyancy force on a prominence loop is the diamagnetic force exerted on the internal currents by the external currents that sustain the pre-existing magnetic field. By spatial transformation between magnetic and mechanical stresses, the diamagnetic force is manifested through a mechanical force acting at various mass elements of the prominence. For a prominence loop in equilibrium, the gravitational force is balanced by the hydromagnetic buoyancy force and the Lorentz force of helical magnetic field is balanced by a gradient force of gas pressure. 相似文献
6.
K. Stpie&#; 《Monthly notices of the Royal Astronomical Society》2009,397(2):857-867
A hydrodynamic model for the energy transport between the components of a contact binary is presented. Energy is transported by a large-scale, steady circulation carrying high entropy matter from the primary to secondary component. The circulation is driven by the baroclinic structure of the common envelope, which is a direct consequence of the non-uniform heating at the inner critical Roche lobes due to unequal emergent energy fluxes of the components. The mass stream flowing around the secondary is bound to the equatorial region by the Coriolis force and its width is determined primarily by the flow velocity. Its bottom is separated from the underlying secondary's convection zone by a radiative transition layer acting as an insulator. For a typically observed degree of contact the heat capacity of the stream matter is much larger than radiative losses during its flow around the secondary. As a result, its effective temperature and entropy decrease very little before it returns to the primary. The existence of the stream changes insignificantly specific entropies of both convective envelopes and sizes of the components. Substantial oversize of the secondaries, required by the Roche geometry, cannot be explained in this way. The situation can, however, be explained by assuming that the primary is a main-sequence star whereas the secondary is in an advanced evolutionary stage with hydrogen depleted in its core. Such a configuration is reached past mass transfer with mass ratio reversal. Good agreement with observations is demonstrated by model calculations applied to actual W UMa-type binaries. In particular, a presence of the equatorial bulge moving with a relative velocity of 10–30 km s−! around both components of AW UMa is accounted for. 相似文献
7.
G. Gislason M. Dryer Z. K. Smith S. T. Wu S. M. Han 《Astrophysics and Space Science》1984,98(1):149-161
A recently developed nonplanar, time-dependent magnetohydrodynamic (MHD) model (Wuet al., 1983) was used to study the interplanetary disturbances produced by a compound event in the heliosphere. That is, a steady-state interplanetary medium is first disturbed by a simulated equatorially-fluctuating current sheet. After a few days (100 hr), the disturbed interplanetary medium is again perturbed by a solar-flare-generated shock wave. Attention is directed toward the differences that are caused by the presence of the equatorially-fluctuating (warped) current sheet. 相似文献
8.
The tree code for the approximate evaluation of gravitational forces is extended and substantially accelerated by including mutual cell-cell interactions. These are computed by a Taylor series in Cartesian coordinates and in a completely symmetric fashion, such that Newton's third law is satisfied by construction and that therefore momentum is exactly conserved. The computational effort is further reduced by exploiting the mutual symmetry of the interactions. For typical astrophysical problems with N=105 and at the same level of accuracy, the new code is about 4 times faster than the tree code. For large N, the computational costs are found to scale almost linearly with N, which can also be supported by a theoretical argument, and the advantage over the tree code increases with ever larger N. 相似文献
9.
Paul E. Nacozy 《Astrophysics and Space Science》1971,14(1):40-51
The numerical integration of systems of differential equations that possess integrals is often approached by using the integrals to reduce the number of degrees of freedom or by using the integrals as a partial check on the resulting solution, retaining the original number of degrees of freedom.Another use of the integrals is presented here. If the integrals have not been used to reduce the system, the solution of a numerical integration may be constrained to remain on the integral surfaces by a method that applies corrections to the solution at each integration step. The corrections are determined by using linearized forms of the integrals in a least-squares procedure.The results of an application of the method to numerical integrations of a gravitational system of 25-bodies are given. It is shown that by using the method to satisfy exactly the integrals of energy, angular momentum, and center of mass, a solution is obtained that is more accurate while using less time of calculation than if the integrals are not satisfied exactly. The relative accuracy is ascertained by forward and backward integrations of both the corrected and uncorrected solutions and by comparison with more accurate integrations using reduced step-sizes. 相似文献
10.
Zi Zhu 《中国天文和天体物理学报》2008,8(1):96-102
From photometric observations and star counts, the existence of a bar in the cen-tral few kpc of the Galaxy is suggested. It is generally thought that our Galaxy is surrounded by a massive invisible halo. The gravitational potential of the Galaxy is therefore made non-axisymmetric generated by the central tfiaxial bar, by the outer triaxial halo, and/or by the spiral structures. Selecting nearly 300 open clusters with complete spatial velocity measure-ments and ages, we were able to construct the rotation curve of the Milky Way within a range of 3 kpc of the Sun. Using a dynamic model for an assumed elliptical disk, a clear weak el-liptical potential of the disk with ellipticity of ε(R0) = 0.060 ± 0.012 is detected, the Sun is found to be near the minor axis, displaced by 30°± 3°. The motion of the clusters is suggested to be on an oval orbit rather than on a circular one. 相似文献
11.
张力 《中国天文和天体物理学报》1995,(4)
使用最近期的ＧＲＯ库中ＥＧＲＥＴ的γ射线数据，研究了逆康普顿散射对中等银纬区银河弥漫γ射线的贡献。采用了两种分析方法：其一为本文的新分析方法，其中银河宇宙与星际气体相互作用的γ射线的发射率ｑ／４π由γ射线数据本身确定；其二为通常的分析方法，即ｑ／４π由使用了局部银河宇宙线强度的观测值的理论估计给出。通过分析，我们获得了两种方法中逆康普顿散射的贡献与能量的依赖关系。结果表明，逆康普顿散射的贡献是大的。 相似文献
12.
13.
The heliocentrifugal motion of coronal loop transients is likely driven largely by the buoyant force exerted by the ambient medium. In the outer corona where the solar wind is well formed, the buoyant force results mainly from the rapid outward decrease in the ambient pressure of the solar wind. The contribution from magnetic buoyancy is not so significant as in the vicinity of the solar surface. Therefore, the pertinent features of the loop transients in the outer corona are basically gasdynamical. As a conspicuous part of coronal expansion, the motion of the compressible masses in the transient loops is largely controlled by thermal forces. The translational motion of heliocentrifugal expansion is driven by the hydrodynamic buoyant force, and the lateral motion of peripheral expansion is driven by the pressure difference between the dense plasma of the ejecta and the tenuous plasma of the ambient medium. 相似文献
14.
The general variational principle obtained by Unno for the study of stellar stability is extended to include the effects of magnetic fields in the general case where the perturbations are nonadiabatic. The unperturbed configuration is non-rotating, but internal motions are possible.A general stability equation is derived, using the modified variational principle. In the simplest case, this equation is identical to the one derived by Unno except for the appearance of magnetic terms in one of the coefficients (through the Lorentz force). If dissipation is neglected, the characteristic equation reduces to the one derived by Frieman and Rotenberg in the stationary case and by Kovetz in the static case.The stability of magnetic stars against homologous oscillations is examined and the usual result for the dynamical mode is obtained. 相似文献
15.
John C. Brown 《Solar physics》1973,29(2):421-427
Non-steady state and non-LTE effects on the ionisation equilibrium of hydrogen in optical flares are considered in terms of a two-level hydrogen atom. It is shown that, just as in the quiet low chromosphere, the ionisation equation is controlled by spontaneous recombination to the second level and by photoionisation from this level by photospheric radiation, and is independent of the nature of the flare energy input mechanism.Adjustment of the ionisation then occurs on the recombination time scale which is short compared to the flare heating time scale. Consequently the ionisation is given by a simple LTE-modification of Saha's equation at the instantaneous electron temperature obtained from the energy equation. These conclusions are contrasted with those of previous authors who, by using one-level atoms only, have omitted the optically thin Balmer continuum which dominates the optically thick Lyman transitions. 相似文献
16.
Tyan Yeh 《Solar physics》1982,78(2):287-316
A magnetohydrodynamic theory is presented for coronal loop transients. It is shown that the heliocentrifugal motion of a transient loop, as exhibited by the translational displacement of the axis of the loop, is driven by the magnetohydrodynamic buoyancy force exerted by the ambient medium. Self-induced hydromagnetic force, which includes the magnetic force produced by the internally driven current and the thermal force produced by the pressure imbalance between the internal and external gas pressures, causes the peripheral expansion of the loop, as exhibited by the lateral broadening and longitudinal stretching. This contention is substantiated by an analysis based on a model structure for a coronal loop.Besides accounting for the acceleration and expansion of a transient loop, this magnetohydrodynamic theory also provides an explanation for the initial ejection of a coronal loop from stationary equilibrium. Magnetic unwinding in consequence of abrupt magnetic activities at the solar surface will cause the periphery of a stationary coronal loop to expand. The increase in volume will enhance the magnetohydrodynamic buyoyancy force to exceed the gravitational force. Once a coronal loop is ejected from the solar surface, it will be continually accelerated and undergo expansion. Eventually a transient loop will blend with the ambient solar wind. This is also indicated by the theory presented in this paper. 相似文献
17.
We model the formation of solar quiescent prominences by solving numerically the non-linear, time-dependent, magnetohydrodynamic equations governing the condensation of the corona. A two-dimensional geometry is used. Gravitational and magnetic fields are included, but thermal conduction is neglected. The coronal fluid is assumed to cool by radiation and to be heated by the dissipation of mechanical energy carried by shock waves. A small, isobaric perturbation of the initial thermal and mechanical equilibrium is introduced and the fluid is allowed to relax. Because the corona with the given energy sources is thermally unstable, cooling and condensation result.When magnetic and gravitational fields are absent, condensation occurs isotropically with a strongly time-dependent growth rate, and achieves a density 18 times the initial density in 3.5 × 104 s. The rapidity of condensation is limited by hydrodynamical considerations, in contrast to the treatment of Raju (1968). When both magnetic and gravitational fields are included, the rate of condensation is inhibited and denser material falls.We conclude that: (1) condensation of coronal material due to thermal instability is possible if thermal conduction is inhibited; (2) hydrodynamical processes determine, in large part, the rate of condensation; (3) condensation can occur on a time scale compatible with the observed times of formation of quiescent prominences.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
18.
19.
Toshiki Aikawa 《Astrophysics and Space Science》1990,164(2):295-307
One of the model sequences which is found to show the period-doubling cascade by Kovacs and Buchler (1988) is re-examined by the hydrodynamic pulsation code with dynamic rezoning of the hydrogen ionization zones (TGRID). It is found that the sequence shows subharmonic bifurcations and chaotic behaviours as the effective temperature, a control parameter in this sequence, is varied, as found by Kovacs and Buchler. The subharmonic cascade, however,is terminated at period 4, and the type I intermittency takes place against the cascade. We conclude that the chaotic behaviour is caused by the intermittency in our model sequence rather than the infinite period-doubling cascade. Finally, we briefly discuss effects of the outburst of highly irregular oscillation due to the intermittency on the tendency in the train of light curves. 相似文献
20.
I. A. Klimishin 《Astrophysics and Space Science》1972,16(3):432-436
It is shown by a certain combination of stella parameters and energy which was released by a outburst, the transfer of that energy outwards is carried out by the thermal wave. 相似文献