共查询到20条相似文献，搜索用时 564 毫秒
1.
We study the relaxation of a compressible plasma to an equilibrium with flow. The constraints of conservation of mass, energy,
angular momentum, cross-helicity and relative magnetic helicity are imposed. Equilibria corresponding to the energy extrema
while conserving these invariants for parallel flows yield three classes of solutions and one of them with an increasing radial
density profile, relevant to solar flux tubes is presented. 相似文献
2.
Similarity solution for the interaction of stellar wind with surrounding interstellar medium is obtained on the base of momentum conservation. The conversion efficiency of kinetic energy of stellar wind into the kinetic energy of expanding shell is derived. The results are compared with the observations on the ring nebulae associated with the Wolf-Rayet star and it is shown that the observed values of energy conversion are in good agreement with the momentum conserving model. 相似文献
3.
Jean-Marc Petit 《Celestial Mechanics and Dynamical Astronomy》1998,70(1):1-21
We investigate the numerical implementation of a symplectic integrator combined with a rotation (as in the case of an elongated
rotating primary). We show that a straightforward implementation of the rotation as a matrix multiplication destroys the conservative
property of the global integrator, due to roundoff errors. According to Blank et al. (1997), there exists a KAM-like theorem
for twist maps, where the angle of rotation is a function of the radius. This theorem proves the existence of invariant tori
which confine the orbit and prevent shifts in radius. We replace the rotation by a twist map or a combination of shears that
display the same kind of behaviour and show that we are able not only to recover the conservative properties of the rotation,
but also make it more efficient in term of computing time. Next we test the shear combination together with symplectic integrator
of order 2, 4, and 6 on a Keplerian orbit. The resulting integrator is conservative down to the roundoff errors. No linear
drift of the energy remains, only a divergence as the square root of the number of iterations is to be seen, as in a random
walk. We finally test the three symplectic integrators on a real case problem of the orbit of a satellite around an elongated
irregular fast rotating primary. We compare these integrators to the well-known general purpose, self-adaptative Bulirsch–Stoer
integrator. The sixth order symplectic integrator is more accurate and faster than the Bulirsch–Stoer integrator. The second-
and fourth- order integrators are faster, but of interest only when extreme speed is mandatory.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
4.
《Chinese Astronomy and Astrophysics》1982,6(3):192-198
We investigate the possibility of an additional acceleration of the high speed solar wind by whistler waves propagating outward from a coronal hole. We consider a stationary, spherically symmetric model and assume a radial wind flow as well as a radial magnetic field. The energy equation consists of (a) energy transfer of the electron beam which excites the whistler waves, and (b) energy transfer of the whistler waves described by conservation of wave action density. The momentum conservation equation includes the momentum transfer of two gases (a thermal gas and an electron beam). The variation of the temperature is described by a polytropic law. The variation of solar wind velocity with the radial distance is calculated for different values of energy density of the whistler waves. It is shown that the acceleration of high speed solar wind in the coronal hole due to the whistler waves is very important. We have calculated that the solar wind velocity at the earth's orbit is about equal to 670 km/sec (for wave energy density about 10?4 erg cm?3 at 1.1R⊙). It is in approximate agreement with the observed values. 相似文献
5.
Titan's atmospheric winds, like those on Venus, exhibit superrotation at high altitudes. Titan general circulation models have yielded conflicting results on whether prograde winds in excess of 100 m s−1 at the 1 mbar level are possible based on known physical processes that drive wind systems. A comprehensive two-dimensional (2D) model for Titan's stratosphere was constructed to systematically explore the physical mechanisms that produce and maintain stratospheric wind systems. To ensure conservation of angular momentum in the limit of no net exchange of atmospheric angular momentum with the solid satellite and no external sources and sinks, the zonal momentum equation was solved in flux form for total angular momentum. The relationships among thermal wind balance, meridional circulation, and zonal wind were examined with numerical experiments over a range of values for fundamental input parameters, including planetary rotation rate, radius, internal friction due to wave stresses, and net radiative drive. The magnitude of mid-latitude jets is most sensitive to a single parameter, the planetary rotation rate and results from the conversion of planetary angular momentum to relative angular momentum by the meridional circulation, whereas the strength of meridional circulation is mainly determined by the magnitude of the radiative drive. For Titan's slowly rotating atmosphere, the meridional temperature gradient is vanishingly small, even when the radiative drive is enhanced beyond reasonable magnitudes, and can be inferred from zonal winds in gradient/thermal wind balance. In our 2D model large equatorial superrotation in Titan's stratosphere can be only produced through internal drag forcing by eddy momentum fluxes, which redistribute angular momentum within the atmosphere, while still conserving the total angular momentum of the atmosphere with time. We cannot identify any waves, such as gravitational or thermal tides, that are sufficiently capable of generating the required eddy forcing of >50 m s−1 Titan-day−1 to maintain peak prograde winds in excess of 100 m s−1 at the 1 mbar level. 相似文献
6.
《New Astronomy》2021
We present an N-body code called Taichi for galactic dynamics and controlled numerical experiments. The code includes two high-order hierarchical multipole expansion methods: the Barnes-Hut (BH) tree and the fast multipole method (FMM). For the time integration, the code can use either a conventional adaptive KDK or a Hamiltonian splitting integrator. The combination of FMM and the Hamiltonian splitting integrator leads to a momentum-conserving N-body scheme with individual time steps. We find Taichi performs well in the typical applications in galactic dynamics. In the isolated and interacting galaxies tests, the momentum conserving scheme produces the same result as a conventional BH tree code. But for similar force accuracies, FMM significantly speeds up the simulations compared to the monopole BH tree. In the cold collapse test, we find the inner structure after relaxation can be sensitive to the force accuracies. Taichi is ready to incorporate special treatment of close encounters thanks to the Hamiltonian splitting integrator, suitable for studying dynamics around central massive bodies. 相似文献
7.
Antonio Alfonso-Faus 《Astrophysics and Space Science》2008,315(1-4):25-29
We prove here that Newton’s universal gravitation and momentum conservation laws together reproduce Weinberg’s relation. It is shown that the Hubble parameter H must be built in this relation, or equivalently the age of the Universe t. Using a wave-to-particle interaction technique we then prove that the speed of light c decreases with cosmological time, and that c is proportional to the Hubble parameter H. We see the expansion of the Universe as a local effect due to the LAB value of the speed of light c 0 taken as constant. We present a generalized red shift law and find a predicted acceleration for photons that agrees well with the result from Pioneer 10/11 anomalous acceleration. We finally present a cosmological model coherent with the above results that we call the Mass-Boom. It has a linear increase of mass m with time as a result of the speed of light c linear decrease with time, and the conservation of momentum mc. We obtain the baryonic mass parameter equal to the curvature parameter, Ω m =Ω k , so that the model is of the type of the Einstein static, closed, finite, spherical, unlimited, with zero cosmological constant. This model is the cosmological view as seen by photons, neutrinos, tachyons etc. in contrast with the local view, the LAB reference. Neither dark matter nor dark energy is required by this model. With an initial constant speed of light during a short time we get inflation (an exponential expansion). This converts, during the inflation time, the Planck’s fluctuation length of 10?33 cm to the present size of the Universe (about 1028 cm, constant from then on). Thereafter the Mass-Boom takes care to bring the initial values of the Universe (about 1015 gr) to the value at the present time of about 1055 gr. 相似文献
8.
9.
《New Astronomy》2021
We consider an interstellar interloper moving at a relatively large distance from a circular binary star. We use the analytical method of separating rapid and slow subsystems, the rapid subsystem being the binary and the slow subsystem being the interstellar interloper. We show that due to the higher than geometrical symmetry of the problem, in addition to the conservation of the energy and the projection of the angular momentum on the axis of the rotation of the binary, the square of the angular momentum is also conserved. In the course of the time evolution, the vector of the angular momentum rotates about that axis at the constant angle to the axis. After obtaining this general counterintuitive result, we focus at the case where the interstellar interloper is coplanar with the binary. We provide an explicit equation of the motion of the interloper. Then we calculate analytically the angle of deflection of the interloper from the straight line. We analyze the difference in the angle of deflection between this three-body problem and the corresponding two-body problem: we show that this difference remains almost constant (a negative constant) at the range of the eccentricities of the interloper trajectory relatively close to unity and linearly increases (by the absolute value, remaining negative) with the eccentricity as the latter becomes much greater than unity. 相似文献
10.
11.
Stability of Binary Asteroids 总被引：1，自引：0，他引：1
D.J. Scheeres 《Icarus》2002,159(2):271-283
The stability and final outcome of a strongly interacting binary asteroid system is considered. We discuss the implications of the system transferring energy and angular momentum between rotational and translational motion while conserving the total system energy and angular momentum. Using these results we can develop a set of sufficient conditions for stability against escape and impact. These allow us to delineate several classes of final outcomes for a binary asteroid system, each of which may have implications for asteroid observations. The effects of energy dissipation on an asteroid binary system are also considered and are shown to be able to change the stability of the system against escape and impact. An example computation for the near-Earth asteroid binary 1996 FG3 is given along with a series of numerical explorations of an evolving binary system consisting of an ellipsoid and a sphere of equal mass. 相似文献
12.
Taeyoung Lee Melvin Leok N. Harris McClamroch 《Celestial Mechanics and Dynamical Astronomy》2007,98(2):121-144
Equations of motion, referred to as full body models, are developed to describe the dynamics of rigid bodies acting under
their mutual gravitational potential. Continuous equations of motion and discrete equations of motion are derived using Hamilton’s
principle. These equations are expressed in an inertial frame and in relative coordinates. The discrete equations of motion,
referred to as a Lie group variational integrator, provide a geometrically exact and numerically efficient computational method
for simulating full body dynamics in orbital mechanics; they are symplectic and momentum preserving, and they exhibit good
energy behavior for exponentially long time periods. They are also efficient in only requiring a single evaluation of the
gravity forces and moments per time step. The Lie group variational integrator also preserves the group structure without
the use of local charts, reprojection, or constraints. Computational results are given for the dynamics of two rigid dumbbell
bodies acting under their mutual gravity; these computational results demonstrate the superiority of the Lie group variational
integrator compared with integrators that are not symplectic or do not preserve the Lie group structure. 相似文献
13.
We discuss the consequences of momentum conservation in processes related to solar flares and coronal mass ejections (CMEs),
in particular describing the relative importance of vertical impulses that could contribute to the excitation of seismic waves
(“sunquakes”). The initial impulse associated with the primary flare energy transport in the impulsive phase contains sufficient
momentum, as do the impulses associated with the acceleration of the evaporation flow (the chromospheric shock) or the CME
itself. We note that the deceleration of the evaporative flow, as coronal closed fields arrest it, will tend to produce an
opposite impulse, reducing the energy coupling into the interior. The actual mechanism of the coupling remains unclear at
present. 相似文献
14.
We determine the possible masses and radii of the progenitors of white dwarfs in binaries from fits to detailed stellar evolution models and use these to reconstruct the mass-transfer phase in which the white dwarf was formed. We confirm the earlier finding that in the first phase of mass transfer in the binary evolution leading to a close pair of white dwarfs, the standard common-envelope formalism (the α-formalism) equating the energy balance in the system (implicitly assuming angular momentum conservation) does not work. An algorithm equating the angular momentum balance (implicitly assuming energy conservation) can explain the observations. This conclusion is now based on 10 observed systems rather than three. With the latter algorithm (the γ-algorithm) the separation does not change much for approximately equal-mass binaries. Assuming constant efficiency in the standard α-formalism and a constant value of γ, we investigate the effect of both methods on the change in separation in general and conclude that when there is observational evidence for strong shrinkage of the orbit, the γ-algorithm also leads to this. We then extend our analysis to all close binaries with at least one white dwarf component and reconstruct the mass-transfer phases that lead to these binaries. In this way we find all possible values of the efficiency of the standard α-formalism and of γ that can explain the observed binaries for different progenitor and companion masses. We find that all observations can be explained with a single value of γ, making the γ-algorithm a useful tool to predict the outcome of common-envelope evolution. We discuss the consequences of our findings for different binary populations in the Galaxy, including massive binaries, for which the reconstruction method cannot be used. 相似文献
15.
16.
Ragab Mohamed Gad 《Astrophysics and Space Science》2004,293(4):453-460
We calculate the energy and momentum distributions associated with a Gödel-type space–time, using the well-known energy–momentum complexes of Landau–Lifshitz and Møller. We show that the definitions of Landau–Lifshitz and Møller do not furnish a consistent result. 相似文献
17.
Patrick Michel Giovanni H. Valsecchi 《Celestial Mechanics and Dynamical Astronomy》1996,65(4):355-371
We discuss the efficiency of the so-called mixed-variable symplectic integrators for N-body problems. By performing numerical experiments, we first show that the evolution of the mean error in action-like variables is strongly dependent on the initial configuration of the system. Then we study the effect of changing the stepsize when dealing with problems including close encounters between a particle and a planet. Considering a previous study of the slow encounter between comet P/Oterma and Jupiter, we show that the overall orbital patterns can be reproduced, but this depends on the chosen value of the maximum integration stepsize. Moreover the Jacobi constant in a restricted three-body problem is not conserved anymore when the stepsize is changed frequently: over a 105 year time span, to keep a relative error in this integral of motion of the same order as that given by a Bulirsch-Stoer integrator requires a very small integration stepsize and much more computing time. However, an integration of a sample including 104 particles close to Neptune shows that the distributions of the variation of the elements over one orbital period of the particles obtained by the Bulirsch-Stoer integrator and the symplectic integrator up to a certain integration stepsize are rather similar. Therefore, mixed-variable symplectic integrators are efficient either for N-body problems which do not include close encounters or for statistical investigations on a big sample of particles. 相似文献
18.
Ariadna Farrés Jacques Laskar Sergio Blanes Fernando Casas Joseba Makazaga Ander Murua 《Celestial Mechanics and Dynamical Astronomy》2013,116(2):141-174
Using a Newtonian model of the Solar System with all 8 planets, we perform extensive tests on various symplectic integrators of high orders, searching for the best splitting scheme for long term studies in the Solar System. These comparisons are made in Jacobi and heliocentric coordinates and the implementation of the algorithms is fully detailed for practical use. We conclude that high order integrators should be privileged, with a preference for the new $(10,6,4)$ method of Blanes et al. (2013). 相似文献
19.
We present numerical simulations of the axisymmetric accretion of a massive magnetized plasma torus on a rotating black hole. We use a realistic equation of state, which takes into account neutrino cooling and energy loss due to nucleus dissociations. The calculation are performed in the ideal relativistic MHD approximation using an upwind conservative scheme that is based on a linear Riemann solver and the constrained transport method to evolve the magnetic field. The gravitational attraction of the black hole is introduced via the Kerr metric in the Kerr–Schild coordinates. We simulate various magnetic field configurations and torus models, both optically thick and thin for neutrinos.We have found an effect of alternation of the magnetic field orientation in the ultrarelativistic jet formed as a result of the collapse. The calculations show evidence for heating of the wind surrounding the collapsar by the shock waves generated at the jet–wind border. It is shown that the neutrino cooling does not significantly change either the structure of the accretion flow or the total energy release of the system. The angular momentum of the accreting matter defines the time scale of the accretion. Due to the absence of the magnetic dynamo in our calculations, the initial strength and topology of the magnetic field determines the magnetization of the black hole, jet formation properties and the total energy yield. We estimate the total energy of accretion which transformed to jets as 1.3 × 1052 ergs which was sufficient to explain hypernova explosions like GRB 980425 or GRB 030329. 相似文献
20.
Plasma and magnetic field data from PROGNOZ-7 have revealed that solar wind (magnetosheath) plasma elements may penetrate the dayside magnetopause surface and form high density regions with enhanced cross-field flow in the boundary layer.The injected magnetosheath plasma is observed to have an excess drift velocity as compared to the local boundary layer plasma, comprising both “cold” plasma of terrestrial origin and a hot ring current component. A differential drift between two plasma components can be understood in terms of a momentum transfer process driven by an injected magnetosheath plasma population. The braking action of the injected plasma may be described as a dynamo process where particle kinetic energy is transferred into electromagnetic energy (electric field). The generated electric field will force the local plasma to , and the dynamo region therefore also constitutes an accelerator region for the local plasma. Whenever energy is dissipated from the energy transfer process (a net current is flowing through a load), there will also be a difference between the induced electric field and the term of the generator plasma. Thus, the local plasma will drift more slowly than the injected generator plasma.We will present observations showing that a relation between the momentum transferred, the injected plasma and the momentum taken up by the local plasma exists. For instance, if the local plasma density is sufficiently high, the differential drift velocity of the injected and local plasma will be small. A large fraction of the excess momentum is then transferred to the local plasma. Conversely, a low local plasma density results in a high velocity difference and a low fraction of local momentum transfer.In our study cases the “cold” plasma component was frequently found to dominate the local magnetospheric plasma density in the boundary layer. Accordingly, this component may have the largest influence on the local momentum transfer process. We will demonstrate that this also seems to be the case. Moreover we show that the accelerated “cold” plasma component may be used as a tracer element reflecting both the momentum and energy transfer and the penetration process in the dayside boundary layer.The high He+ percentage of the accelerated “cold” plasma indicates a plasmaspheric origin. Considering the quite high densities of energetic He+ found in the boundary layer, the overall low abundance of He+ (as compared to e.g. O+) found in the plasma sheet and outer ring current evidently reduces the importance of the dayside boundary layer as a plasma source in the large scale magnetospheric circulation system. 相似文献