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A. V. Kolesnichenko 《Solar System Research》2011,45(3):246-263
The problem of the influence of vortex helicity on the synergic structuring of cosmic matter in it, as well as the appearance
of the effect of negative viscosity in three-dimensional gyrotropic turbulence, were studied in the framework of the fundamental
problem of simulating the evolution of a rotating astrophysical nonmagnetic disk—in particular, the accretion disk—surrounding
the Sun at the early stage of its existence. The evolution equations for averaged vorticity and vortex helicity, as well as
rheological relations for the turbulent flow of heat and asymmetrical tensor of the turbulent stress in helical turbulence,
were obtained. The demonstrative dependence of helicity on the rotation velocity, density (temperature) gradients, and turbulent
energy of the disk gas was established. The role of helicity in the appearance of the inverse Richardson-Kolmogorov energy
cascade from small vortices to larger ones and the related process of the generation of the power-consuming macroscale coherent
vortex formations appearing in gyrotropic turbulence at high Reynolds number were discussed. The results of the numerical
experiments confirming the real existence of the inverse energy cascade in helical turbulence were analyzed. It was assumed
that the relatively long-term decay of turbulence in the solar protoplanet cloud can be due to the absence of the reflective
symmetry of the anisotropic field of the turbulent velocities with respect to its equatorial plane. As the concept of the
inverse energy cascade in three-dimensional helical turbulence is more and more reliably confirmed in numerical experiments,
accounting for this effect affecting the structure and dynamics of the astrophysical nonmagnetic disk becomes important during
its simulation. 相似文献
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A. V. Kolesnichenko 《Solar System Research》2010,44(4):334-347
This paper considers the modern approach to the thermodynamic modeling of developed turbulent flows of a compressible fluid
based on the systematic application of the formalism of extended irreversible thermodynamics (EIT) that goes beyond the local
equilibrium hypothesis, which is an inseparable attribute of classical nonequilibrium thermodynamics (CNT). In addition to
the classical thermodynamic variables, EIT introduces new state parameters—dissipative flows and the means to obtain the respective
evolutionary equations consistent with the second law of thermodynamics. The paper presents a detailed discussion of a number
of physical and mathematical postulates and assumptions used to build a thermodynamic model of turbulence. A turbulized liquid
is treated as an indiscrete continuum consisting of two thermodynamic sub-systems: an averaged motion subsystem and a turbulent
chaos subsystem, where turbulent chaos is understood as a conglomerate of small-scale vortex bodies. Under the above formalism,
this representation enables the construction of new models of continual mechanics to derive cause-and-effect differential
equations for turbulent heat and impulse transfer, which describe, together with the averaged conservations laws, turbulent
flows with transverse shear. Unlike gradient (noncausal) relationships for turbulent flows, these differential equations can
be used to investigate both hereditary phenomena, i.e., phenomena with history or memory, and nonlocal and nonlinear effects.
Thus, within EIT, the second-order turbulence models underlying the so-called invariant modeling of developed turbulence get
a thermodynamic explanation. Since shear turbulent flows are widespread in nature, one can expect the given modification of
the earlier developed thermodynamic approach to developed turbulence modeling (see Kolesnichenko, 1980; 1998; 2002–2004; Kolesnichenko
and Marov, 1985; Kolesnichenko and Marov, 2009) to be used in research on a broad class of dissipative phenomena in various
astro- and geophysical applications. In particular, a major application of the proposed approach is the reconstruction of
the processes in the preplanetary circumsolar disk, which might help solve the fundamental problems of stellar-planetary cosmogony. 相似文献
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We formulate a complete system of equations of two-phase multicomponent mechanics including the relative motion of the phases, coagulation processes, phase transitions, chemical reactions, and radiation in terms of the problem of reconstructing the evolution of the protoplanetary gas-dust cloud that surrounded the proto-Sun at an early stage of its existence. These equations are intended for schematized formulations and numerical solutions of special model problems on mutually consistent modeling of the structure, dynamics, thermal regime, and chemical composition of the circumsolar disk at various stages of its evolution, in particular, the developed turbulent motions of a coagulating gas suspension that lead to the formation of a dust subdisk, its gravitational instability, and the subsequent formation and growth of planetesimals. To phenomenologically describe the turbulent flows of disk material, we perform a Favre probability-theoretical averaging of the stochastic equations of heterogeneous mechanics and derive defining relations for the turbulent flows of interphase diffusion and heat as well as for the “relative” and Reynolds stress tensors needed to close the equations of mean motion. Particular attention is given to studying the influence of the inertial effects of dust particles on the properties of turbulence in the disk, in particular, on the additional generation of turbulent energy by large particles near the equatorial plane of the proto-Sun. We develop a semiempirical method of modeling the coefficient of turbulent viscosity in a two-phase disk medium by taking into account the inverse effects of the transfer of a dispersed phase (or heat) on the growth of turbulence to model the vertically nonuniform thermohydrodynamic structure of the subdisk and its atmosphere. We analyze the possible “regime of limiting saturation” of the subdisk atmosphere by fine dust particles that is responsible for the intensification of various coagulation mechanisms in a turbulized medium. For steady motion when solid particles settle to the midplane of the disk under gravity, we analyze the parametric method of moments for solving the Smoluchowski integro-differential coagulation equation for the particle size distribution function. This method is based on the fact that the sought-for distribution function a priori belongs to a certain parametric class of distributions. 相似文献
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An heuristic way of modeling the turbulent exchange coefficients for Keplerian accretion disks surrounding solar-type stars is considered. The formulas for these coefficients, taking into account the inverse effects of dust transfer and potential temperature on the maintenance of shear turbulence, generalize to protoplanetary gas–dust clouds the expression for the turbulent viscosity coefficient in so-called a-disks which was obtained in a classic work by Shakura and Syunyaev (1973). The defining relationships are derived for turbulent diffusion and heat flows, which describe, for the two-phase mixture rotating differentially at an angular velocity O(r, z), the dust and heat transfer in the direction perpendicular to the central plane of the disk. The regime of limiting saturation by small dust particles of the layer of “cosmic fluid” located slightly above (or below) the dust subdisk is analyzed. 相似文献
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This paper considers, in the context of modeling the evolution of a protoplanetary cloud, the hydrodynamic aspects of the theory of concurrent processes of mass transfer and coagulation in a two-phase medium in the presence of shear turbulence in a differentially rotating gas–dust disk and of polydisperse solid particles suspended in a carrying flow of solid particles. The defining relations are derived for diffuse fluxes of particles of different sizes in the equations of turbulent diffusion in the gravitational field, which describe the convective transfer, turbulent mixing, and sedimentation of disperse dust grains onto the central plane of the disk, as well as their coagulation growth. A semiempirical method is developed for calculating the coefficients of turbulent viscosity and turbulent diffusion for particles of different kinds. This method takes into account the inverse effects of dust transfer on the turbulence evolution in the disk and the inertial differences between disperse solid particles. To solve rigorously the problem of the mutual influence of the turbulent mixing and coagulation kinetics in forming the gas–dust subdisk, the possible mechanisms of gravitational, turbulent, and electric coagulation in a protoplanetary disk are explored and the parametric method of moments for solving the Smoluchowski integro-differential coagulation equation for the particles' size distribution function is considered. This method takes into account the fact that this distribution belongs to a definite parametric class of distributions. 相似文献
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An attempt is made to construct a phenomenological model of turbulence as a self-organization process in an open system. The representation of a turbulized continuum in the form of a thermodynamic complex consisting of two subsystems—the subsystem of averaged motion and the subsystem of turbulent chaos, which is considered, in turn, as a conglomerate of vortex structures of different space–time scales—made it possible to obtain, by methods of nonequilibrium thermodynamics, the defining relationships for the turbulent fluxes and forces that describe most comprehensively the transport and structurization processes in such a continuum. Using two interpretations of the Kolmogorov parameter (as a quantity that describes the rate of dissipation of energy into heat and as the rate of transfer of turbulent energy in the eddy cascade), the defining relationships were found for this quantity, thereby making the thermodynamic approach self-sufficient. An introduction into the model of internal parameters of the medium, which characterize the excitation of macroscopic degrees of freedom, made it possible to describe thermodynamically the Kolmogorov cascade process and to obtain a variety of kinetic equations (of the Fokker–Planck type in the configuration space) for the functions of distribution of small-scale turbulence characteristics, including the unsteady kinetic equation for the distribution of probability of dissipation of turbulent energy. As an example, a detailed derivation of such relationships is given for the case of stationary turbulence, when a tendency toward local isotropy is observed. In view of the wide occurrence of this phenomenon in nature, one might expect that the developed approach to the problem of modeling strong turbulence will find its use in astrophysical and geophysical applications. 相似文献
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We present a new experimental platform for studies of turbulence and turbulent mixing in accelerating and rotating fluids.
The technology is based on the ultra-high performance optical holographic digital data storage. The state-of-the-art electro-mechanical,
electronic, and laser components allow for realization of turbulent flows with high Reynolds number (>107) in a relatively small form-factor, and quantification of their properties with extremely high spatio-temporal resolutions
and high data acquisition rates. The technology can be applied for investigation of a large variety of hydrodynamic problems
including the fundamental properties of non-Kolmogorov turbulence and turbulent mixing in accelerating, rotating and multiphase
flows, magneto-hydrodynamics, and laboratory astrophysics. Unique experimental and metrological capabilities enable the studies
of spatial and temporal properties of the transports of momentum, angular momentum, and energy and the identification of scalings,
invariants, and statistical properties of these complex turbulent flows. 相似文献
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A. V. Kolesnichenko 《Solar System Research》2008,42(5):391-404
A stochastic-thermodynamic approach to the derivation of the generalized fractional Fokker—Planck—Kolmogorov (FFPK) equations
is considered. The equations describe turbulent transfer processes in a subsystem of turbulent chaos on the basis of fractional
dynamics, which takes into account the structure and metric of fractal time. The actual turbulent motion of a fluid is known
to be intermittent, since it demonstrates the properties that are intermediate between the properties of regular and chaotic
motions. On the other hand, the process of the flow turbulization may be non-Markovian because of the multidimensional spatiotemporal
correlations of pulsating parameters; in a physical language, this means that the process has a memory. The introduction of
fractional time derivatives into the FFPK kinetic equations, used to find the probability distribution functions for different
statistical characteristics of structured turbulence, makes it possible to use an unified mathematical formalism in considering
the effects of memory, nonlocality, and time intermittence, with which we usually associate the presence of turbulent bursts
against the background of less intense low-frequency oscillations in the background turbulence. This study is aimed at creating
representative models of space and natural media. It is a development of the synergetic approach to the modeling of structured
turbulence in astrogeophysical systems, which has been developed by the author in a series of papers (Kolesnichenko, 2002–2005). 相似文献
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We study the scattering of low-energy cosmic rays (CRs) in a turbulent, compressive magnetohydrodynamic (MHD) fluid. We show that compressible MHD modes – fast or slow waves with wavelengths smaller than CR mean free paths induce cyclotron instability in CRs. The instability feeds the new small-scale Alfvénic wave component with wavevectors mostly along magnetic field, which is not a part of the MHD turbulence cascade. This new component gives feedback on the instability through decreasing the CR mean free path. We show that the ambient turbulence fully suppresses the instability at large scales, while wave steepening constrains the amplitude of the waves at small scales. We provide the energy spectrum of the plane-parallel Alfvénic component and calculate mean free paths of CRs as a function of their energy. We find that for the typical parameters of turbulence in the interstellar medium and in the intercluster medium the new Alfvénic component provides the scattering of the low-energy CRs that exceeds the direct resonance scattering by MHD modes. This solves the problem of insufficient scattering of low-energy CRs in the turbulent interstellar or intracluster medium that was reported in the literature. 相似文献
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A study is made of axisymmetric, low sonic-Mach-number flows of a viscous fluid with angular momentum outside of a black-hole. The viscosity is an eddy viscosity due to turbulence in the sheared flows. Self-similar solutions arise naturally, reducing the Navier-Stokes equations to a set of nonlinear ordinary differential equations. These equations are solved analytically for flows of constant specific angular momentum and numerically for more general flows. For flows with non-constant specific angular momentum, the momentum flux density includes a planar discontinuity which is interpreted as an accretion disc. In general, two flow regions appear on each side of the disk, corresponding to accretion onto the disk and jet-like outflows along the ±z-axes. Physical interpretations of the solutions show that these flows arise in response to point sources of axial momentum at the origin directed in the ±z-directions. The power needed to maintain this momentum input is assumed to come from the mass accretion onto the black hole.The hydrodynamic flows are generalized to include a magnetic field. In the limit of infinite electrical conductivity, the possible types of flow patterns are the same as in hydrodynamic case. The magnetic field alters the relative amounts of reversible and irreversible momentum and angular momentum transport by the flow. For a flow with turbulent viscosity, the magnetic field acts to reduce the level of the turbulence and the effective value of the eddy viscosity. 相似文献
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Identifying generic physical mechanisms responsible for the generation of magnetic fields and turbulence in differentially rotating flows is fundamental to understand the dynamics of astrophysical objects such as accretion disks and stars. In this paper, we discuss the concept of subcritical dynamo action and its hydrodynamic analogue exemplified by the process of nonlinear transition to turbulence in non‐rotating wall‐bounded shear flows. To illustrate this idea, we describe some recent results on nonlinear hydrodynamic transition to turbulence and nonlinear dynamo action in rotating shear flows pertaining to the problem of turbulent angular momentum transport in accretion disks. We argue that this concept is very generic and should be applicable to many astrophysical problems involving a shear flow and non‐axisymmetric instabilities of shearinduced axisymmetric toroidal velocity or magnetic fields, such as Kelvin‐Helmholtz, magnetorotational, Tayler or global magnetoshear instabilities. In the light of several recent numerical results, we finally suggest that, similarly to a standard linear instability, subcritical MHD dynamo processes in high‐Reynolds number shear flows could act as a large‐scale driving mechanism of turbulent flows that would in turn generate an independent small‐scale dynamo. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Recent advances in understanding of the basic properties of compressible Magnetohydrodynamic (MHD) turbulence call for revisions of some of the generally accepted concepts. First, the MHD turbulence is not so messy as it is usually believed. In fact, the notion of strong nonlinear coupling of compressible and incompressible motions is not tenable. Alfven, slow and fast modes of MHD turbulence follow their own cascades and exhibit degrees of anisotropy consistent with theoretical expectations. Second, the fast decay of turbulence is not related to the compressibility of fluid. Rates of decay of compressible and incompressible motions are very similar. Third, the viscosity by neutrals does not suppress MHD turbulence in a partially ionized gas. Instead, MHD turbulence develops magnetic cascade at scales below the scale at which neutrals damp ordinary hydrodynamic motions. The implications of those changes of MHD turbulence paradigm for molecular clouds require further studies. Those studies can benefit from testing of theoretical predictions using new statistical techniques that utilize spectroscopic data. We briefly discuss advances in development of tools using which the statistics of turbulent velocity can be recovered from observations. 相似文献
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An analysis is presented of the hydrodynamic aspects of the growth of protostellar disks from the accretion (or collapse) of a rotating gas cloud. The size, mass, and radiative properties of protostellar disks are determined by the distribution of mass and angular momentum in the clouds from which they are formed, as well as from the dissipative processes within the disks themselves. The angular momentum of the infalling cloud is redistributed by the action of turbulent viscosity on a shear layer near the surface of the disk (downstream of the accretion shock) and on the radial shear across cylindrical surfaces parallel to the rotation axis. The fraction of gas that is fed into a central core (protostar) during accretion depends on the ratio of the rate of viscous diffusion of angular momentum to the accretion rate; rapid viscous diffusion (or a low accretion rate) promotes a large core-to-disk mass ratio. The continuum radiation spectrum of a highly viscous disk is similar to that of a steady-state accretion disk without mass addition. It is possible to construct models of the primitive solar nebula as an accretion disk, formed by the collapse of a slowly rotating protostellar cloud, and containing the minimum mass required to account for the planets. Other models with more massive disks are also possible. 相似文献
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Bruce G. Elmegreen 《Earth, Moon, and Planets》1978,19(2):261-277
The interaction between a strong stellar wind carrying no intrinsic angular momentum and a surrounding disk nebula is investigated. We analyze the shape and stability of the wind-nebula interface, the strength and direction of the ensuing mass motions and the time scales for nebular disruption. The resultant time scale is given by Equation (44). The dominant physical process is one of nebular accretion onto the central star due to turbulent viscosity in the disk. The turbulence will be driven in the upper layers of the disk by the wind. We note that if the accretion supplies mass for the wind (after the absorption of stellar energy), then the particle fluxes may undergo a runaway increase until the energy or momentum flux in the wind is limited by the total stellar luminosity. This may explain the origin of strong, pre-Main-Sequence winds.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978. 相似文献
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Magnetically mediated disk outflows are a leading paradigm to explain winds and jets in a variety of astrophysical sources, but where do the fields come from? Since accretion of mean magnetic flux may be disfavored in a thin turbulent disk, and only fields generated with sufficiently large scale can escape before being shredded by turbulence, in situ field production is desirable. Nonlinear helical inverse dynamo theory can provide the desired fields for coronae and outflows. We discuss the implications for contemporary protostellar disks, where the (magneto-rotational instability (MRI)) can drive turbulence in the inner regions, and primordial protostellar disks, where gravitational instability drives the turbulence. We emphasize that helical dynamos are compatible with the magneto-rotational instability, and clarify the relationship between the two. 相似文献
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Yu. I. Fedorov 《Kinematics and Physics of Celestial Bodies》2011,27(1):1-16
Acceleration of cosmic rays interacting with the anisotropic magnetohydrodynamic turbulent medium is studied. Particle acceleration
is caused by a large-scale electric field arising in a turbulent medium due to the α-effect. A comparison is made of equilibrium
spectra of cosmic rays, characteristic of the specific acceleration mechanism, with the energy distribution of particles corresponding
to the statistical Fermi acceleration. 相似文献