10,830 research outputs found
Radiative Bulk Viscosity
Viscous resistance to changes in the volume of a gas arises when different
degrees of freedom have different relaxation times. Collisions tend to oppose
the resulting departures from equilibrium and, in so doing, generate entropy.
Even for a classical gas of hard spheres, when the mean free paths or mean
flight times of constituent particles are long, we find a nonvanishing bulk
viscosity. Here we apply a method recently used to uncover this result for a
classical rarefied gas to radiative transfer theory and derive an expression
for the radiative stress tensor for a gray medium with absorption and Thomson
scattering. We determine the transport coefficients through the calculation of
the comoving entropy generation. When scattering dominates absorption, the bulk
viscosity becomes much larger than either the shear viscosity or the thermal
conductivity.Comment: 17 pages. Latex with referee style file of MNRAS (mn.sty). MNRAS, in
pres
A prospectus for a theory of variable variability
It is proposed that the kind of stellar variability exhibited by the Sun in its magnetic activity cycle should be considered as a prototype of a class of stellar variability. The signature includes long 'periods' (compared to that of the radial fundamental model), erratic behavior, and intermittency. As other phenomena in the same variability class we nominate the liminosity fluctuations of ZZ Ceti stars and the solar 160 m oscillation. We discuss the possibility that analogous physical mechanisms are at work in all these cases, namely instabilities driven in a thin layer. These instabilities should be favorable to grave modes (in angle) and should arise in conditions that may allow more than one kind of instability to occur at once. The interaction of these competing instabilities produces complicated temporal variations. Given suitable idealizations, it is shown how to begin to compute solutions of small, but finite, amplitude
Generation of large-scale winds in horizontally anisotropic convection
We simulate three-dimensional, horizontally periodic Rayleigh-B\'enard
convection between free-slip horizontal plates, rotating about a distant
horizontal axis. When both the temperature difference between the plates and
the rotation rate are sufficiently large, a strong horizontal wind is generated
that is perpendicular to both the rotation vector and the gravity vector. The
wind is turbulent, large-scale, and vertically sheared. Horizontal anisotropy,
engendered here by rotation, appears necessary for such wind generation. Most
of the kinetic energy of the flow resides in the wind, and the vertical
turbulent heat flux is much lower on average than when there is no wind
Destabilizing Taylor-Couette flow with suction
We consider the effect of radial fluid injection and suction on
Taylor-Couette flow. Injection at the outer cylinder and suction at the inner
cylinder generally results in a linearly unstable steady spiralling flow, even
for cylindrical shears that are linearly stable in the absence of a radial
flux. We study nonlinear aspects of the unstable motions with the energy
stability method. Our results, though specialized, may have implications for
drag reduction by suction, accretion in astrophysical disks, and perhaps even
in the flow in the earth's polar vortex.Comment: 34 pages, 9 figure
On the penetration of meridional circulation below the solar convection zone
Meridional flows with velocities of a few meters per second are observed in
the uppermost regions of the solar convection zone. The amplitude and pattern
of the flows deeper in the solar interior, in particular near the top of the
radiative region, are of crucial importance to a wide range of solar
magnetohydrodynamical processes. In this paper, we provide a systematic study
of the penetration of large-scale meridional flows from the convection zone
into the radiative zone. In particular, we study the effects of the assumed
boundary conditions applied at the convective-radiative interface on the deeper
flows. Using simplified analytical models in conjunction with more complete
numerical methods, we show that penetration of the convectively-driven
meridional flows into the deeper interior is not necessarily limited to a
shallow Ekman depth but can penetrate much deeper, depending on how the
convective-radiative interface flows are modeled.Comment: 13 pages, 5 figures. Subitted to Ap
Effect of Rossby and Alfv\'{e}n waves on the dynamics of the tachocline
To understand magnetic diffusion, momentum transport, and mixing in the
interior of the sun, we consider an idealized model of the tachocline, namely
magnetohydrodynamics (MHD) turbulence on a plane subject to a large
scale shear (provided by the latitudinal differential rotation). This model
enables us to self-consistently derive the influence of shear, Rossby and
Alfv\'{e}n waves on the transport properties of turbulence. In the strong
magnetic field regime, we find that the turbulent viscosity and diffusivity are
reduced by magnetic fields only, similarly to the two-dimensional MHD case
(without Rossby waves). In the weak magnetic field regime, we find a crossover
scale () from a Alfv\'{e}n dominated regime (on small scales) to a Rossby
dominated regime (on large scales). For parameter values typical of the
tachocline, is larger that the solar radius so that Rossby waves are
unlikely to play an important role in the transport of magnetic field and
angular momentum. This is mainly due to the enhancement of magnetic
back-reaction by shearing which efficiently generates small scales, thus strong
currents
The tachocline revisited
The solar tachocline is a shear layer located at the base of the solar
convection zone. The horizontal shear in the tachocline is likely turbulent,
and it is often assumed that this turbulence would be strongly anisotropic as a
result of the local stratification. What role this turbulence plays in the
tachocline dynamics, however, remains to be determined. In particular, it is
not clear whether it would result in a turbulent eddy diffusivity, or
anti-diffusivity, or something else entirely. In this paper, we present the
first direct numerical simulations of turbulence in horizontal shear flows at
low Prandtl number, in an idealized model that ignores rotation and magnetic
fields. We find that several regimes exist, depending on the relative
importance of the stratification, viscosity and thermal diffusivity. Our
results suggest that the tachocline is in the stratified turbulence regime,
which has very specific properties controlled by a balance between buoyancy,
inertia, and thermal diffusion.Comment: Invited review for the meeting Dynamics of the Sun and Stars:
Honoring the Life and Work of Michael J. Thompson (Boulder, Colorado, 24-26
September 2019
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