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CATEGORIES:DAMTP Astro Mondays
SUMMARY:Zero net flux MRI-turbulence in disks – specific a
nisotropy of nonlinear processes\, sustenance and
dependence on magnetic Prandtl number - George Mam
atsashvili - Helmholtz-Zentrum Dresden-Rossendorf
DTSTART;TZID=Europe/London:20210426T140000
DTEND;TZID=Europe/London:20210426T150000
UID:TALK155902AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/155902
DESCRIPTION:We study the sustenance and effect of magnetic Pra
ndtl (Pm) number for\nMRI-turbulence in accretion
disks with a zero net magnetic flux in the\nsheari
ng-box. The zero flux case is unique\, as there is
no characteristic\nlength-scale for MRI that woul
d grow purely exponentially and hence the\ninstabi
lity is instead of a subcritical type\, being ener
getically powered by\nlinear nonmodal/transient me
chanism of perturbation growth. This transient\ngr
owth of MRI is “imperfect” in the sense that it is
not able to ensure\nlong-term sustenance of the t
urbulence. This should be compensated by a\nneces
sary positive nonlinear feedback\, regenerating no
nmodally growing MRI\nmodes. To examine the existe
nce of such a feedback and ultimately understand\n
the self-sustenance process\, we first perform num
erical simulations with\nSNOOPY code and then do a
detailed analysis of the turbulence dynamics in\n
Fourier space. Our main results can be summarized
as:\n\n1. It was shown that the flow shear leads t
o anisotropy of nonlinear processes\nin Fourier sp
ace. A key factor for the sustenance is the existe
nce and\ndominance of a topologically new type of
a nonlinear process – an angular\n(i.e.\, over wav
evector orientations) redistribution of modes in F
ourier space\,\nwhich we call the nonlinear transv
erse cascade in contrast to the classical\ndirect/
inverse cascade in classical (Kolmogorov\, Iroshni
kov-Kraichnan)\ntheories of isotropic turbulence w
ithout mean shear flow. The transverse\ncascade th
at we revealed is the generic nonlinear process of
decisive\nimportance in different kinds of shear
flows.\n\n2. The sustenance of the MRI-turbulence
is achieved by the interplay of the\nlinear nonmod
al growth of MRI and the nonlinear transverse casc
ade. These two\nprocesses mainly operate at length
scales comparable to the box size (disk\nscale he
ight) which we refer to as the vital area of the t
urbulence in Fourier\nspace. The usual direct casc
ade merely transfers the energy of these modes\nfr
om the vital area to large dissipative wavenumbers
(small scales).\n\n3. At large Pm\, the transvers
e cascade prevails over the direct one\, keeping\n
most of the mode energy contained in small wavenum
ber (large scales) modes.\nWith decreasing Pm\, th
e action of the nonlinear transverse cascade weake
ns\, so\nthat it can no longer oppose the action o
f the direct cascade\, which transfers\nenergy of
small wavenumber modes to higher wavenumber ones\,
where it is\nefficiently dissipated\, leading to
the decay of the turbulence. Thus\,\ndecreasing of
Pm results in the topological rearrangement of th
e nonlinear\nprocesses when the action of the dire
ct cascade begins to dominate over the\naction of
the key agent for the turbulence sustenance - the
nonlinear\ntransverse cascade.
LOCATION:Online
CONTACT:Cleo Loi
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