Oral, Chairs: A. P. Boss and J. J. Lissaurer, Thursday, September 4, 2003, 1:30-3:20pm, DeAnza I-II

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*I. Mosqueira (NASA Ames/SETI Institute), S. Kassinos (Univ. of Cyprus/Stanford Univ.), K. Shariff, J. N. Cuzzi (NASA Ames)*

For Keplerian disks to accrete a source of enhanced angular
momentum transport must be present in the disk. A key issue
is whether, in the absence of ``stirring'', hydrodynamic
shear turbulence can be self-sustaining in Keplerian disks
or has to be transient. Simulations by Hawley et al. (1999)
with resolutions up to 256^{3} using two codes with
different dispersion properties showed no evidence of a
non-linear shear instability. These authors interpreted this
result to mean that the stabilizing Coriolis force easily
overcame the non-linear terms in the Navier-Stokes equation
and led to the complete viscous decay of the turbulence.
Recently, on the basis of phenomenological arguments, it has
been claimed (Longaretti 2002) that this result stems from a
lack of resolution of the numerical codes, which results in
too low a Reynolds number for the non-linear instability to
be observed. Even if true, however, the mechanism for
sustaining turbulence remains to be elucidated. Kato and
Yoshizawa (1997) took steps in this direction by treating
non-linear pressure-strain fluctuating terms in a one-point
Reynolds-stress closure model, and argued that these terms
serve to re-distribute the energy of the fluctuations in
shear-driven anisotropic flows, and can counter the effects
of the energy sink due to the Coriolis term at sufficiently
small scales, thus possibly resulting in self-sustaining
shear turbulence. Yet, these authors did not model the
dissipation of turbulence.

More recently, Kassinos and Reynolds (2003, 2001) and
Reynolds, Kassinos and Langer (2002) have greatly improved
on prior one-point closure models of rotating shear flows by
adding turbulent structure information to the modeling of
the production and redistribution of turbulent kinetic
energy, as well as explicitly including the effects of
rotation in the dissipation of turbulence. In particular,
the dissipation rate \epsilon is obtained from a
large-scale enstrophy equation, which differs from standard
local treatments of the dissipation equation by
incorporating the inhibition of the energy cascade from
large to small scales due to the scrambling effects of
inertial waves in rotating frames of reference (Cambon et
al. 1997). Here we report on the results of direct numerical
simulations with resolution of 512^{3} (and possibly
1024^{3}) conducted by Kassinos et al. We will also discuss
structure-based modeling of unbounded turbulent shear flows
rotated about a spanwise axis, the success in fitting low to
medium Reynolds number numerical simulations, current
indications as to the regime of stability in Keplerian
flows, and the prospects for such modeling to provide
reliable high Reynolds number results.

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Bulletin of the American Astronomical Society, **35** #4

© 2003. The American Astronomical Soceity.