[Nek5000-users] large CFL numbers

[nek5000-users at lists.mcs.anl.gov]

Hi Frank,

If you're constrained by the convective CFL (only) and not
by, say, free-surface motion or capillary waves, etc., then
you can use the characteristics scheme, which allows CFL of
2-5, say, vs. 0.5.

IFCHAR T

in the .rea file --- Look at the results in

  www.mcs.anl.gov/~fischer/users.pdf


Characteristics is not working for free-surface flows but
it should work for your stress-induced flows, I believe.

Paul


On Fri, 23 Apr 2010, nek5000-users at lists.mcs.anl.gov wrote:

> Hello all,

I am wondering if there is any fully implicit scheme in NEK5000 that
would allow the use of much larger CFL numbers.  The question arises due
to the different behavior of certain flows driven by surface tension
gradients in contrast to bluff body or fully turbulent flows.  Many
surface tension driven flows have fluid motion with a time scale that is
very large relative to the time scale based on the velocity and length
scale of the system.  In other words the Strouhal number is ~50X smaller
than for bluff body flows.  If using an explicit scheme for the
convective terms, the result is that the time step is restricted by the
stability constraints of the convection scheme to be ~50X smaller than
is needed to accurately resolve the temporal scales of the flow.

Cheers,
Frank

-- 
Frank Herbert Muldoon, Ph.D. Mechanical Engineering
Technische Universität Wien (Technical University of Vienna)
Inst. f. Strömungsmechanik und Wärmeübertragung (Institute of Fluid
Mechanics and Heat Transfer)
Resselgasse 3
1040 Wien
Tel: +4315880132232
Fax: +4315880132299 
Cell:+436765203470
fmuldoo (skype)
http://tetra.fluid.tuwien.ac.at/fmuldoo/public_html/webpage/frank-muldoon.html

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