[overture] Re: cgins parameters
- From: Bill Henshaw <henshaw@xxxxxxxx>
- To: "overture@xxxxxxxxxxxxx" <overture@xxxxxxxxxxxxx>
- Date: Fri, 20 Jan 2012 10:07:54 -0800
Hi Alessandro,
Alessandro Orchini wrote:
Hi all,
I have a few questions about cgins.
1) I read the Reference Manual "Cgins: A Solver for the Incompressible
Navier–Stokes Equations" and I also looked at the reference "On the
smallest scale for the incompressible Navier-Stokes equations", but I
can't figure out if and how much the artificial diffusion (second
order) influence the simulations.
For example, if I run a simulation with \nu=1.0e-4, is it ok to choose
values for artificial diffusion ad21 = ad22 = 1? How can I know /a
priori/ if they're too big?
Suppose you want to solve the INS equations with a given value of nu. If
you want to
have a fully resolved computation then you should choose the grid
spacing according
to the formula on page 13 of the Cgins ref. manual:
h = C * sqrt( nu/ Du )
The local grid spacing h is related to the local gradient of the
velocity, Du. The constant C
seems to normally be about 1. Thus you really can't tell the correct
value for h
without knowing what the solution looks like (approximately).
If you choose h in this way then you should need NO artificial dissipation.
If you can't afford to use a very fine grid then you can use
artificial dissipation.
You can think of this as an LES turbulence model. To have as small
effect as possible
you should choose the dissipation to be as small as possible BUT large
enough so the
solution remains stable. These leads to the recommendations in the ref.
manual.
If you want to know if the dissipation is affecting your result then you
should run
the problem with a finer grid and see if the things you are interested
in change. If they
don't change very much then you are probably OK.
2) I'd like to know precisely how the "project initial condition"
option actually work, does anyone know if there's some reference about
it, or where the source code is located in Overture?
Unfortunately, I don't recall ever writing up the precise details of the
projection.
However, if you want to know where the projection is computed in the
code a good guess would be
provided by using "grep" to find out who is printing the messages that
are written to
the screen when the projection is performed:
projectVelocity>>> iteration=0, (new div)/(old div)= 0.065, divergence
after projection= 1.61e+00
projectVelocity>>> iteration=1, (new div)/(old div)= 0.127, divergence
after projection= 2.05e-01
projectVelocity>>> iteration=2, (new div)/(old div)= 0.902, divergence
after projection= 1.85e-01
projectVelocity>>> iteration=3, (new div)/(old div)= 0.998, divergence
after projection= 1.84e-01
You would then find that this is done in the file cg/ins/src/project.C
Regards,
Bill
Thanks for the help
Alessandro
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