Non repeatability issue

[Aldo Bonfiglioli]

Barry,
thanks for your prompt reply.

> 1)    Have you made runs where you require, say -ksp_rtol 1.e-12 to  
> eliminate the effects of
> not solving the linear systems accurately?

I am trying this right now, although it may require very many linear
iterations, particularly in the last Newton steps, when the CFL number
becomes infinite.
The systems arising from the fully coupled RANS eqns appear to be fairly 
ill-conditioned:
even in the 2D cases we generally need to increase the level of fill 
beyond 0
to obtain reasonable convergence of the linear solver.

In the 3D case (internal flow through an elbow) the previous plots refer 
to, there are
about 200,000 nodes times 5 dof per node and
I have been using BJ+ILU(1) with GMRES(60).

> 2) Have you run the exact example that you ran with geometric  
> decomposition also with
> the parmetis decomposition? Is that what you sent? (This is too  
> eliminate any fundamental
> differences with the two problems.) 

Precisely.

> 3) In your plots you plot L_2 norm of the mass residual while Newton  
> is running on all
> equations. This means the Newton's criteria for progress is based on  
> || u,v,m .....||
> as it chugs along. What do plots of || u,v, m....|| (that is what  
> Newton calls the residual
> when you use -snes_monitor) look like, are they also unstable? Are  
> they decreasing?

All residuals appear to be fairly un-stable. This is shown in the 
enclosed plot.
The strategy to ramp the CFL number looks at the ratio btw the current 
non-linear
residual _of one of the conservation eqns_ and the initial residual.
The Newton strategy adopted in the code is NOT implemented through 
PETSc's TS
(just for "historical" reasons).

> Sometimes people scale some equations stronger than others if those  
> are the residuals
> they are most interested in pushing down. What happens if you scale  
> the mass residual
> equations by some factor (say 100 or 1000) in your FormFunction? 

Never tried. We had once tested on a different problem row scaling
 without seeing noticeable differences. But I guess this may be
different from what you mention.


Regards,
Aldo

-- 
Dr. Aldo Bonfiglioli
Dip.to di Ingegneria e Fisica dell'Ambiente (DIFA)
Universita' della Basilicata
V.le dell'Ateneo lucano, 10 85100 Potenza ITALY
tel:+39.0971.205203 fax:+39.0971.205160

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