Non repeatability issue

Barry Smith bsmith at
Wed Mar 12 12:26:28 CDT 2008


     I would scale the turbulence equation to match the scaling of the  
rest of the equations.

      I suspect you are ramping up the CFL number too quickly. For a  
"continuation" type
Newton method to work well (in my mindset anyways), you need to have  
the solution
well converged before moving on to the next "continuation" point.  
Worth experimenting


On Mar 12, 2008, at 11:02 AM, Aldo Bonfiglioli wrote:

> 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
> 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
> <convhst.pdf>

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