Non repeatability issue

[Aldo Bonfiglioli]

>
>
> 1) You have way too many Newton steps. Newton is quadratically
>convergent, so if
>      you have 100+ steps, it means you are very very far from the
>solution when you
>      begin. In this region, Newton is a really bad algorithm and can
>be very very
>      sensitive to perturbations. I would never expect it to be
>reproducible. From the
>      well-known fracint demo, it can be chaotic.
>

The initial solution from which Newton's algorithm is started
is obtained using a fixed point iterative scheme in which the
turbulence transport eqn is solved de-coupled
from the mean-flow eqns.
This de-coupled solution strategy usually stalls (in terms of residual
decrease) after a residual drop of a few orders of magnitude.
At this stage Newton is invoked.
 From a qualitative view-point (say at plotting level)
the initial solution is close to the fully converged one, but
I will try to quantify this and check how mutually far or close are the two.
Even for the 2D RANS eqns we have noticed a fairly "long"
plateau in terms of residual convergence history before
quadratic convergence occurs. This is also in line with
the few references on the subject I have in mind.
3D Euler is far more benign in our experience.

>  2) I would also question whether the system is actually stable.
>Instability can result
>      in occasional divergence based upon small perturbations.
>
We know this might occur. At this stage we are still
trying to make sure there are no implementation bugs
in our code.

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