Re: [femm] Convergence problem with pure iron?

[Dave Squires <djsquires@xxxxxxxx>]

David Meeker wrote:

> uI've also been looking into ways to fix the sensitivity in the BH curves. It
> is easy to programatically check if the curves are OK, but I haven't decided
> whether or not I like the fix that I have implemented in my development
> version--I just repeatedly smooth the BH points using a 3-point moving
> average filter until the resulting fit is single valued. Now, this has the
> advantage that it always works in the sense of providing a smooth,
> single-valued BH curve that won't hang the solver. However, it is possible
> for a repeatedly smoothed curve not to match up that well with the original
> BH data points...any suggestions are welcome.
>
> Dave.

Dave,
I wrote a program some years ago for creating airfoils and smoothing the
surface curves. Sometimes I would have bad data points that gave the
sine(x)/x ringing effect. To fix this I would simply edit out the offending point

and do another cubic spline curve fit to get the new point between the
original ones. In essence I let the cubic spline algorithm do the curve fit
to get me an "accurate" data point between the others. To do this you
have to know which point is bad, take it out, re-interpolate and get back
a new point and re-insert it into the data. It seemed to work really well
and the airfoil analysis program then gave good results of airflow, pressure
gradient, and lift. Any bad data point would have amplified effects in
the flow solver and it was obvious when it was fixed "right" by letting
the spline interpolator calculate a "good" data point.
That would probably be easier to do. You just need a routine to determine
the bad point and then recalculate a new one between existing ones.
In fact if you want to be sure then take out two or more data points and
recalculate new ones using the cubic spline algorithm. Only at the knee
of a curve do you have to be careful to only take out one point at a time
if needed.

Regards,
Dave Squires