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# Re: [femm] Convergence problem with pure iron?

- To: femm@yahoogroups.com
- Subject: Re: [femm] Convergence problem with pure iron?
- From: Dave Squires <djsquires@plix.com>
- Date: Tue, 15 May 2001 10:27:55 -0700
- References: <9dmnch+rdgg@eGroups.com> <3B013504.6D77C3C3@foster-miller.com>

    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

- References:
  - Convergence problem with pure iron?
    - From: uwenh@gmx.de
  - Re: [femm] Convergence problem with pure iron?
    - From: David Meeker <dmeeker@foster-miller.com>

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