Re: [ATM] Thermal mirror deformation -fan on both sides

["Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>]

About http://home.hetnet.nl/~artm/atm/articles/cooling.html

Ric wrote:
> I'm somewhat surprised more comments haven't been
> advanced on this specific example of the effects of

Yes, so was I!
But from the low activity I guessed that a large part of the list members
was on holidays...


Tony wrote:
> I don't think it's a matter of how important the work is but is it a
useful,
> validated tool? It's interesting but I don't think anyone really knows
what
> to do with it. Everyone knows that performance will be degraded while the
> mirror is equalizing to ambient. My personal feeling is that actual field
> measurements would be more useful.

I know that most of the ATMs are pragmatic, and are more inclined to
building than theorizing.
Personally, I believe that not only the observation that methods work is
important, but also the reasons why they work. Understanding the underlying
mechanisms can open up completely new ways of doing things, which you would
not think of while building by intuition. (Think of another FEM based tool:
PLOP)

There have been many discussions on the list about thermal behaviour of
mirrors, most were driven by qualitative arguments and subjective evidence.
What I have tried is to quantisize behaviour, to give at least a feeling for
the magnitude of the effects of convection and radiation. Of course the
model is only as good as it is, but at least it gives you a vehicle to
analyze the effect of changes you make to it. For example, I have been
playing with non-uniform convective cooling profiles.

A corroboration of the results can be found in the afore mentioned
observations by Wolfgang Rohr (www.astro-foren.de/showthread.php?t=6454).
This leads me to believe that the results of the FEM model at least are in
the ballpark.

> a flow of air that surrounds the mirror, front back and sides. The actual
> airflow is much more complicated then Jan's simple infinite diameter disk
> model suggests and is also subject to turbulence and dead areas induced by

Yes, of course it is, but as a first approximation of cooling times it still
gives a good estimate of time. The simple model assumes a medium with very
high heat capacity, which free-flowing air certainly is not. Lots of forced
airflow approach it though.

The FlexPDE model discussed later in the article gives you a more elaborate
picture. Just try it, and play with it: FlexPDE is not so hard to
understand, especially when you have an example to work with:
home.hetnet.nl/~artm/atm/resource/cooling.zip
You can download FlexPDE from: www.pdesolutions.com/ along with the free
student license you will need to run it.


    Arjan te Marvelde
    www.udjat.nl




_______________________________________________
ATM mailing list http://www.atmlist.net/