Re: [ATM] synthetic jets

[Gary Seronik <gseronik@netway.com>]

Hi Don:

At 06:57 AM 6/2/2004 -0700, Don Clement wrote:
>Georgia Tech has come up with a method of breaking up thermal boundary
>layers by creating an airflow with tiny vortices. Synthetic jets arrays are
>created using electromagnetic or piezoelectric drivers vibrating a diaphragm
>at 100 to 200 Hz. The drivers look like small loud speakers. There are no
>friction parts to the drivers. I was thinking that this technology could
>replace fans blowing across the face of mirrors to break up the thermal
>boundary layer.

I'm not familiar with this method and it may indeed have some benefits for
us, but what could be simpler than a $5 fan?

That issue aside, it's worth pointing out that no matter how you break up
the thermal boundary layer, so long as it persists, you're going to have
trouble -- and the only way to get rid of it is to get your mirror down to
ambient and keep it there.  The main objective of having a fan is to cool
the mirror down so that a thermal boundary layer can't form.  By blowing
across the face of the mirror you can accomplish this goal and usually
achieve the secondary (and less important) goal of breaking the boundary
layer into a finer pattern.  For my money, this is only marginally better
than a smooth, slowly moving one.  Bryan Greer's excellent two-part article
in the May and June Sky&Telescopes goes into detail about this.


Gary

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