Re: ATM 'Frozen' liquid mirrors

[Suzanne Sarlette/Gerald Pearson <suegerry@mut1.muscanet.com>]

Richard Schwartz wrote:
> 
> -- [ From: Richard Schwartz * EMC.Ver #2.5.02 ] --
> 
> > The more precise and smooth the surfaces are, the thinner can be the film
> of
> > air that separates them.  The thinner the film, the smaller the air flow
> which
> > is required.  I suspect that, for conical surfaces in particular, a
> thinner
> > film also means less "wobble".
> 
> Sorry to lurk on this thread, but you have implied that there is some
> mechanism to damp out "wobble".  Is this some sort of electromagnetic damper
> , or is it inherent in the air bearing itself?  What is the mechanism?
> 

I was thinking in terms of simple geometry.  Say for example that you
have 2 surfaces 10" in diameter.  If they have an average separation of
0.010", then one surface could tip by 0.001 radian = 0.057 degree = 3.4
minutes relative to the other without touching.  If the separation is
only 0.0001", then the maximum tip would be only 1/100 of this.

I was also thinking in terms of a mechanical "fluid dynamical" mechanism
that might work like this:  In areas where the 2 surfaces are closer
than average, the air flow between the surfaces would tend to get
"squeezed off", the _speed_ would drop, this would reduce the Bernoulli
effect in this region, and the net outward pressure (air pressure minus
Bernoulli "suck") would increase, tending to push the surfaces apart. 
In areas where the 2 surfaces are farther apart than average, the air
flows easily and fast, increasing the Bernoulli effect, tending to suck
the surfaces together.  
        This is speculation on my part.  I do _not_ know a great deal about
fluid dynamics.  I'm a chemist with a heavy background in physics and
math, with a fair amount of practical experience in electronics and
scientific computer programming.  
        But there's got to be _some_ mechanism to dynamically stabilize air
bearings, or there would be nothing to prevent a "touch", and this would
mean that air bearings could not possibly work as well as we all know
that they do.  


> > of the Bernoulli (sp?) effect that tries to pull together 2 surfaces when
> air
> > is flowing between them.
> 
> I remember something about Reynold's number determining whether viscous or
> inertial forces dominate the picture. I think Bernoulli effect is an
> inertial force that is of little or no importance when the problem
> dimensions are very small.

FWIW, we see the Bernoulli effect in operation in conical-surface NMR
"spinners" -- cone apex angle perhaps 120 deg, outer dia about 25 mm,
inner dia about 15 mm, supported weight something like a few tens of
grams.  Temperature-regulated air flows up over the 5 mm dia sample tube
passing through the rotor ("spinner") for the purpose of "thermostating"
the sample temperature, and this cooling/heating air tries to pop the
sample & spinner up out of the stator.  One of our NMR spectrometers
normally uses a high "cooling gas" flow rate.  With _no_ "spinner air"
flowing through the tiny holes in the conical surface of the stator, the
sample is indeed lifted by some amount;  I don't know exactly how much,
because we cannot see this directly because it's all happening inside a
52 mm dia. room-temperature tube which passes through the center of a
superconducting magnet -- but we can easily see that the sample must be
above the small transmitter/receiver coil (10 mm high) which it normally
sits inside of, because the NMR signals disappear!  When we turn on at
least a small amount of "spinner air", we see the NMR signals again,
presumably because the spinner was sucked down against the stator by the
Bernoulli effect, thereby pulling the sample tube back into the
transmitter/receiver coil.

        -- Gerry