Re: shock wave as pressure builds in a vacuum?

[Stefan Jeglinski <jeglin@4PI.COM>]

> That's a start in the right direction.
> I believe the key variables are

<snip>

Thank you John for a concise analysis.

>  > What is a reasonable v?
>  > Not large, intuition tells me 10cm/sec into a vacuum is a reasonable
>  > guess.
>
> My intuition differs.
>
> I would expect that catastrophic venting due
> to a broken window would lead to air moving
> in with velocity on the order of 340 m/s.

Yes, Roger Haar privately pointed out to me earlier that my intuition
was decidedly bad on this point. And actually, the issue is whether
the window would break when the chamber is catastrophically vented,
not the other way around. I see that Roger may have experience in
exactly the sort of issue I speak of, although he speaks of a
[presumably] tougher Be window.

> You can get the same answer by approximating
> ordinary static air in a box as a "standing
> wave" composed of 340 m/s rightward flow
> superposed with 340 m/s leftward flow.  The
> flow turns around at the boundaries.  This
> turnaround is what produces pressure on the
> boundaries.  If you remove the right-hand
> boundary, the rightward flow just exits
> stage right, ballistically.  Reality is a
> little more complicated than this, since
> there is a broad distribution of velocities.
> Still, the velocities are peaked around the
> characteristic velocity.

This is easy to picture, thanks. Roger's analogy was a piston (in an
engine) pulling in air. Putting 34000cm/sec straight into the
calculation I presented yields 35 psi... or 17 without my factor of 2.


> SJ also questiond my "sound wave" analysis.
>
> I think I muffed a factor of two.

I dunno --- 35, 28, 17, pretty good for this level of analysis...


> If the chamber has a shape resembling a horn,
> it could be very effective at focussing this
> sound onto the membrane.

Fortunately perhaps, it is anything but, usually with lots of
baffling but no direct protection of the window. Turbulence kicking
around dust or small dirt particles is also a definite issue. Often
these windows (when broken) will show evidence of projectile
puncture. But not all, hence the original question.

Thanks all for the insights. I've corrected my intuition about air
rushing into a vacuum.


Stefan Jeglinski