Re: shock wave as pressure builds in a vacuum?

[Brian Whatcott <betwys@SBCGLOBAL.NET>]

At 03:52 PM 5/28/2003 -0400, Stefan Jeglinski , you wrote:
> /snip/
>
> FWIW, the membrane is an ultrathin polymer window that allows x-rays
> in the 100eV range to pass. /snip/
> Normally, venting to atmosphere is done slowly, but there are
> instances of window breakage during catastrophic venting. The physics
> question is whether it is plausible that catastrophic venting alone
> can break the window, which is designed to withstand a 1 atm
> differential under steady state conditions

/snip/

> Let's do back of the envelope, zeroth approximation. Force due to a
> fluid cylinder of cross-sectional area A moving at velocity v,
> density rho, that impinges on a flat surface, and assuming worst case
> elastic collision, is 2.A.rho.v^2. Normalize A to get pressure,
> assume rho is atmospheric density at RT (the in-rushing air is from
> the atmosphere, approx 1 gram per 1000 cc). What is a reasonable v?
> Not large, intuition tells me 10cm/sec into a vacuum is a reasonable
> guess. My calculation of collision pressure is 2e-6 N/cm^2, or 3e-6
> psig. The more likely inelastic collision (cf wind against a sail),
> and compression effects make this even smaller (right?). Even if I
> increased v by 3 orders of magnitude, I'm still relatively small at
> 0.3 psig. Please check my math and try not to embarrass me too much
> if you find a mistake :-)

/snip/

> Stefan Jeglinski


Likening Stefan's situation to the classical problem of rapid inrush of
room temperature air into an insulated vacuum bottle, where a temperature
 rise of more than 100K is expected, the thin polymer membrane will
possibly not have been selected for strength at elevated temperature,
and so may be weakened  to its yield point locally.    If that were the case,
a high temperature film might suit the purpose.
(Inveterate list members may recall that a certain UBC Emeritus posed a
contrasting problem: if a tubeless tire leaks at some rate, what is the
 temperature rise? In that case it was negligible)

Brian Whatcott   Altus OK