From: terry@asl.dl.nec.com
Subject: Re: Responses to Dale Bass
Originator: terry@aslss01
Organization: (Speaking only for myself)
Date: Thu, 7 Jan 1993 23:48:26 GMT

Hi folks,

In article <1993Jan7.182337.19186@murdoch.acc.Virginia.EDU>
crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes:

> In article <1993Jan7.080252.15953@asl.dl.nec.com>
> terry@asl.dl.nec.com writes:
> 
> | I was interested mainly in _very_ thin shock wave media, not the
> | conventional thick stuff...
>
> Why the 'thick' vs. 'thin' stuff?  A shock is a shock.  They are usually
> pretty thin.  A vacuum bubble collapsing does not necessarily create a
> shock, it is also not necessarily energetic.


Assume the following (highly non-equlibrium) initial conditions:

  (1) A very sharp, gaseous "bubble boundary" with a high degree of
      spherical symmetry,

  (2) A bubble interior that is an extremely hard vacuum, and

  (3) An initial velocity profile in which the gaseous surface is moving
      radially inward at a even rate either equal to, or in excess of,
      the velocity of sound in that gas under those surface conditions.


First question:  _Do_ you call this a "shock wave," or not?  I have
always understood a shock wave to be a result of pushing an object
through another media at a rate beyond its normal sonic velocity.

But what if the "object" is the wave front itself?  And the media is
a vacuum?  What is the correct terminology when there will be _no_
additional matter "piling up" at the front of the wave?


Second question:  Will the inboud surface velocity of the whatever-you-
choose-to-call-it of my hypothetical scenario:

  (a) Always rapidly slow down

  (b) Sometimes remain at the same velocity

  (c) Sometimes accelerate rapidly


For a what I always thought was a "shock wave", I'd say that (b) is a
pretty reasonable answer.  The media will limit the velocity.  Also, no
part of the original surface will ever reach the center of the bubble when
the wave reaches the center.

I do not see the hard vacuum scenario as being nearly as intuitive.  For
one thing some part of the original interior surface _will_ be the same
matter that reaches the center.  But only a very _small_ portion of that
surface will ever reach it.  One way or another the surface molecules in
that case must "compete" to reach the center.  Most will lose, a few will
"win."

The ones that "win" _must_ be accelerated to some degree relative to the
ones that "lose," else the competition cannot be resolved.  If there is
enough diversity of inward momentum, such a "competition" will be resolved
trivially by selection of the faster components (already a violation of
your second law concerns, of course, but please don't forget Hilsche (sp?)
vortex tubes).

If there is not enough diversity of the momentum profiles, I say that you
_will_ get an acceleration effect as in (c), whatever you wish to call it.
I call it wedge-out, and I maintain (as originally in the UC draft) that
this effect is _quantitatively_ different from milder effects such as a
"classic" shock wave in which such a "competition" does not exist -- there
will never be any of the original matter making it to the center.


Well?  Is there room for further analysis there, or not?  I am truly
interested in your opinion on this, Dale.

				Cheers,
				Terry


P.S. -- Dieter; thanks; yes, words are dangerous.  (And so are equations
        when they lack common sense and good analysis to back them up!)

	But I've nailed one or two items with this style of information-based,
        "search space" theorizing (e.g., the prediction of hydrogen forming
        atomic bands in metals), and I'm starting to get a bit more stubborn
	about it having some real value as an approach to physical problems.

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