From: eckard blumschein <blumschein@et.uni-magdeburg.de>
Reply-To: blumschein@et.uni-magdeburg.de
Organization: uni-magdeburg
Newsgroups: sci.physics.plasma
Subject: Re: Far or near field?
References: <36B1E208.426F@et.uni-magdeburg.de> <mansell-2901991629170001@teacozy.nhn.ou.edu> <36B93DB3.31B0@et.uni-magdeburg.de> <36B9B84E.71D91A9D@earthlink.net> <36B9CD30.7306@et.uni-magdeburg.de> <mansell-0402991539320001@teacozy.nhn.ou.edu>


mansell@ou.edu wrote in alt.sci.physics.acoustics

> I hope the references prove useful. I would also be interested in your
> results. The lightning research community has had the assumption that the
> shock wave is created by a rapid thermal expansion of the heated air in
> the channel. The paper by Graneau, on the other hand, claims that the
> primary force must be electrodynamic in origin. His pictures of an arc
> discharge show that the shock wave propagates outward in a disk with a
> wedge-shaped cross-section, which would not be expected from an isotropic
> thermal expansion.
>
> But perhaps you are mainly interested in the propagation rather than the
> actual creation of the shock wave? Are you looking at how the sound field
> changes as a function of the electrical energy dumped into the discharge?
>
The outer appearance of the picture by Graneau is intriguing to me in so
far as I identify exactly the same shape with an unstable mode of arc in
GMA welding. I also observed the internal disk shape. However in this
case at about 400 Amps only, the latter can be seen (in videos taken
with 40500 fps) as a rapidly speading shock wave obviously consisting of
metal vapor (see a paper of mine in Proc. of ASM/AWS Conf. Trends in
Welding Research, Pine Mountain 98). Spreading requires less than 25
microseconds, maybe 10 microseconds, after electromagnetic force pinched
off a small short circuiting bridge of liquid metal. Within the next
some 100 microseconds this flat blast is further spreading into all
directions. If this scenario is repeating, it looks like flames flaring
up. Obviously, there are irregular sequences of eruptions, and these
'plasma jets' are responsible for the irregular cloudy shape, roughly
reminding of a potato rather than a sphere and exhibiting single
prevalent transient vortices. Having expanded and got cooler, the metal
vapor does no longer radiate but it might be visible in back-light as
shadow reminding of a vortex of smoke. On certain condition, the
shockwave is not so flat disk-shaped but more spherical from the
beginning. In this case, sound emission is much more intense.

I further guess, inertia of metal vapor is to blame for the high
pressure that worried Graneau in his experiments with short arcs.

Anyway, Graneau's phenomenon obviously relies on metal vapor and
geometry of electrodes. For that reason, I guess, it cannot be used at
all as model of long electrode-less arcs. I have to admit, I have no
background in any theory of lightning. Nonetheless, I tend to confirm
Graneau's objection to the neglect of electromagnetic forces, since in
already in arc welding increasing current density leads to several modes
of arc instability (corresponding to the theory by Murty). Current is
much higher in lightning. I myself see the plasma channel in lightning
as somewhat meandering due to electromagnetic forces rather than
zigzag-shaped. The proof of Graneau's idea could be performed very
easily by a high speed camera. It must be visible whether or not the
plasma channel is moved to the side more rapidly than it is spreading.

What about my intentions, yes, an electrical engineer like me has to
look for mitigation of annoying sound as well as use of informative
sound signals by power electronic means. At present time, my main
objection is to find out how to mimic the superior performance of
welding by ear.