Re: FUN: high-speed electrostatic air-threads

[William Beaty <billb@eskimo.com>]

On Tue, 16 Jun 1998, Leigh Palmer wrote:

> > oppositely charged ions, then I would suspect Pinch Effect.  But if
> > negative air is leaving a negative needle, where is the opposite flow of
> > positive air?
>
> It is flowing toward the needle. What's wrong with that?

My difficulty is in explaining the origin of opposite ions out in the
stream of air.  I have a needle at large negative potential wrt earth, and
a 30cm stream of fast-moving air of 1mm diameter being shot from the
needle's tip.  I assume that negative charges are leaving the needle and
travelling along with the air (corona leakage current.)  If positive ions
are flowing backwards upstream towards the needle, what is the origin of
these positive ions?  The negative ions I understand, I can see a corona
at the needle tip, and I know that electrical plasmas can emit air-ions if
the plasma is given a net charge. 

Also, if positive ions were to be created within the length of the air
stream, charge conservation has equal negative ions simultaneously
arising, and so these positive ions would in no way cancel any *net*
charge in the air stream. Their presence might just increase the carrier
mobility within the substance of the stream.  If the reason for the
positive ions is to cancel the negative charge of the stream so that
"pinch" can occur, then pair-creation of ions out in the air stream does
not help, it does not reduce the charge of the stream.

Now that I'm talking about all this, I suddenly have another
fluid-dynamics image which could explain the narrow stream. When a pool of
gasoline burns, the flames tend to be drawn inwards towards the center of
the pool, and then a narrow column of hot gas rises rapidly upwards from
the center.  It's the "firestorm" effect that the US inflicted on the city
of Dresden in WWII (as an experiment!!)  I think it occurs because the
bouyancy forces cause the hot gas from the fire to move much faster than
it is being generated, and so a flow pattern is established where it
becomes "stretched" into a thin column. 

If the ambient e-field causes the charged air to leave the needle tip much
faster than the needle is "emitting" charged air, and if the shape of a
thin stream leads naturally to efficient high-speed flow, then the charged
air would not fan radially outwards as an expanding cloud, it would
encounter inward-rushing air from all directions but one, and would move
in that direction as a narrow high-speed threadlike jet.

I've been imagining the charge density of the air stream as being fairly
high, and then wondering why it is not disrupted by self-repelling
electrical force.  Maybe the charge density is extremely low.  A high
velocity stream with low charge density per meter would have equal current
to a low velocity stream with high charge density per meter.  These being
equal, perhaps other nonlinear effects might cause the air flow pattern to
be stable in one mode or the other.  It might "choose" a narrow,
threadlike, high-speed flow in some instances, and a wide, slow,
sausage-like flow in others.


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