Aerodynamics

To: tesla-at-poodle.pupman-dot-com
Subject: Aerodynamics
From: Tesla List <tesla-at-stic-dot-net>
Date: Sat, 29 Mar 1997 19:34:35 -0600
Approved: tesla-at-stic-dot-net
Subject: 
        Aerodynamics
  Date: 
        Sat, 29 Mar 1997 11:54:57 -0500 (EST)
  From: 
        richard hull <rhull-at-richmond.infi-dot-net>
    To: 
        tesla-at-pupman-dot-com


All,

I have always held that rotary gaps of the simple type, as built by the
majority of the Tesla community, often flame, flare, or exhibit "follow
around" for several reasons.  The most obvious is poor design and lack
of
forethought in the tank circuit itself.  The other reason is Aerodynamic
fluid flow problems!

We all want to seen the rotary as a big fan in addition to a spark gap.
This fan cools the electrodes and keeps a steady flow of churning air at
the
stationary points. How many of us view the system as an series of
instantaneous events?

If we invision the moving point as a slab sided cylinder in motion, the
fluid flow dynamics will show a low pressure region extending well
behind
the point.  the faster the point moves the lower the pressure in the
region
and the more extended it becomes.  Furthermore, we all know that in air,
at
reduced pressures, the electric conduction is more readily supported due
to
reduced mean free path.  This remains the case until we get to below 1
or 2
mm of pressure and then the situation reverses and the resistance of the
arc
goes up quickly due to reduced charge carriers.

Thus, as the points are firing and the gap tries to spearate, the low
pressure region following in the wake of the moving cylinder electrode
allows conduction to continue long after the points are beyond normal
spark
over.  All this seems to occur in the low pressure region.

The biggest flamer gaps I have seen have been high speed gaps with big
electrodes!   Realizing what the cause might be, I have always used
electrodes smaller than 1/4" and lately in magnifier work, have opted
for
.180 tungsten rod up to 12KW.  I also opt for a much slower gap or rotor
speed and a larger wheel diameter.  I often operate at no more than 1800
rpm
on a 1 foot rotor and use multiple gaps in series with the rotary and
run
lethal rotary gap spacings in the .010" range.  I never have experienced
flare, flaming or "follow around" up to the maximum of 15 KW which I ran
on
Nemesis a few times.

We might do well to think about this problem a bit.  I am planning some
aerodynamically designed electrodes in future.

Richard Hull, TCBOR