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Re: slow UGF

Steve Pushak wrote:
> There was a discussion of this very idea in TOA in which the authors
> discarded the idea as impractical. They decided that you could not
> produce a uniform flow via an UG plenum since the water always seeks the
> path of least resistance and 95% of the flow would pass through a very
> small area. Still, if you have a pressure differential you will still
> have some slow motion of water through the grains of the substrate in
> the other zones and it will be much lower than in the primary flow zone.
> I think the idea would be to have it as slow as possible.

I agree completely that a slow UGF may give rise to the condition where a
large part of the flow is funneled through a small part of the substrate.
My knee-jerk assessment of heater cables is that, since their energy is
spread more evenly in the substrate, the flow they generate will be more
evenly distributed.

However, flow by convection isn't a very stable process.  Convective flow
gave rise to the Lorenz attracter - one of the classic examples of chaos
theory.  I've also done a little theoretical work with convective flows
and know that conditions for convective flow are fairly demanding.  In
short, heating cables may actually fail to produce convective flow, or may
produce irregular patterns of convection that concentrate much of the flow
into small areas with large intervening dead areas.

Before concluding that heater cables produced a better pattern of flow
than slow ugf, I'd really like to see some data.  Does anyone have a
usable way to actually measure flow in a substrate?

Roger Miller