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Re: reverse water sprinkler

On Tue, 7 Dec 1999, brian whatcott wrote:

At 16:08 12/7/99 -0500, David Strasburger wrote:
// a question about a water sprinkler and
which way would it spin if you ran it backwards. ///

I see that the asymmetry of the "sucking" action when compared to the
"jet" action is a major issue here. Long ago when thinking about
Feynman's Lawn Sprinkler it dawned on me that the forces inside the pipes
are not the central issue.

When air is sucked into the end of a pipe, the flow takes the form of a
contracting sphere centered upon the mouth of the pipe (assuming that a
tornado-type vortex does not develop.) On the other hand, if air is
ejected from the mouth of a similar pipe, it takes the form of a long
narrow jet. At high Reynolds numbers the intertia of the air is
important, and the pattern of flow near an orfice depends upon the
direction of flow (sucking versus blowing.) If you play with the inlet
hose and outlet hose of a shopvac, you will quickly notice that the outlet
hose produces a significant reaction force, but the inlet hose does not.
That narrow jet of air coming from the outlet hose is the cause. The
"contracting sphere" which surrounds the mouth of the inlet pipe simply
doesn't produce much of a momentum change in the air of the outside
environment.

If "sucking" could produce a long, narrow stream in the water in front of
an inlet pipe, then backwards underwater lawn sprinklers would spin!
Also, it would become possible to "suck out" a candle flame from several
inches away.


I don't understand fluid mechanics well, but here are some unverified
suspicions: a narrow jet of fluid must be surrounded by a thin "shell" of
vorticity. Another way to visualize the same situation: a narrow jet of
fluid can be conceptually decomposed into a long column of superimposed
"smoke ring" vorticies flying outwards.

If the above is correct, then the underwater lawn-sprinkler problem has a
simple conceptual solution: entropy. An underwater orfice can launch a
"smoke ring", but you'll have to wait a very long time before you see a
"smoke ring" spontaneously assemble itself and fly backwards into the same
orfice.

But does the backwards lawn sprinkler move even a tiny bit? I suppose
this could depend on any asymmetry of the "contracting sphere" water flow.
If the inlets at the tips of the sprinkler were in the shape of long, thin
nozzles, then there would be almost nothing in the way of the contracting
sphere-shaped flow, and there should be very little momentum delivered to
the spinner.


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