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Q & A: water dynamics

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Most recent answer: 07/04/2009
Q:
I was lifeguarding at my pool when I noticed that the fill line which shoots water into my pool did not follow the kinematics equaations I learned in highschool physics. Sure the rough center of the stream of water fit the equations, but the outside edges of the stream were spraying off in different directions after about 10 cm from the tube exit. I was wondering if the interactions of two fluids would cause this effect,if it had something to do with the torque of a fluid while underpressure, or some other explanation.
- Brock Burdyl (age 17)
Houston, Tx
A:
Great question.

I don't think the interaction with the other fluid, air, has a big effect in this case. The most likely reason is that the flow through the nozzle is fast enough to go turbulent. In turbulent flow, small deviations of the velocity of parts of the fluid from the average actually pick up energy and grow a bit, so that the flow is a mixture of a range of velocities, in a random-looking pattern. Turbulence is driven by the differences in flow speed that are found, for example,  between the middle and the edges of the nozzle. Once you have turbulent flow, there will be bits of fluid heading every which-way, and some of them will fly apart from the main stream.

If I'm right, then if the flow is slow enough the spray will stop because the flow will cease to be turbulent.  Wikipedia has a discussion of the 'Reynold's number', which determines whether or not turbulence occurs.

Mike W.


(published on 07/04/2009)

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