Rifling - what I think it actually did

Threads about water gun modifications.
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cantab
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Rifling - what I think it actually did

Post by cantab » Thu Jun 26, 2008 1:29 pm

On the Physics section of the website, you mention a dubious claim of 'rifling' a water gun, by driving a screw into the nozzle, having improved performance.

This might actually work, though not by imparting spin to the water. I'm assuming the screw was driven in then removed. In which case, the inside of the nozzle/barrel is rough, rather than smooth as it was before. Surprisingly, this could actually reduce internal drag, thus increasing water flow in the gun. Same principle as why golf balls have dimples.

I wouldn't recommend doing it to any gun you care about - there are far better mods to do. Just saying that it might actually work.

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SSCBen
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Re: Rifling - what I think it actually did

Post by SSCBen » Thu Jun 26, 2008 1:54 pm

Interesting theory, but I don't think that would increase flow because it'd make the water more turbulent. There's not really any drag on the stream anyway. There's drag on gold balls, yes, but not a water stream.

The screw was supposed to be driven in and then kept in from what I remember. And this combined with some other power modifications on an XP 310 was supposed to make it shoot 70 feet. We know that's not going to work now, but back in 2002 and 2003 no one knew any better.

For kicks I've attached the old modification guide that's probably not available anywhere else. It doesn't explain much sadly. I vaguely remember visiting Codemaster's (the guy who made the modification) second website and he had some more information there. I think that website is actually still online but it didn't contain much information. I'll also attach some old forum posts from WaterWar.net with information about the modification when I find them.
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Silence
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Re: Rifling - what I think it actually did

Post by Silence » Thu Jun 26, 2008 2:27 pm

Golf ball dimples work slightly differently. When you have a ball moving through air, there's a "boundary layer" of disturbed air around the ball. In the front of the ball, the layer sticks to the surface because of the force of the oncoming air. But around the edges, "separation" happens - the boundary layer suddenly separates from the surface at a high angle. This leaves a larger "wake" that adds to the drag because it's like having an object that's actually the size of the wake, not just the size of the ball.

The dimples create turbulence right on the surface, which helps the boundary layer stick to the surface a bit longer, even though it's turbulent. The end result is the wake is smaller and there's less drag. My explanation probably doesn't make any sense without a diagram, but this effect is fairly well documented...I'll find a link after today's TopCoder contest.

If anything, a spinning stream will be torn apart more quickly by inertia/centripetal force.

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Drenchenator
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Re: Rifling - what I think it actually did

Post by Drenchenator » Fri Jun 27, 2008 10:15 pm

The boundary layer is very important in many things in fluid mechanics. To describe it basically, it's where there is a velocity gradient and therefore shear stress, related by the first equation here. To state it in a few words, the boundary layer is where viscosity matters.

The fluid mechanics has a good diagram of infinite flat plate that shows how the uniformed velocity field is disturbed by the plate and where the boundary layer forms from this. Basically, it slows down the flow close to the plate because of the no-slip condition and this carries through, creating a velocity gradient and therefore a boundary layer.

I don't think any kind of rifling would work. It would just disrupt the velocity profile, something that a nice smooth pipe does not.
The Drenchenator, also known as Lt. Col. Drench.

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