Paintball Spin Physics - Getting to the final Answer

[Lurker27]

I misspoke, what I meant was that the seams induce random spins, which in turn generate small, uncertain magnus forces.

The vortex shedding itself has an oscillatory nature, and so, on average, cancels itself out. I mentioned the Baker rifle before, and I feel that the comparison still holds...spherical ammunition beig improved by rifling.

http://project.seas.gwu.edu/~fsagmae/papers/AIAA_Apr_2002.pdf

http://project.seas.gwu.edu/~fsagmae/papers/AIAA-99-3806.pdf

Two excellent papers relevant to our discussion...The most important thing to note is the assertio that the mean force from vortex shedding will always be nonzero, but in general approaches zero.

I personally dont believe us to be at the very threshold of paintball accuracy yet.

[hitech]

I haven't had a chance to read all of both those articles, but I will try. The first one looks very interesting. I did read a little and this jumped out at me:

The variability in the orientation of the vortex loops is also found to increase with Reynolds number. Thus, as the Reynolds number increases the preference toward any particular azimuthal orientation diminishes, and the wake is expected to slowly approach a statistically axisymmetric state.

If I read correctly, the top R number they tested was 1000. The R number for a paintball at 300 fps is closer to 10,000 IIRC. That would suggest that at paintball speeds the vortex shedding is almost completely random. Tom's testing of paintball trajectories support this.

[aut911]

I have noticed that some paintballs have a smooth shell while others (more expensive) have a rough shell to them. Would the rough shell infact reduce drag? I know that goldballs rely on the same thing but I am wondering if the rough shells would make a difference on such a "non dense" projectile. the only problem with adding fins or blunt noses is the market.
A customer would have to buy a whole new gun, buy more expensive paintballs and more then likely have to reload more often. I built some "finned" paintballs to see if they would in fact work better than a regular paintball and they do. It would be funt to have a sniper that has a longer range than anyone else on the team. could make war games more fun. I have seen another type of paintball fin(not AGD) that makes a paintball look like a very blunt dart. It is used in law enforcement I believe. I would like to find out where to get a few thousand of those for an experiment, and mabye later, converting a pump gun into a real sniper.

aut

[Lurker27]

Hitech: random perhaps, but the key phrase is "approaching statistically axisymmetric". Isn't he saying that as the reynolds number increases, the wake becomes more symmetrical about the path of the paintball, ie, LESS buffetting about in random directions.


I'm reding some studies of cylinders as bluff bodies too, its very interesting stuff, but I am still not fully convinced of the built-in inaccuracy that we "see" currently.

I really have to shoot a hammerhead.

[hitech]

To me the key phrase is

The variability in the orientation of the vortex loops is also found to increase with Reynolds number.

That is more random, not less. I'm also working backward from Tom's testing. His VERY extensive testing showed the flight pattern to be random. His testing also showed that spinning a paintball about it horizontal axis did not improve the "accuracy". He built a machine that would spin the breech and barrel. It was capable of high RPMs (at leas 10k) and it did not make a difference. I believe that random vortex shedding is the reason.

BTW, I have read that for a specific R number range (can't remember the exact numbers, but it's higher than a paintball at 300 fps) the vortex shedding appears to stop. It is a small range and no one seems to know why.

I still haven't had a chance to read (and digest) those articles. I will try to as they are interesting. There seems to be very little testing with spheres, and that's what we are dealing with.

Cylinder vortex shedding is much easier to study. However, it tends to create predictable loops (alternating between sides).

[Lurker27]

Random will tend to congregate near zero net force, though. Perhaps if we could shed vortices at a higher rate, our groupings would be tighter.

Interesting stuff. Foudn this regarding Reynolds numbers:

Wakes behind spheres are observed to be steady for Reynolds numbers below 300-400. Above this limit (which also depends on the surface finish) vortices break off and are periodically released to form vortex loops that are connected like in a chain.

At Re above 6000 the vortex shedding is very periodic, with Strouhal number ranging from 0.125 to 0.20, the largest figure being a limit at high Reynolds numbers (Achenbach, 1974). Similar wakes can be observed behind particles falling in water. Effects of the surface geometry have been studied for the evaluation of the aerodynamic performances of sports balls (Metha, 1985).

Looking at Figure 3 in the first link I posted, force diagrams for spheres at different reynolds numbers. The distribution shows the least pull away form the origin as as the reynolds number increases.

Even with all this, I still think that non-spin barrel systems are providing less than optimal accuracy, if only via analogy to a knuckleball.

[hitech]

I think the problem here is that the articles you found (sorry, still haven't read all of them) deal with Reynolds numbers MUCH smaller than those for a paintball. The Reynolds numbers for a paintball at 300 fps is ~113000. Back on page six (I think) "we" concluded that at Reynolds numbers that high the vortex shedding orientation is "chaotic", or random.

I do want to read the articles you posted. New information on this subject is always interesting and appreciated.

[Lurker27]

I'm no arguing the random nature of those vortices, but the tests involved suggest, to me, that the pull along any azimuthal axis will be pretty minimal.

Minimal enough to not explain ALL of the inaccuracy in the bathbeads we're shooting.

[Lurker27]

Another thought

I'm not sure if we have looked into the effects of rifling on the vortex shedding yet...so...

http://www.ias.ac.in/currsci/apr102004/1033.pdf

Another idea is to create a mareker with an elongated breech and a detent sytem which allowed 2 balls to be fired at the same time, creating a projectile more similar to a cylinder.

Tournament legality could be preserved for one shot, one pull, by storing hte pulls and only firing in clusters of 2.

[hitech]

I'm not sure if we have looked into the effects of rifling on the vortex shedding yet...so...

http://www.ias.ac.in/currsci/apr102004/1033.pdf

Spinning a sphere with the axis perpendicular to the flow will produce significant lift at high enough RPMS (i.e. flatline barrel). Spinning a sphere with the axis parallel (as in rifling) does not produce the same results. Any research on cylinders is useless as it is always with the axis perpendicular. I've never seen any research on the effects of axis parallel spin on vortex shedding. And certainly not at the Re numbers for paintball flight. Other than Tom's experiments. And they did not capture any data on vortex shedding anyway.

Another idea is to create a marker with an elongated breech and a detent system which allowed 2 balls to be fired at the same time, creating a projectile more similar to a cylinder.

Tournament legality could be preserved for one shot, one pull, by storing the pulls and only firing in clusters of 2.

I'd love to see test results for that. It's quite possible that closely "drafting" paintball will avoid the vortex shedding problem. Basically it changes the shape of the object.

It would never be tournament legal.

[Lurker27]

Why not? one ball per pull, right? Just let them collect in series of 2. Impractical yes, Illegal...no-ish.

[bjjb99]

I'm no arguing the random nature of those vortices, but the tests involved suggest, to me, that the pull along any azimuthal axis will be pretty minimal.

Minimal enough to not explain ALL of the inaccuracy in the bathbeads we're shooting.

The plots in the first paper you listed indicate that at high Re values the time-averaged lateral force on the test sphere is nearly zero. I would expect this as the orientations of the shed vortices approaches a purely random distribution. This does not necessarily mean that said lateral forces have not significantly deflected the flight path of the sphere during the time-averaged period.

Example: I get in my car, accelerate to 60 miles per hour (positive force), travel at a constant velocity for a while (zero force), decelerate to zero (negative force), and then exit the car. My time-averaged force is zero from the point when I get in the car to when I exit.

Am I anywhere near my starting point?

Zero net force does not equal zero net displacement.

The plots you're seeing in the first paper show essentially a random-walk in force as a function of time. If you took sample points from those plots, inserted them as forces into a model to calculate position as a function of time, and ran the numbers, you'd see that the final X,Y displacement is far from zero, even though the net force is near-zero.

BJJB

[eric1337]

i think the key to range is putting as much fill in the paintball as possible so that the fill does not fly to the back of the ball and make it decelerate so terefore less fill travel inside of the sphereequals more range and less deceleration i think from playing ping pong that when the ball starts spinning faster then the ball is moving forward then the spin takes over and grips the atmosphere also if u had a deadball wind would affect it with horizontal spin affecting the trajectorie the trajectorie is more likely to be strait in windy conditions a bit of topspin would likely provefor the most accuracy ping pong balls show u how spin affects trajectorie very well i recommend if agd does spin testing they use orange ping pong balls because there medium wait and have exposed seems

half of this is prbly spelled wrong i have a grade eight education lol

[Rokudon]

uuhhhh, i have very limited knowlege, and doubt i should be posting on deep blue, but in the beginning, it was talk about the seams creating a minor magnus effect (and btw, any terms i mite use r from wat i read) that was previously believed to have an effect on accuracy... well, now there's talk about the vortex shredding, and then about oscillation cancelling out the effect. so.. i was wondering if it were possible to add intentional seams along the ball to create oscillation, or create more turbulence (to minimize the vorticies)? and to keep it in the right orientation, create a weight bias to the back of the ball so it would revert itself to the right direction while inflight? feel free to flame me, i'm just bored with a ton of grd 10 ecology hwk... and btw eric1337, u mite b in grd 8, but the use of periods was probably introduced aliitle earlier in ur education....?

[captainsarcastic]

uuhhhh, i have very limited knowlege, and doubt i should be posting on deep blue, but in the beginning, it was talk about the seams creating a minor magnus effect (and btw, any terms i mite use r from wat i read) that was previously believed to have an effect on accuracy... well, now there's talk about the vortex shredding, and then about oscillation cancelling out the effect. so.. i was wondering if it were possible to add intentional seams along the ball to create oscillation, or create more turbulence (to minimize the vorticies)? and to keep it in the right orientation, create a weight bias to the back of the ball so it would revert itself to the right direction while inflight? feel free to flame me, i'm just bored with a ton of grd 10 ecology hwk... and btw eric1337, u mite b in grd 8, but the use of periods was probably introduced aliitle earlier in ur education....?

This has been tried in several variations. First, look at tippmans flatline system. These never were very successful in breaking into the open market. Flatline cockers were as close as it got and I have never seen one at a field. Imperial also tried to promote consistent spin with the Undertow bolts. I have owned two bolts and have had good success with both. Whether or not it is superior to a traditional venturi bolt however is another matter. It is also hard to compare to say a NDZ bolt (for impulses) owning to what I perceive as a lesser manufacturing standard of IPB bolts from their high degree of hand craftsmanship.