Paintball Spin Physics - Getting to the final Answer

[jtoothman25]

since its off by 10, it'd make the final answer off by a factor of 10. Thus, instead of over 100,000 being the final answer, it'd be 10,000. And like you said, it hardly matters, and doesnt change the fact that its HIGHLY turbulent. We need this number to get down around 2000-3000 for laminar flow, which would make the balls shoot more straight (in theory). Since in the equation, we have speed, this shows us that lowering our Velocity on our marker, that we can get closer to laminar flow.. but to get there, we'd have to cut 300 down to 100 aparently, and that'd get us nowhere. I guess with my Emag, i dont care about spin, i just shoot enough paint that it covers anything, "over there". Also, what matter is it at what temperature we shoot at? what if you go out on a cold day, versus a warm day? I played in the rain recently, and it didnt seem to effect the flight of the ball as much as I anticipated.

[bjjb99]

Originally posted by [5x5]

bj would you please check your viscosity value. At 25 C and 760 mm I believe the viscosity is 0.000185 poises.

If you check back around page 4 or 5 of this thread you will see that the units I'm using are N-s/m^2 (i.e. Pascal-seconds) not poises. A poise is 0.1 Pascal-second, so it's perfectly consistent for you to see a factor of ten difference between your value and mine... it's all in the units. Here is the URL from which I obtained my viscosity value:

http://www.grc.nasa.gov/WWW/K-12/airplane/airprop.html

BJJB - "Nothing to see here... move along. "

[bjjb99]

Originally posted by jtoothman25
We need this number to get down around 2000-3000 for laminar flow, which would make the balls shoot more straight (in theory).

That would be true if we were talking about flow through pipes. Flow around spheres may or may not follow the same behavioral relationships as a function of reynolds number. For example, the vortex shedding from a sphere goes from periodic to chaotic at reynolds numbers between 350 and 375... far lower than the 2000-3000 range for laminar flow in pipes. Here's a URL for reference, courtesy of Hitech:

http://www.alumni.seas.gwu.edu/~fsagmae/AIAA_Apr_2002.pdf

Originally posted by jtoothman25
Also, what matter is it at what temperature we shoot at? what if you go out on a cold day, versus a warm day? I played in the rain recently, and it didnt seem to effect the flight of the ball as much as I anticipated.

Temperature can affect air density, though the effect may not be significant when compared to other factors.

BJJB

[jtoothman25]

I think this article might be of significance to us. Its of an extremely large object, but http://naca.larc.nasa.gov/reports/19...i?page0003.gif might be fun. I know 60 inches isnt what we are playing with here, but, its hardly a huge part of the equation if you exchange .34 where the 60 was.

[mquinn96]

Simple Question:
Would spinning the paintball along its axis of travel (like a riffled shot) effect the vortex shedding? If not, why were civil war era cannons riffled when they too were shooting solid round balls. Also, the spin of the fluid inside should be discounted for this question since it only involves surface effects.
-Mike Quinn

[FatMan]

Originally posted by AGD
DO all guns shoot different? In what way?

AGD

I liked the idea that bjjb came up with back a bit - about low rates of spin affecting the pattern of vortex shedding.

I have always felt that two balls leaving a gun under the same conditions will travel the same except for variations due to flight dynamics ... which appears to have been shown to be significant enough to account for observed variance in shot to shot placement.

It has always been less clear how much the gun affects the initial flight conditions. I've seen the data posted in the data thread, but that doesn't include a study of paint-to-breech fit, or show how often balls spin at various rates, from different setups, or if muzzle blast is an issue.

For example - will I get the same performance with a 4" smooth-bore barrel as a 12" ported step-bore barrel?

My gut reaction is that MOST of that is not important - maybe some of it to efficiency or loudness, but the only thing that has seemed to have much impact on my shooting (and I've shot both Typhoons and Mags quite a lot) is the consistency of the paint to bore match, and then it seems to primarly result in consistent muzzel velocity.


I'd love to see data that proves or disproves that!

FatMan

[pbjosh]

Man, I need to get in here more often...........

I realize I wasn't concise. When I said "all guns shoot different" I was talking about shot groupings. We are talking about the shot groupings of one Paintball gun compared to another.

AGD, I ask in responce, Do all paintball guns shoot (have consistant shot groupings) the same?

No.

There is still something that makes a Stingray NOT very accurate. I might be into Glen Palmers' "Imperical" datagathering here, but some guns are NOT accurate. If you wanted to quote the PCRI accuracy tests of a bench mounted gun, then it should be apparent that all guns, even bench mounted, do not have the same accuracy. Same with all paint. Remember all the 'Best paint of the Year' tests?

And why? As much as we have looked at the ball in flight to a really decent level, there still is something that makes some guns NOT dead-on-hit-the-target-every-time. WHY?

Remember when the Angel came out? It was fast, but the 'Cocker was more accurate. Did people say that because they could see verticie shedding? If we are to believe that the ONLY issue that affects the gun is what happens AFTER the ball is shot, I could make a crappy barrel, out of tolerance components and a bunch of other bad design specs, and "bench mounted", the gun would shoot EXACTLY the same as any other gun out there, if I used the same paint. In reality though, it wouldn't shoot well. WHY?

Barrel and tolerance are issues, but something else is really at work here. Something is happening to that ball BEFORE it leaves the barrel. Something that makes a 'Cocker and another marker with 'Cocker threads shoot differently even if you used the Same barrel and paint. WHY?

If you have just the tiniest amount of paint in the barrel it can traumendously affect the accuracy. Why is THAT? There would be no affect as it goes throught the air because the small amount of paint on the ball wouldn't affect it. So what is different? Is there NO difference?

I think there is.

Not all Paintball guns shoot the same, even with the same barrel and paint. Unless you have played with only 1 gun in so long you have forgotten, they all shoot a bit different. WHY?

Josh

[hitech]

Tom, I'm slowly begining to see what you meant. Now I understand why you never released the information in the first place.

[spantol]

Before I wade in here, I just want to state that I'm way, way out of my element here, and that I'm extremely impressed with where you folks have taken this so far.

That said, I don't think Tom's received the answer he was looking for with his last question just yet. I'm starting from a rather ignorant position, so I need to respond to that question with another question.

Let's say you have a Cocker, a Matrix, and an X-Mag, each identical barrels and air systems. Clearly, each of these markers "shoots differently" in that the firing process differs in each case. That said, once the ball has accelerated to 300 fps and left the barrel, how could the firing process possibly make any difference on where that ball ends up? I would be exceedingly grateful to anyone who could explain this to me (preferably using small words :) )

Further, the claim that any one firing system is inherently and quantitatively superior to another is a rather bold one. Carl Sagan said it best--"Extraordinary claims require extraordinary evidence."

[hitech]

Originally posted by spantol
...That said, once the ball has accelerated to 300 fps and left the barrel, how could the firing process possibly make any difference on where that ball ends up?...(preferably using small words :) )

Sure.

It cannot.

Actually, the only thing it could affect is the spin imparted on the paintball. None of the firing systems are different enough in that respect to make any difference.

[nerobro]

Wow, i'm late joining this conversation.....

Originally posted by AGD Let's see, mysterious force "X" affects the paintball randomy in flight, knuckleball effect happens randomly in flight....

has anyone considdered looking at fluid analysis programs? We could examine a circle in 2d. The turbulant wake of a boat, ball, whatever will tug at the ball a little bit. this tug is random. Like the turbulance behind a semi truck. Given that this is random, it should even out in the end. And a few pages later we see comments on the "drunken walk" sounds like a fine scientific term ;-)

*looks at hiteks post on 1/6/03* That's exactly what I'm describing. but the vortex shedding is happening in 3d.. and just a few pages later our friends show us 3d vortex shedding. Really pretty if you ask me ;-)

I personally think there might be a chance that a drafting paintball might stay in line better. (thats why I always shoot fast )

Well, you can be sure that a shed vortex would provide a turbulant "ball" of air that the paintball will happily fall into. I would imagine the wake of a paintball would be like having a ball sit under a water faucet. If you've done the experiement where you fill a pot with water and put a ball under the flow from the faucet, the ball will stay under the faucet instead of being pushed over the edge of the pan.

As posted by Spantol

Further, the claim that any one firing system is inherently and quantitatively superior to another is a rather bold one. Carl Sagan said it best--"Extraordinary claims require extraordinary evidence."

Given that the balls landing pattern is "perfectly" random. I don't forsee how we can alter this pattern. Not without changing the velocity of the ball to change the way the ball is flying. As Glenn noted that at certian velocitys balls are just more accurate. appearantly 300fps is a poor choice of velocity from an accuracy point of view. *makes a funny face* Now how could we alter the paintballs to change the the range of velocitys where they are more accurate.......

What about changing the size of the ball? Course we have seen that heavier balls are more accurate, but that's just becuase they resist the forces that are developed by the peeling off of vorticies. Though i'm not equipped to do the math, would changing the paint size to say.. .70 or .72 make a signifigant difference?

I"m still a little bothered by Glenn's statement that the ball swept the powder out near the base of the barrel. methinks I'll be doing some testing on my own.

[bjjb99]

Originally posted by nerobro
Wow, i'm late joining this conversation.....

Hey there! Better late than never, though. :)

Originally posted by nerobro
has anyone considdered looking at fluid analysis programs? We could examine a circle in 2d. The turbulant wake of a boat, ball, whatever will tug at the ball a little bit. this tug is random. Like the turbulance behind a semi truck. Given that this is random, it should even out in the end. And a few pages later we see comments on the "drunken walk" sounds like a fine scientific term ;-)

It's pretty much been established that we cannot use a 2 dimensional case to examine what's happening to a spherical paintball. The results from these two cases just don't converge. A 2-D case (infinitely long cylinder moving through a fluid) results in a periodic vortex shedding, while the 3-D case results in a chaotic vortex shedding. The former results in a sinusoidal force profile acting on the object, while the latter results in a random-walk force profile.

Originally posted by nerobro
Well, you can be sure that a shed vortex would provide a turbulant "ball" of air that the paintball will happily fall into. I would imagine the wake of a paintball would be like having a ball sit under a water faucet. If you've done the experiement where you fill a pot with water and put a ball under the flow from the faucet, the ball will stay under the faucet instead of being pushed over the edge of the pan.

Similarly, a ping pong ball will remain in a vertical column of air ejected from a vacuum cleaner running in reverse.

However, this is a finite column of moving air, and the ball has to contend with boundary layer effects as it reaches the edges of the moving air column. In the case of a paintball flying through air, the boundary layer effects you'd see in the ping pong ball case are so far away from the paintball as to have zero contribution. You know that the ball under water (or ping pong ball in an air column) dances about, but remains in the stream of moving fluid. What if the moving fluid column was ten feet in diameter? That's more along the lines of what a paintball is "seeing".

Just some random comments from a local madman.

BJJB

[nerobro]

Going by the simulations I've been seeing, and the 3d modeling... the wake created by the leading ball is fairly tight. Say on the order of 1.5-2x the diameter of the ball. I think it would be interesting to see flight data from rapidly fired paintballs. And then again, i'ts now 8:20 am, i've been up all night so I may be talking out of my rear ;-) I'll take a better look a bit later.

And one more thought, dont' the voteicies still peel off at regular intervals, just they peel off in random directions?

[hitech]

Originally posted by nerobro
And one more thought, dont' the voteicies still peel off at regular intervals, just they peel off in random directions?

First off, welcome to the discussion. Second, yes, I believe that is true.

[bjjb99]

The frequency of vortex shedding is related to the Strouhal number, which (working from memory here) is related to the Reynolds number. A fast-moving paintball will shed vortices more frequently than a slow-moving one. However, I believe the spatial separation between successive vortices is actually very close to equal for the two cases.

As for the orientation of the vortices, their orientations relative to the axis of travel is probably best described as chaotic. For all intents and purposes we can assume they are random with a uniform distribution in azimuth about the paintball's axis of travel.

BJJB