Fluid Mechanics in Paintball

[ShooterJM]

Hey guys I've been plotting trajectories and impact velocities of paintballs. Bluntly, I'm not so hot at remembering anything in Fluid Mechanics. So help me out here.

Can I make the assumption that:

A) The Magnus Effect on a smooth sphere will not produce the reverse Magnus Effect anamoly with any regularity whatsoever. Explanation: I don't have a flatline and have never seen one used. Occasionally do the balls drop down instead of a flat trajectory?

B) Air pressure does not have a great dampening effect on the RPM's of a spinning paintball mid-flight. Assume no wind, rain, etc.

Thanks.

[ShooterJM]

Originally posted by ShooterJM

Can I make the assumption that:

A) The Magnus Effect on a smooth sphere will not produce the reverse Magnus Effect anamoly with any regularity whatsoever.

B) Air pressure does not have a great dampening effect on the RPM's of a spinning paintball mid-flight. Assume no wind, rain, etc.

Thanks.

Um. Guess I'll just go ahead and make the assumption:confused:

[soilent green]

be patient deep blue doesn't see as much action as the others sorry if this reply wasn't deep blue appropriat just encouraging this guy but as for B. I think the ball from a flatline slows down (its RPM) gradually through its flight

[ShooterJM]

Originally posted by soilent green
be patient deep blue doesn't see as much action as the others sorry if this reply wasn't deep blue appropriat just encouraging this guy but as for B. I think the ball from a flatline slows down (its RPM) gradually through its flight


Yeah, I've noticed that. Actually, I was rather surprised that they let me post, let alone start a new thread. I was kind of expecting to see it deleted or moved quickly. As for the RPM's changing, I think you're correct, but I don't think it's enough to make changes to my program/equations. Thanks!

[AGD]

Shooter, please explain the Magnus effect in detail.

thanks

AGD

[vf-xx]

Paraphrased from the 4th ed Fluid Mechanics textbook byt Frank M. White:

"The magnus effect is negative lift which is proportional to stream velocity and vortex strength. From the streamline patter (I wish I had a scanner) we can tell that the velocity on the top of the cylinder/ball is less and therefore the pressure is higher from Bernoulli's equation; this explains the force. This uses the inviscid theory."

I"m just quoting, we haven't gotten this far in the class yet. Hope it helps.

[ShooterJM]

Originally posted by AGD
Shooter, please explain the Magnus effect in detail.

thanks

AGD


Well there are two main theories on what causes it. As I'm not the greatest about explaining things, here's a passage that I ruthlessly stole from various Advanced Fluid Mechanics textbooks and reworded to be a bit more concise.

Magnus force results from the asymmetric difference of the boundary layer displacement thickness caused by the combined spinning and flow past the sphere. Actually a sphere can impart a spinning motion to only a very thin layer next to the surface. The motion imparted to this layer affects the manner in which the flow separates from the surface in the rear. Boundary layer separation is delayed on the side of the spinning object that is moving in the same direction as the free stream flow, while the separation occurs prematurely on the side moving against the free stream flow. The wake then shifts toward the side moving against the free stream flow. As a result, flow past the object is deflected, and the resulting change in momentum flux causes a force in the opposite direction
This phenomenon is influenced by the conditions in the thin layer next to the body, known as the boundary layer, and there may arise certain anomalies in the force if the spin of the body introduces anomalies in the layer, such as making the flow turbulent on one side and not the other. One such is the reverse Magnus effect which may occur for smooth spheres. (Rough balls such as cricket balls, baseballs, golf balls and tennis balls, do not show this effect. )

In summary, the magnus effect explains why backspin can cause upward lift force on a moving sphere. It's just that when a smooth sphere is used the opposite can sometimes occur (the balls take a dive).

I figured that since the flatline barrel system imparted backspin (and the Z-body as well) perhaps someone would know how often this occurs.

[ShooterJM]

Originally posted by vf-xx
Paraphrased from the 4th ed Fluid Mechanics textbook byt Frank M. White:

"The magnus effect is negative lift which is proportional to stream velocity and vortex strength. From the streamline patter (I wish I had a scanner) we can tell that the velocity on the top of the cylinder/ball is less and therefore the pressure is higher from Bernoulli's equation; this explains the force. This uses the inviscid theory."

I"m just quoting, we haven't gotten this far in the class yet. Hope it helps.

Yeah that's it! I think that assumes the sphere is spinning towards the flow pattern though. Depending the direction of the spin, the delayed boundry layer seperation would change location.

Good luck in the class, it's really fun! Tough, but fun!

[Croix71]

/boggle

reading this post made my head spin... :eek:

[ShooterJM]

Originally posted by Croix71
/boggle

reading this post made my head spin... :eek:


Sorry. Guess it doesn't really matter. I just want to be able to factor in every little thing into my calculations.

[Croix71]

It's all good. We had an interesting conversation about the whole subject of sphere's and Bernoulli's equation.

We came up with this question.

Would it be possible to have a paintball exit the barrel with a backward rotation (i.e. like you hit a tennis ball with 'slice') so as to have a longer flatter parabolic trajectory?

The opposite of this would be having the paintball exit the barrel with a forward rotation (i.e. like if a tennis ball is hit with a 'top spin'). I think the result would be a steeper parabolic trajectory.

A lot would depend on how fast the the paintball is rotating.

[ShooterJM]

Originally posted by Croix71
It's all good. We had an interesting conversation about the whole subject of sphere's and Bernoulli's equation.



Yeah, It's quite interesting. I'm tempted to go out and buy a Z body just so I can run some tests on it. From those I've spoken to the question of whether backspin can be put on a paintball is moot. It can be done. This is not to say that I've witnessed it at all, just reports from users of the flatline and Z-body. It's the whole smooth sphere anomaly that gets me. I can't figure out how to calculate the frequency of it's occurence.

I guess another angle to take would be to try to figure out exactly at what point a smooth sphere is considered smooth. By slightly introducing regular
imperfections along the surface, more seams perhaps, I wonder if a paintball would no longer be considered smooth. It would probably be a pain to manufacture though.

[Croix71]

Agree with ya there, would the benefit worth the cost.

[ShooterJM]

Originally posted by Croix71
Agree with ya there, would the benefit worth the cost.

Well I'm working on that cost thing. :D Have a meeting next monday to get some more information on manufacturing costs, etc. Actually I even have phase one venture capital if I can figure out just two more things.

[AGD]

The Magnus effect as you are describing affects spinning spheres. Most paintballs fired from markers have little spin and when they do it is usually not enough to distort the airflow enough in a particular direction. The testing we have done shows that the small induced spin on paintballs does not correlate with where it hits the target when the spin is below 6000 rpm.

AGD

[ShooterJM]

Originally posted by AGD
The Magnus effect as you are describing affects spinning spheres. Most paintballs fired from markers have little spin and when they do it is usually not enough to distort the airflow enough in a particular direction. The testing we have done shows that the small induced spin on paintballs does not correlate with where it hits the target when the spin is below 6000 rpm.

AGD

Out of curiosity was there no correlation whatsoever, or just no positive correlation? Did you find that there was a constant or exponential type of drop off in RPMs during flight? If it's trade information or something like that don't worry about it. Just trying to figure out if my calculations are wrong.

[CRySyS]

Tom's talking about spinning the ball along the axis it is traveling down. Like what the Armson Stealth tries to do with rifleing or what AGD did when they spun(sp?) a barrel at high rpm and then fired the ball out of it. But spinning the ball along the perpendicular horizontal axis, like slice on a tennis ball as you said, can and does create lift altering the trajectory of the ball. Backspin will give the ball upward lift that fights gravity; forespin will aid gravity and driv ethe ball down.

The Tippmann flatline works and it works well but to answer your initial question, of all the flatlines I've put together and shot I don't remember ever seeing any balls just drop down. I have seen odd shots rise higher so I propose that the reverse magnus effect is present most of the time but is not strong enough to reverse the backspin lift, and odd shots that do not experience the reverse effect would then go flying higher than normal. I can neither proove nor deny that statement, It's only my little theory I came up with to explain and corolate what I see with what you said. I'm on my way to S. Padre Island for PACS so I'm not going to proove it but I can't wait to get back and see where this thread goes. ;)

[Croix71]

The testing we have done shows that the small induced spin on paintballs does not correlate with where it hits the target when the spin is below 6000 rpm.

hehe thanks TK, I was curious how many rpms it would take to effect a paintball.


forespin will aid gravity and driv ethe ball down

I agree with ya there CRySyS. If the paintball is spinning forward fast enough (not sure how many rpms it would take)it could generate enough lift to rise but after reaching it highest point it will drive the ball down sacrificing distance.

Sooooo, we need to find a way to get a paintball to spin more than 6000 rpms, with a back spi, and make it safe and affordable. :D

[eskimo]

? if you could make paintballs shaped like golf balls i.e. dimpled, then could you control the ball? What about just altering the casing/shell? If it's a gelitain layer with fluid inside, then why not just make balls with dimples? Patent anyone?

[ThePatriot]

If i recall correctly, paintballs in severe cold get dimples on them. This dimpling normally causes the paint to shoot HORRIBLY, so i do not think dimpling a paintball would show any advantage what so ever.

[eskimo]

I think the big difference is the regularity of the dimples, if it's more structured, like a golf ball, as apposed to one large dimple, then I would guess that you could model the flight of a paintball like a golf ball?? The other factor mentioned is cold, and when the ball gets colder you introduce a solid, or more solid. I think that's a whole different post. I still have the question, "Does the paint inside the ball act independant of the shell? or do the shell and fill act as one mass?" I have seen sides that support both.

[Croix71]

I would think that doing a comparison between the two will yield such a minute difference it would considered negligible.

[ShooterJM]

Originally posted by eskimo
I think the big difference is the regularity of the dimples, if it's more structured, like a golf ball, as apposed to one large dimple, then I would guess that you could model the flight of a paintball like a golf ball?? The other factor mentioned is cold, and when the ball gets colder you introduce a solid, or more solid. I think that's a whole different post. I still have the question, "Does the paint inside the ball act independant of the shell? or do the shell and fill act as one mass?" I have seen sides that support both.

Yes regularity is key in dimpleing. Basically the dimples cause the laminar layer of air to disrupt slightly making it turbulant. This delays boundry layer seperation at the rear of the golf ball and gives increased range. Size of the dimples is a big factor too. The smaller the dimples, the less unused turbulance there is and therefore the more efficient the design. However, if the dimples are too small their effect is negligable because there isn't enough disturbance in directional flow to make it non-laminar. One process has been patented, but as far as I can tell it didn't work. Too expensive with bad shooting paint. It would seem gel doesn't have the structural integrity to retain shape during the drying process.

[CRySyS]

I think you misunderstood me. When I said forespin will create lift I meant lift toward the ground. Anti-lift you could call it. Forespin will not lift away from the ground unless the whole reverse magnus effect thing did something. Because the lower pressure side is facing down that is direction the ball will pull. With backspin the low pressure side is up, opposite of gravity, which counteracts gravities pulling effect making a flat straight trajectory. This is what you see comming out of a flatline. I don't know how fast a flatline spins the ball but I think its as much as 6000 rpm. Keep in mind I have absolutely no clue, I may be wrong, the thing may fling them out at 50000 rpm and I wouldn't know. The Mk I eyeball can see what it want to see. I tried to simulate the flatline barrels effect on the paintball calculator (http://home.surewest.net/hoelkers/dyrgcmn/trajectory.html) but it doesn't work properly. A flatline will actually lift up at higher velocities before comming back down. Thats why flatline Tippmann are chronoed in at 260 or less, to prevent the ball from flying up up and away. If anyone knows of a better calculator to use I'd love to hear of it. Didn't NASA have one on their kids site?

[CRySyS]

Ooo, look what I just found...

physics of paintball (http://lennon.pub.csufresno.edu/~nas31/nsa/pballResults.html)

I havn't read the page yet but the last graph shows the wave effect I described where the ball rises and then falls.

[ShooterJM]

Originally posted by CRySyS
Ooo, look what I just found...

physics of paintball (http://lennon.pub.csufresno.edu/~nas31/nsa/pballResults.html)

I havn't read the page yet but the last graph shows the wave effect I described where the ball rises and then falls.

Excellent web page! I'm going to have to read all that when I get a chance!

[AGD]

The paintball physics page is pretty good and very factual. He is working from theory and our test results show a few slightly different things but it's 95% right on. Someone try and get him here to deep blue.

The fluid inside the paintball acts independently of the shell (at first). If the barrel induces a spin on the shell of the ball the fluid does not spin with it instantaneously. We have photographed the spinning ball slowing down after leaving the barrel in flight. The shell slows down while the fill speeds up and at some point they match.

Dimpled paintballs don't fly any better than regular ones. We made them and shot them. I'll try and find the pic.

AGD

[BlackVCG]

Here's an interesting discussion from awhile back that correlates with this discussion.

Theroretical Paintball Physics (http://www.automags.org/forums/showthread.php?s=&threadid=8862)

[ShooterJM]

Originally posted by AGD
The paintball physics page is pretty good and very factual. He is working from theory and our test results show a few slightly different things but it's 95% right on. Someone try and get him here to deep blue.

Dimpled paintballs don't fly any better than regular ones. We made them and shot them. I'll try and find the pic.

AGD

It's that 95% part that's bothering me. I checked and the last update to the page was in 2000. I did email him and extend an invitation to him though, it'd be nice to hear his theories, especially if they've changed.

I'm not sure how much information you'd care to give out on the dimpled paintballs experiment, Tom, but I'm very interested in the details. I've been trying to calculate the perfect pattern, size, frequency and volume of paintball dimples for the last 3 months. In addition to shell material changes, manufacturing process differences and fill. Did you refrence existing patants? Ah, the suspense is killing me.

[eskimo]

Does the fluid's properties inside the paintball have different effects? Do more dense fills "fly" better than less dense?

[ShooterJM]

Originally posted by eskimo
Does the fluid's properties inside the paintball have different effects? Do more dense fills "fly" better than less dense?

I personally don't know. I would guess that the more viscous the fill, the closer it approaches the characteristics of a solid, if you assume that the volume of the fill is the same. Cross winds might not have as great an effect with a heavier paintball. Theoretically, if you filled a paintball with extraordinarily dense fil,l it would tend to follow the ballistics of round musket shot.

All things being equal (fill amount, etc) a heavier paintball would probably require higher pressure or increased volume to hit 300 fps. Guess if you went with increased volume you could formulate a more brittle shell so as to hurt less at close range.

How's that for a rambling response that probably should have been "beats me man"