# Thread: Paintball Spin Physics - Getting to the final Answer

1. Confused Member
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## Re: from precision shooting:

Originally posted by ezrunner
When fired by a human, all shot groups are a donut. The shooter
with the smallest donut wins.

Observationally, paint behaves the same, this is due to the
natural mechanics of the body.

Question: AGD/Tom.. or Glenn,

"Does a paintball marker, when held in a fixed mechanism, exhibit
this same tendency, or is there a pattern grouping that is somewhat
uniform over a reasonably large sample set?"

-rob
Yes, shooting from a firm and fixed hold on the barrel generates doughnut shaped shot patterns. The less spin the ball has when it enters the air stream, the smaller the doughnut.
If the balls leave the barrel with the seem either directly in line with or directly across the line of flight, the shot pattern tends to be more lineal in nature.

2. Not a shedder of vortices
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## Re: from precision shooting:

Originally posted by ezrunner
When fired by a human, all shot groups are a donut..."Does a paintball marker, when held in a fixed mechanism, exhibit this same tendency, or is there a pattern grouping that is somewhat
uniform over a reasonably large sample set?"
Here are some shot patterns that Tom posted:

http://www.automags.org/~TomAGD/spin...aterns2_lg.jpg
http://www.automags.org/~TomAGD/spin...paterns_lg.jpg

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## thanks

hitech:

Those pics are just the ticket. It seems that there is a central grouping in each of the pictures, just below a donut of sorts.

Now, to me it suggests that the paint is grouping in a certain area as those balls left the barrel in the same manner (it seam/etc).

The others seem to be fliers from various mechanical factors.

I do not think that paint "spins" or that there is some magic force.

I am very likely wrong, but I think we are best off with a good,
double regulated air source, and a consistent paint with a tight barrel to paint match.

After exams I plan to do some rudimentary testing with several markers (electros mags and cockers).

We may even toss in a spyder or two.

-rob

4. AGD
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Originally posted by bryan
I dont know much about the magnus effect, but i do know that a knuckleball goes crazy in flight.
Bryan,

Ah, I see you have brought up the point about a force we have not discussed in detail yet!! Bravo!! Let's see, mysterious force "X" affects the paintball randomy in flight, knuckleball effect happens randomly in flight....

Could there be ANY connection? I'm just not sure....I have to wait and see how the forum examines this.

AGD

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## Dance technique and paintball physics

*Edit*Why post when under the influence?*Edit*

Ok, I'll shut up now.
Last edited by Crimson_Turkey; 12-15-2002 at 10:41 AM.

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I was reading the original post and i completly oppose the statement the ball distorts as it is coming out of the barrel. Not only have tests been performed that show this but it just makes more sense to me that the ball stays round. The main one being the fact in order for a round object to get flat on one side it must increase in size somewhere. So it would in turn be like a basket ball with diameter of 14 inchs gets flat on one side and then the diameter of it one way will be >14. Obviously if this happened the ball would become too big for the bore of the barrel so in reality u wouldn't want a perfect paint to bore match you would want a paint too small and it has been proven that that is simply not the case. This has been tested by me. I used paint that was too small in my barrel and chopped every 20-25 balls then when i got paint the correct size I chopped 1 in half a case.

7. Not a shedder of vortices
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[5x5] asked some questions in his long post. I wanted to make sure they were not "lost" and was hoping they would be answered. So, here goes:

1. I am having a problem understanding the 114 data particularly the 114side image. Tom indicates that the distance was 60 ft for the data shown, but based on the distances shown and the distances between strobe shots mentioned in another photo, the distances come out closer to 88 inches. What am I missing?

AGD: I have not looked up the actual dimensions. The actual layout of the test range was changes several times and it would take a lot of looking to figure it out right now. It isnt 88 inches its more like 60 feet with the first strobes at the barrel and the second set 2/3rds down range.

2. Did all shots show the balls spinning or were there a large fraction that did not spin at all? I guess I would expect that all balls would have some spin but with a low average spin velocity, under 5K.

AGD: most were under 5k and a small percentage had no spin.

Several questions come up as I contemplate the data; please don't take them as a harangue.

3. Are air currents possible?
AGD: NO, the tests were in a temp controlled room inside a bigger building.

4. Were there any attempts to look for trends in shot pattern versus shot number?
AGD: No, each shot was a single shot, not part of a string.

5. What was the distance of the target from the muzzle and was the target center perfectly collinear with the barrel axis?
AGD: about 60 feet and the barrel was laser aligned to the string and the target.

6. Also what is the scaling?
AGD: The ball circles represent the actual ball size.

7. Are the circles accurate shadow representations of a paintball diameter? The reason I ask the latter three questions is because this gives me a way to possibly cross check the accuracy of my trajectory calculator under definitely controlled conditions.
AGD: Yes.
Last edited by AGD; 12-17-2002 at 02:06 AM.

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Pardon me for butting in, I'm a paintball newbie. I am also a tinkerer and reloader for varmint rifles. One thing that struck me in this thread was this comment from AGD:

"Yes BC can affect things in flight. Problem is that all spheres have the same drag coefficient with is something like .7 or .8. The nylon balls being perfectly round to better than 1000th of and inch and missing the seam should fly straight and true but they don't. In fact the shot group for nylon balls is hard to differentiate from regular paint."

If a concentric nylon ball doesn't shoot better than a paintball, and you can't reliably spin paintballs, should we look elsewhere for accuracy?

One thing that I have yet to see in ANY discussion on any forum is reference to the crown of the barrel. I shoot for accuracy with anything from a .17 hornet to a 6BR and I can state without a doubt that the condition of my barrel crown is vital to accuracy, due to the effect that the expanding gases can have on the departing bullet. If that is true for a bullet leaving the muzzle at >4000FPS, I would think that a paintball at <300 FPS would be affected greatly.

Think about it - the gases are expanding much faster than the paintball once the ball exits the barrel. In a rifle, the burning powder is expanding at 15KFPS, while the bullet is moving at around 1/5 of that velocity. I don't know the velocity of expanding HPA or CO2, but it HAS to be faster than 300FPS. That means that the paintball is exposed to a very volatile airstream at the most important moment - when it leaves the muzzle.

I guess my point is that there are a lot of factors in markers that can be addressed to improve accuracy before ball spin. We "sweet-spot" triggers, who has actually tested their gun to determine where the VELOCITY sweet spot is? In a rifle, the difference of .1 grain of powder can reduce a group by 50%.

Or, I could be talking out of my hat, in which case I apologize for my intrusion on Deep Blue. I just bought a used Automag classic - does that get me in the club?

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aegis has several very good points that many people over look. but, then again, thats also why most barrels these days have some sort of porting. another factor is that the muzzle of most paintball barrels is very thin walled, where as your .17 is probably sporting a barrel .750+" and has a much larger surface to use for gas deflection. i've thought about trying an 11 or 90 degree target crown, but never bothered too as i simply dont have the means for performing an accurate/deffinative test. i'll try it on my blazer next time i've got a chance to chop up some barrels, and see if i cant rig up some sort of vise to shoot out of, or get a good rest together.
as for velocity sweet spotting it probably varies so greatly with paint changes that it would be really hard to work with. a markers valving should be able to release a pretty consistant volume of air if you have a good set up, which, just like powder charge will affect how a projectile is launched, but the variance in ball size and weight would force you to reset your pressure and velocity for every batch of paint. or you could get really hard core and sit around the house before you play weighing and sizing each ball to make sure they will all fly the same. also, you may find that your sweet spot is either to fast for same play, or slow to the point that what you gain in accuracy you loose in range. the 6br is a good example of that, very very accurate, but traveling at a slow speed compared to more popular varmint rounds. sorry to throw a book at you guys, but he's right, a consistant projectile launch is usually much more important than what said projectile is doing down range. (at least for the distances we work with, its hard to keyhole a paintball)
Last edited by Rust; 12-24-2002 at 09:20 PM.

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For starters; I think that the reason that my powdered barrel tests showed different results than what seems to be expected is that I was doing the tests using CO2 and not compressed air. The higher density CO2 has much different propulsion characteristics. When I did the same tests using compressed air and with the same regulator settings, the velocity went down about 10% and the streaks in the powder did not indicate a full perimeter wipe. What it did demonstrate was that there was less upset to the ball and the width of the marks narrowed back down to thin lines quicker than when using CO2. Lost a few points in velocity consistency as well.
Now, it has been said here that compressed air is a "better" propellent for paintballs but I'm going to need someone to prove that to me.

Also:
"Originally posted by AGD

If a barrel did it's job perfectly every time and 100% of the spread was due to external forces then it would be a waste of time to try and improve it. "

I seem to recall that this was predicated on the consideration of barrels that are "sized and honed to perfection" but I cannot see that "perfection" has yet been defined in the context of this thread. Are we assuming that a straight bore of any given size, with an absolute mirror finish is perfect for launching paintballs ? What if something else apears to yeild better results? Certainly, there are several other factors besides bore size, finish and length involved in giving a paintball its best opportunity to go where it is aimed.

Again from AGD
"Importantly we see it overcoming the slower spin WHILE THE BALL IS IN FLIGHT. This is the next point of debate so I will start. If the X force happened in the barrel or say a foot from it, then the spin no matter how small should affect the flight path in a direction consistent with the axis of rotation as it flies down range. To state it another way, if the ball in flight was not being affected by X then spin should be the major factor causing deviation. From another point of view, the force X has to be happening while the ball is in flight because in 114 it pushes the ball in two different directions while its going down range. So in order to argue against this you have to explain how something in the gun or barrel can affect the ball down range as we see in 114. The one thing the barrel can do is impart spin to the ball and that affects it down range but since we have that under control you have to come up with something else. Fire away. "

There is nothing at all mysterious about the forces that act on a paintball in flight. (even though I don't know how to explain it in scientific terms) It is all a matter of how the ball is presented to the airstream and the results of air flow around a not perfectly round object. The only "random" element in the equation is the projectile itself and the speed and direction of the rotation that it is given or allowed to have when in-flight forces take over. A perfectly round ball would be some improvement but makining them heavier to increase on the ballistic coefficient would show greater improvement. "BC" is a calculated description of a projectile's ability to sustain its energy and resist deflection from external forces. A calculation for the BC of a projectile is based on size, shape, weight and velocity. With the factors being established as "multipliers", the variables don't just add up, they multiply up to significant differences in the outcome of the shot. Aero-dynamics is only part of a BC #.(I've got some heavy teflon balls that will litterally drive tacks at 300 fps and higher, when a lighter weight nylon or delrin ball shows very eratic flight patterns at similar velocities.) I've also seen instances where a gun will shoot paintballs very straight at up to -lets say- 280 fps and the size of the shot group will consistently get larger as velocity increases and then at some point in excess of 400 fps the shot pattern decreases sharply again. I think I know what is going on but I haven't the slightest clue of how to write a formula to demonstrate or prove it.

In short, what goes on down range is greatly influenced by what happens inside the barrel as well as shortly after the ball leaves the barrel. A ball spinning slowly while in flight seems to be subject to a planing effect as the seem and size differential is presented to the air stream at different angles.
Boundry layer and other aero-dynamics stuff applies. At higher rpm's centrifugal forces have to be factored in.

11. Miso Horny
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Originally posted by AGD
Ok back to the battle,
Glenn, I am purposely leaving the valving air blast thing out of this becasue it's one of the things on the table to be examined at the end.
Time to examine this yet?

12. AGD
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Originally posted by Paladin
There is nothing at all mysterious about the forces that act on a paintball in flight. (even though I don't know how to explain it in scientific terms)
Glen,

The discussion so far has lead to the knowledge that spin does not correlate to where the ball hits down range. The fact that the ball changes direction in mid flight indicates that something, "X", is affecting the ball in flight. If you want to say that the barrel is causing this you have to make a reasonable argument on what it is. We all know the barrel can induce spin but this has been discounted in this case study.

Your claim of seam problems is discounted because shooting round nylon balls of the same weight shows no improvement in accuracy.

We were proceeding down a path that was getting somewhere but we seemed to have stalled as we got closer to the truth.

AGD

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has anyone thought about turbulence caused by air resistance on certain surfaced of the ball? What if those abnormal flight paths were caused by some spots of the paintball being more oily/dry than others? The air would flow over the smooth oily surfaces batter than the dry surfaces, which could, mixed with a slight offset spin from that knuckle ball throw, put the ball off course. THe nylon balls should ot have this effect though since htey are not oilly in any way.. so could it be the barrel? Maybe parts of the barrel have an oily residue where others do not.

All I'm saying is that maybe we are not looking at every variable here.

What about hte surface of hte ball AFTEr it leaves the barrel? maybe rubing against the barrel in certain spots while exiting caused the ball to have a slight dry spot from friction or in the case of geletain balls.. a slight flat spot.. causeing that spot to effect the flight path.

Just ideas.

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I would tend to say that the oilyness/condition of the surface of the paintball, except in extreme cases, has little to do with the issue since nylon balls, which hopefully were not greasy, did not show any improvement over a normal, oily paintball. However, making a nylon ball that had one half of it rough, and the other a mirror finish could prove interesting. I think that oily spots in the barrel could definitely have an affect on accuracy. My best analogy is bowling. When you bowl the first half of the lane is slick, and oiled, but when it reaches the other half of the lane, the ball curves in whatever direction you spun it. Thats how the pro bowlers get those cool looking hooks on their balls. Testing for that in a barrel should be easy since you could simply wash a barrel really well, or blow some oil through it after oiling a gun.

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## maximizer

I don't know if I have much to add, but I have used backspin bolts for the last 10 years. I bought a bolt for the Cocker and VM the second they came out in 92 so I have some experience.

First, when they both have the same barrels, paintball and air, they seemed to shoot exactly the same. Granted, any spin added by the open-bolt design was probobly negated by the fast backspin.

When using a .700 barrel, they would shoot flat for a 150 feet or so, then usually curve left/up/right/down in some random way. I don't know if this meant the spin was destabilizing? Because of this, I don't think backspin bolts/barrels increase effective range, only maximum range.

Also, if using a standard .688-.694 barrel, the ball would curve randomly up/down/left/right as soon as it left the barrel. Obviously friction was effecting the spin somehow. You could still use a .692 barrel if it was cut down to 2-3".
In fact, I had barrels of various length that I could effectively control the backspin and FPS. (Which is important if I play at a field with a lower FPS limit).

Most people tell me they find backspin bolts/barrels useless for indoor/speedball/etc paintball, and only use them for long range woods ball...
In my experience, I've found the reverse is true. They don't get you any more effective range, only a flat shot, so I would just use mine for indoor/village/speedball style paintball where I could shoot through a series of windows/bunkers without having to 'curve' it in.

As to the knuckle ball effect- That refers to the ability of a 'rough spinning object' to greatly reduce its drag. That is why the seams and dimples on base/golf balls can fly faster if they are spinning, but only if they are moving at a specific speed. I thought the magic number for a seamless paintball is 600-800fps? Adding seams and dimples lowers that number, but how is that measurable? I suppose if we gradually inceased the fps on a paintball, we would probobly find a speed in which the range suddenly jumped, but the backspin would have to be kept at a constant to keep it from curving upward...

Nick

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The fact that the ball changes direction in mid flight indicates that something, "X", is affecting the ball in flight.
Well, i usally don't tend to get involved in these types of discussions, but since this thread was mentioned on another forum and i am sitting here w/ nothing to do but read today i though i would flex my brain on this topic.

COULD force "X" be caused by a shift in the center of gravity within the paintball itself? since paintballs are liquid filled and the contents prone to separation / settlment over a undetermined period of time.

SINCE every other varible seems to have been accounted for. Inconstencies of the PBs in question i.e. seams, dimples,ectera. ball spin, atmospheric conditions, air sources co2 or hpa, barrel movement, and so on.....

It would be reasonable to assume after elimating all of these EXTERNAL forces that what else would be left to be accounted for....the internal forces of the fill being " stirred" while in flight due to ball spin. Since each ball being fired have its own unique properties( size, weight, shape,fill constency, and "x-factor")....

Anyway thats my best guess

17. FS: Magical elves (\$5 US)
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Originally posted by s.centralpirate
... COULD force "X" be caused by a shift in the center of gravity within the paintball itself? since paintballs are liquid filled and the contents prone to separation / settlment over a undetermined period of time. ...
No, because:
Originally posted by AGD
... In fact the shot group for nylon balls is hard to differentiate from regular paint. ...

18. Not a shedder of vortices
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The mysterious "X" force appears to be the von Karman Vortex Street phenomenon. This phenomenon causes an oscillating turbulent wake behind a cylinder (or sphere). An oscillating wake would most certainly cause a knuckle ball effect. I have not been able to find a good explanation of why this occurs, but it appears to be a well-known phenomenon, at least among fluid dynamics guys. Here are a couple of links that demonstrate this phenomenon:

http://www.csa.ru/~ignatiev/examples_2d_1.html
http://kuwahara.isas.ac.jp/KARMAN/KR1.HTM

Added: Another example of this phenomenon is the vibration of a taut wire as air flows past it. If/when the oscillation frequency of the wake matches the resonant frequency of the wire (the frequency the wire vibrates at) the wire vibrates. We have all seen (heard) this before.

Okay, it appears that I can't find an explanation of why this occurs because no one is sure. Here are some theories:

http://www.pcnet.com/~jdutka/thesis/thesis1.html
Last edited by hitech; 01-02-2003 at 07:27 PM.

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Originally posted by Paladin
There is nothing at all mysterious about the forces that act on a paintball in flight. (even though I don't know how to explain it in scientific terms)
--------------------------------------------------------------------------------
OK, now HiTech has labeled it for us but I tend to think of it as an effect rather than a "force". The actual force involed is air pressure and the way it varies around the projectile as the current flow behind the ball "oscilates".

Originally posted by AGD

Glen,

The discussion so far has lead to the knowledge that spin does not correlate to where the ball hits down range.
Well, you have not yet convince me of that. So far, all that I have seen in that regard is an agreement that a high speed spin (like what is generated by the Flatline barrel) will overcome or diminish the effect of "X". There is a large gap between the high speed rotation and no rotation at all that seems to have been overlooked; or in this case, "discounted". I'm quite convinced that the rate of rotation that the ball has when it leaves the barrel does in fact have an effect on the ball in flight but I can only argue my personal observations. In short, the less rotation, the tighter the shot group.

Originally posted by AGD

The fact that the ball changes direction in mid flight indicates that something, "X", is affecting the ball in flight. If you want to say that the barrel is causing this you have to make a reasonable argument on what it is. We all know the barrel can induce spin but this has been discounted in this case study.
I've not said that I think the barrel causes "X"; just that "X" is not the only factor involved in the flight of the ball.
Certainly "X" affects a ball in flight but the properties of the ball and its movement after the gun is done with it have an influence on the magnitude of the force created by "X". That is what I was eluding to when I mentioned having followed various objects in freefall and had the opportunity to witness first hand and close up, the variable effects of air flow over an object.

Originally posted by AGD
Your claim of seam problems is discounted because shooting round nylon balls of the same weight shows no improvement in accuracy.

The seam itself is far less of an influence than the not -round shape that influences the shape and frequency of the wake behind the ball. Pressure builds and falls in varying degrees relative to the shape of the object and the speed of its rotation. In freefall, "X" apears to have more influence on the line of flight of a true sphere than on something like a paintball and rigid objects are affected differently than flexible ones are.

Originally posted by AGD

We were proceeding down a path that was getting somewhere but we seemed to have stalled as we got closer to the truth.

AGD
Actually, I'm still not sure just where it is we are going with this.
What is it that the truth is going to conclude ?

20. Not a shedder of vortices
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Glenn,
are you aware of the phenomenon know as vortex shedding or the von Karman Vortex Street? I believe this is the major problem with paintball accuracy. It causes an oscillating wake behind a cylinder (or sphere). Here is an example of that wake:

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That definitely looks like it would cause the ball to "S" curve....

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Originally posted by hitech
Glenn,
are you aware of the phenomenon know as vortex shedding or the von Karman Vortex Street? I believe this is the major problem with paintball accuracy. It causes an oscillating wake behind a cylinder (or sphere). Here is an example of that wake:
Yes, in fact I am aware of and somewhat familiar with the phenomenon. I just didn't know the name for it or even how to describe what I do know about it. I learned much about objects in flight about 30 years ago, while trying to follow or catch things in freefall at velocities very similar to firing paintballs. Alas, I have only emperical obsevations to work with.

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Originally posted by hitech
Glenn,
are you aware of the phenomenon know as vortex shedding or the von Karman Vortex Street? I believe this is the major problem with paintball accuracy. It causes an oscillating wake behind a cylinder (or sphere). Here is an example of that wake:
Yes, in fact I am aware of and somewhat familiar with the phenomenon. I just didn't know the name for it or even how to describe what I do know about it. I learned much about objects in flight about 30 years ago, while trying to follow or catch things in freefall at velocities very similar to firing paintballs. Alas, I have only emperical obsevations to work with but I believe that the more stable the object is when flight begins, the less drastic the results of the oscillating wake/vortex.

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Originally posted by hitech
Glenn,
are you aware of the phenomenon know as vortex shedding or the von Karman Vortex Street? I believe this is the major problem with paintball accuracy. It causes an oscillating wake behind a cylinder (or sphere). Here is an example of that wake:
In the attached images, what does the "R=xx" reference indicate ??

25. Not a shedder of vortices
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Originally posted by Paladin
Yes, in fact I am aware of and somewhat familiar with the phenomenon...Alas, I have only empirical observations to work with but I believe that the more stable the object is when flight begins, the less drastic the results of the oscillating wake/vortex.
From what I have been able to read it has to do with the reynolds number (in our case, the fps). I even found an equation that predicted the effect. All the testing I have found was for stable objects. There are even pictures of the effect occurring in the wind wake of an island. Based on all the flow patterns I have looked at (the internet is great for find this kind of thing, search for "Karman Vortex") it would appear that this phenomenon whould have a large effect on a paintballs trajectory.

Just for fun, here is a picture of the vortex shedding around an island (the bottom is a 3d view, use a red lens over your left eye).

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## question

does the positioning and posible
movement of the seam of the ball
affect these patterns of airflow?

In hull designs (boats) there is
much attention paid to these small
details, are we seeing the same
thing?

i.e., if the ball is in flight
and turbulence is being induced
as air flows over the seam, and
that turbulence changes as the
ball is rotated in response to
the turbulence.

-rob

27. Not a shedder of vortices
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Originally posted by Paladin
In the attached images, what does the "R=xx" reference indicate ??
That is the reynolds number. I don't know how to calculate it. However someone here said at 280 fps it is ~ 10^5. It is a function of speed, so as the paintball slowed the number would decrease. I think the effect would be more apparent "down range" as the paintball slowed.

28. AGD
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Hitech....those pictures (falling to knees).....ARE BEAUTIFULL!.....(tears come to eyes)..we have arrived at the altar...

AGD
Last edited by AGD; 01-03-2003 at 11:07 PM.

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How's about an animation of this effect

Apparently...

This animation shows the shedding vortical structures at Re = 300 computed using Jeong and Hussain's lambda-2 method. Along with the hairpin vortices revealed by the numerical streaklines, oppositely-oriented induced hairpins can be observed shedding from the sphere.
You can imagine how the effect of this would cause the ball to follow that 's' shaped path.

manike

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I've been reading up a bit about how the Reynolds number is calculated, and I figured I'd toss in a basic formula [ref. 1]:

Re = r * d * v / u, where

r is the radius of the sphere in question (could be diameter... there's some dispute over this)
d is the density of air
v is the velocity of the sphere relative to the air, and
u is the viscosity of air

For our paintball case,

r = 0.34 inches = 8.64x10-3 meters
v = 280 fps = 85.3 m/s
d = 1.229 kg/m^3 (air density at sea level, according to a NASA website [ref. 2])
u = 1.73x10-5 N-s/m^2 (air viscosity at sea level, from same NASA site as air density value)

Plugging in our paintball-related values gives us a Reynolds number of right around 52000 (or twice that if the paintball's diameter is supposed to be used instead of its radius). The highest Reynolds number in the photograph presented was Re = 161, and the animated model presented used a Reynolds number of Re = 300.

I am leery about extrapolating behavior seen at these Reynolds number values up into the regime occupied by a paintball moving at 280 fps. If my constants and Reynolds number forumla are reasonably accurate, then an Re = 300 paintball would be moving at around 1.6 fps. That's two full orders of magnitude we're extrapolating over here, and it makes me nervous.

If we can get our hands on some Re = 25000 or higher pictures, I'd be more inclined to believe that Karman vortices were a major contributor to the paintball's behavior in flight. As it stands now, I remain at least somewhat skeptical.

BJJB

References:
[1] - http://www.aero.ufl.edu/~uhk/STOKES2.pdf
[2] - http://www.grc.nasa.gov/WWW/K-12/airplane/airprop.html

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