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Nerf, Nozzles, And Fluid Dynamics

Can nozzles be used effectively in Nerf?

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#1 imaseoulman

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Posted 18 August 2010 - 04:48 PM

This is something I've wondered about for quite some time. Typically we only give thought to maximizing air flow, limiting dead space, and generally creating flow paths with the least resistance possible. What I wonder about is the possibility of using nozzles to increase the speed of the air entering and expanding in the barrel, thereby increasing acceleration of the dart. I don't have a very strong background in physics or engineering (I majored in psychology) but I have had classes on fluid dynamics and their applications.

In steam plants, we utilize "air ejectors" to help create a vacuum to which we exhaust steam after it leaves the turbine. The air ejectors "suck" air out of a system by accelerating steam through a nozzle to such a high speed that the pressure drops to a near perfect vacuum and through eduction and entrainment "pulls" the air out of the system it's attached to.

For those of you familiar with fluid dynamics, we know that mass flow rate = density * area * speed. So as the fluid enters the nozzle and it becomes more restrictive (the diameter goes down) the speed goes to maintain constant mass flow rate. The unique thing about nozzles is that after it reaches the critical pressure ratio in the throat (most restrictive part of convergent divergent nozzle) as the fluid expands again, it continues to accelerate, instead of slowing down again.

Here is a more detailed explanation with pressure-velocity and mollier diagrams.

So, for those of you with more understanding of physics and machining than I have, is there a way to incorporate this into Nerf applications? Could it allow shorter barrels due to increased acceleration? Is there any way to use the convergent divergent idea?

I put this in the off topic section because it's theory and not yet home made or general nerf. Thanks for any feedback.
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#2 nostyleguy

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Posted 18 August 2010 - 04:57 PM

I'm pretty sure the pitfall here is that air compresses much more easily than steam and certainly water. Therefore any attempt to 'accelerate' the air through a tightening nozzle just serves to build up high pressure behind the nozzle, without actually being accelerated through it. This is why air 'flow' is so important in nerf. High pressure is good, but not if it doesn't ever contact your dart.
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#3 imaseoulman

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Posted 18 August 2010 - 05:09 PM

There's something to that, but air is used in these things all time. If you google search "air ejector" or "air ejector theory," there are plenty of examples of it. I don't know how efficient they are and, again, if it can be applied, but you certainly can accelerate air through a nozzle.
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#4 Brutal 770

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Posted 18 August 2010 - 06:36 PM

Here's the thing though, high pressure is good but you need a significant volume of air. Think of it this way an AT3K tank has more volume than an AT2K tank making its range a bit better. Well if you add a nozzle it will act as a regulator, limiting the amount of air that passes through to the dart. Also it may come out at a high pressure but it will almost instantly convert to a lower pressure slightly higher than the barrel or space. Now to apply this to nerf though you could take a hopper clip, place this nozzle or regulator behind a sort of input tank, and a quick action valve in front of the input tank making a semi auto gun. Of course you would have to limit the volume of the of the input tank so you wouldn't have to wait a while for the tank to fill.

Edited by Brutal_770, 18 August 2010 - 06:37 PM.

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#5 Doom

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Posted 18 August 2010 - 06:52 PM

imaseoulman, the device you refer to is called a De Laval nozzle. This nozzle does increase the velocity of the gas beyond the speed of sound. But it dramatically decreases the pressure of the gas as well (See the graph on the link). You can’t get that increase in kinetic energy for free, after all; energy is conserved.

You seem to think that what accelerates the projectile is moving gas. This is not true; applied forces to the projectile cause acceleration. The most significant force applied to the projectile is pressure. So, decreasing pressure decreases acceleration.

With that being said, dynamic pressure's effects can be significant. I have run simulations and have found dynamic pressure effects to be essentially negligible for most Nerf guns. That might not be true here. I will say that the effects of dynamic pressure would have to be extremely significant to make up for the reduction in static pressure. Edit: Dynamic pressure is pressure from movement of gases. This is what needs to be significant for imaseoulman's idea to work. And as I explained here, it almost certainly is not significant.

I shouldn't have to add that flow restrictions like this will reduce the mass flow rate, which in turn will reduce performance.

So, from my understanding of the physics this proposal will most likely reduce performance.

Here’s some more discussion of the idea at Spudfiles: http://www.spudfiles...ddy-t16641.html

If you're interested in improving performance, I suggest studying interior ballistics.

Edited by Doom, 19 August 2010 - 04:38 PM.

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#6 TantumBull

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Posted 18 August 2010 - 06:54 PM

I don't have much background in this theoretical stuff, I'm just speaking out of practical experience.

If this worked, than wouldn't all our airflow mods on the 2k, the recon and its clones, etc actually be hindering performance? Or are you referring specifically to nozzle with a funnel, rather that a wall with a smaller opening?
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#7 Lion

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Posted 18 August 2010 - 08:30 PM

It's not exactly what you're asking, but when I mod my NF's I found I lost rang from drilling out the AR's so I added in a washer where the AR use to be so that the air had more time to compress before the dart left the barrel. That gave me standard moded NF ranges.
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#8 imaseoulman

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Posted 19 August 2010 - 04:21 AM

Thanks, Doom. You hit what I was looking for. I was hoping somebody could run some simulations, or at the very least run some equations. I do partly disagree with your pressure vs. moving gas argument. I could have a misconception here, this isn't exactly my forte, but I can think of several actual examples where high velocities of a gas quickly accelerate objects while at low pressure. Also, in our applications, it's almost impossible to separate the two.

As for energy conservation, that is one concept I understand very well. At least once a week I use mechanical energy balance equations, focusing on the conversion of internal energy to kinetic energy (flow work to velocity or pressure to speed) and of "matter" into "energy."

I have assumed the effects would be negligible for a Nerf gun, but was just wondering if somebody with more knowledge and resources than I could make it work.

@TantumBull: No. Widening and straightening flowpaths decreases the conversion of kinetic energy to internal energy, but more importantly, it increases mass flow rate. The idea behind the 2K mods is to dump as much as air possible into the barrel before the dart leaves. That also kinda goes along with what Doom was saying.

And also, Doom, we usually don't refer to them as de Laval nozzles (though he did invent them), which I find kinda funny, because we do use DeLaval oil purifiers. Anyway, thanks for the input.

Edited by imaseoulman, 19 August 2010 - 04:31 AM.

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#9 Zack the Mack

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Posted 19 August 2010 - 09:14 AM

Could high-velocity air speed up a dart in an extremely loose (blowgun-style) barrel, where the dart doesn't seal the barrel? Put another way, could we propel a dart using air velocity instead of air pressure, and prevent some of the dart-on-barrel friction?
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#10 Lt Stefan

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Posted 19 August 2010 - 10:53 AM

Could high-velocity air speed up a dart in an extremely loose (blowgun-style) barrel, where the dart doesn't seal the barrel? Put another way, could we propel a dart using air velocity instead of air pressure, and prevent some of the dart-on-barrel friction?


I believe, and someone can correct me if I'm wrong, but that is what a HAMP does.
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#11 Ice Nine

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Posted 19 August 2010 - 11:20 AM

Could high-velocity air speed up a dart in an extremely loose (blowgun-style) barrel, where the dart doesn't seal the barrel? Put another way, could we propel a dart using air velocity instead of air pressure, and prevent some of the dart-on-barrel friction?


I believe, and someone can correct me if I'm wrong, but that is what a HAMP does.


That is the equivalent of saying your mom sucks such good dick, not because she forms a great seal with her lips, but because her diaphragm moves air so fast that it generates massive negative pressure ramming into the back of her throat.

Basically, consider yourself corrected.
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#12 Zorns Lemma

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Posted 19 August 2010 - 11:22 AM

Could high-velocity air speed up a dart in an extremely loose (blowgun-style) barrel, where the dart doesn't seal the barrel? Put another way, could we propel a dart using air velocity instead of air pressure, and prevent some of the dart-on-barrel friction?


I believe, and someone can correct me if I'm wrong, but that is what a HAMP does.


PROTIP: Hamps do better with looser barrels because they don't generate enough pressure to push through high pressure-of-friction. They definitely will not generate enough pressure to create any sort of high velocity AIR HAMMER OF DART PUNCHING DOOM.

It's like how blowguns work best with a smooth fit. Too snug and you can't get the dart through. Too loose and you're just blowing air.

Or how your mom sucks dick. If her lips are too dry then nothing moves in or out, but if she doesn't both seal and lubricate with saliva, you just hear inhaling noises.

Edited by Zorn's Lemma, 11 September 2010 - 11:01 PM.

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#13 Doom

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Posted 19 August 2010 - 06:46 PM

I'm going to try to keep this simple as barely anyone understands my posts to begin with.

Most people here seem to not understand fundamental physics and confuse correlation with causation.

Acceleration is caused by applied forces. Newton's second law states this. Pressure is by large the most significant force in Nerf guns. De Laval nozzles reduce pressure very significantly and by consequence they will almost certainly not improve performance. I think it's far more likely that a working De Laval nozzle will reduce performance.

And yes, high speed flow is necessary for high muzzle velocities. But simply generating a high speed flow will not make a high speed gun. Making that connection is confusing correlation for causation.

I do partly disagree with your pressure vs. moving gas argument. I could have a misconception here, this isn't exactly my forte, but I can think of several actual examples where high velocities of a gas quickly accelerate objects while at low pressure.


You seem to imply that I claim that movement of a gas does not apply a force. Do you know what dynamic pressure is? I specifically mentioned dynamic pressure as I could see someone using that to justify the use of a De Laval nozzle. I was under the impression that you knew what dynamic pressure is because you "have had classes on fluid dynamics and their applications."

My argument is this: Dynamic pressure's effect on the projectile is negligible for most Nerf guns. It would need to be extremely significant for your nozzle idea to work. And I doubt this will create a supersonic flow near the projectile.

Let's assume for the sake of argument that a De Laval nozzle works in a Nerf gun to increase the velocity at the inlet of the barrel. Does this mean that the velocity of gas next to the projectile is also high? No. Would it surprise you to learn that the flow in the valve of most air guns is at the sonic velocity? The speed of sound is generally the velocity valve flow equations "choke" at. Just because the velocity at one spot is high doesn't mean that it will also be high at the projectile. The gas can slow down.

Indeed, the velocity of the gas near the projectile is almost certainly very close to the velocity of the projectile. There is a rule of thumb in fluid mechanics called the "no-slip condition."

All of this assumes a De Laval nozzle will even work correctly in a Nerf gun, by the way. From what I understand, the inlet air of a De Laval nozzle based hypersonic wind tunnel is generally heated to about 1400 K. You'd probably have to do similar things to get the nozzle to work right. Certainly, some of the work is already done as I bet adiabatic compression makes the average springer have a peak gas temperature of about 600 K or so, but that's less than half of the typical operating temperature.

Also, the inlet pressure of the nozzle should always be above the critical pressure. The pressure almost certainly will drop below the critical pressure during the shot. So the nozzle will become nothing more than a flow restriction during at least part of the shot.

There's a lot more I could write, but I'll leave it at this.

If you think I'm wrong and a nozzle will improve performance, go and try it! I'd be interested to hear what you find.

Edit: Don't get me wrong. I think this idea would be worth investigating, but I wouldn't expect any increases in performance. One similar idea (for spud guns) I've had in the past involved a vortex tube. I came to the conclusion after some investigation that it wouldn't do what I wanted it to do (i.e. significantly head the gas beyond what adiabatic compression could do) and it would increase dead space by a very significant amount. Which was a shame, because I thought the idea was badass initially. :P

Edited by Doom, 19 August 2010 - 10:21 PM.

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#14 Lt Stefan

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Posted 19 August 2010 - 08:57 PM

Could high-velocity air speed up a dart in an extremely loose (blowgun-style) barrel, where the dart doesn't seal the barrel? Put another way, could we propel a dart using air velocity instead of air pressure, and prevent some of the dart-on-barrel friction?


I believe, and someone can correct me if I'm wrong, but that is what a HAMP does.


PROTIP: Hamps do better with looser barrels because they don't generate enough pressure to push through high pressure-of-friction. They definitely will not generate enough pressure to create any sort of high velocity AIR HAMMER OF DART PUNCHING DOOM.

It's like how blowguns work best with a smooth fit. Too snug and you can't get the dart through. Too loose and you're just blowing air.

Or how your mom sucks dick. If her lips are too dry then nothing moves in or out, but if she doesn't both seal and lubricate with saliva, you just hear inhaling noises.



Since Hamps aren't powered by high pressure, what powers the dart?

It is obviously the "High airflow," but even that won't cause a dart to shoot far. If you close/pull/whatever the plunger slowly, it isn't going to do shit. The faster you pull it the farther the dart goes. Therefore, it is the air velocity, not the pressure or volume that fires the dart.

So basically hamps "propel a dart using air velocity instead of air pressure."
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#15 Ice Nine

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Posted 19 August 2010 - 09:09 PM

Since Hamps aren't powered by high pressure, what powers the dart?

It is obviously the "High airflow," but even that won't cause a dart to shoot far. If you close/pull/whatever the plunger slowly, it isn't going to do shit. The faster you pull it the farther the dart goes. Therefore, it is the air velocity, not the pressure or volume that fires the dart.

So basically hamps "propel a dart using air velocity instead of air pressure."


Didn't you read the two posts telling you that you were really fucking stupid? Or anything Doom posted?

High airflow is a slight misnomer as airflow generally gets limited to 1/2" SCH40 pipe before the barrel but HAMPs unload a massive amount of air into the barrel setup. This is in the same way that a low pressure but high-volume burst of air from lungs allows blowguns to work.

I don't need to explain blowing to YOU, though. That runs in your family.

In any case, air velocity is totally not what fires the dart, and you should feel bad for thinking that. You don't have a couple O2 or N2 molecules flying at high velocity and transferring kinetic energy to the dart, you have a large volume of air at lowish pressures displacing the dart.

In closing: Heed advice given to you several threads ago and take all the dicks out of your mouth.

Edited by Ice Nine, 19 August 2010 - 09:12 PM.

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But Zeke guns tend to be like proofs by contradiction

Theoretically solid but actually non-constructive

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#16 Doom

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Posted 19 August 2010 - 09:16 PM

Since Hamps aren't powered by high pressure, what powers the dart?

It is obviously the "High airflow," but even that won't cause a dart to shoot far. If you close/pull/whatever the plunger slowly, it isn't going to do shit. The faster you pull it the farther the dart goes. Therefore, it is the air velocity, not the pressure or volume that fires the dart.

So basically hamps "propel a dart using air velocity instead of air pressure."


You're wrong. As I've detailed, dynamic pressure (i.e. pressure from movement of the gas) is negligible for most Nerf guns. So the movement of the gas alone is not enough.

The pressure is low in a HAMP, yes, but the barrel is long. This allows for the dart to accelerate for a longer time. This is a very simple principle.

The reasons why if you move the plunger slowly the gun performs worse are the same reasons why low flow valves perform worse. The pressure would not build up fast enough, so the dart starts moving down the barrel, leading the area up to the dart to act like dead space, which makes pressure even harder to build up in the barrel.

Edit: Most dynamic pressure terms depends on the Mach number squared. What's the Mach number of a Nerf dart? The upper limit would be approximately 0.2. That makes M^2 equal to 0.04, which is basically negligible. And I should note that's the Mach number at the muzzle; the velocity obviously is slower before the dart reaches the muzzle.

Edited by Doom, 19 August 2010 - 09:25 PM.

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#17 Zorns Lemma

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Posted 19 August 2010 - 09:28 PM

Since Hamps aren't powered by high pressure, what powers the dart?

It is obviously the "High airflow," but even that won't cause a dart to shoot far. If you close/pull/whatever the plunger slowly, it isn't going to do shit. The faster you pull it the farther the dart goes. Therefore, it is the air velocity, not the pressure or volume that fires the dart.

So basically hamps "propel a dart using air velocity instead of air pressure."


PROTIP: things move because they have acceleration.
PROTIP2: forces accelerate things.
PROTIP3: pressure is how force propagates through fluids.
PROTIP4: velocity never does shit. velocity describes the movement of stuff but does not describe how stuff moves*
ADDENDUM: fluid movement does generate pressure*
COROLLARY: these effects in our low tech homemade scrap is negligible*

PROTIP5: think in the reverse. When you are sucking dick and slurping up semen does it make more sense that you are generating negative overpressure in your lungs and that this pressure differential is what moves things, or that you are somehow creating really fast moving air particles that are velcro'd to your appetizer.

*velocity and mass together will have dynamic effects, as any change in momentum is also associated with force-time. However, collisons of air particles and your dart is 1) SUPER LOW MASS and also 2) reasonably long time so these effects are fairly minimal. These would only matter if you had SUPER HIGH SPEED, but then you'd need pretty good initial speed first before accelerating it with cross-sectional area changes, and the whole point is that HAMPs work because there is a lot of gas trying to expand, and not because the gas is expanding very fast.
Again, it's like how you suck dick. When your diaphragm moves to actuate your lungs, it is not because it is moving extremely fast, but because of the large cross-sectional area, a small movement generates a lot of volume change, which results in pressure changes.

Edited by Zorn's Lemma, 19 August 2010 - 09:40 PM.

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#18 imaseoulman

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Posted 20 August 2010 - 04:37 AM

Woe, doom, simmer. I misunderstood that part of your first post. We are in agreement. I really didn't think a convergent-divergent nozzle would help in nerf (negligible at best) but I thought I'd throw out the question for discussion. But as for the inlet temperature, it works at much lower temperatures. I can't give you numbers because they're classified, but they are significantly lower than that.
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#19 Doom

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Posted 20 August 2010 - 08:09 AM

I'm just playing devil's advocate, really. The idea might have some merit if it's engineered right, but I don't think that's too likely. Nonetheless, it's an interesting idea, and it's good that you are thinking about these issues

If you have the means to test this, I'd be interested in seeing what happens.

As for the minimum temperatures being classified... that's interesting. I was just quoting off what I understood to be typical operating temperatures as I assumed there was a reason they were that high. I could see the nozzle working at lower temperatures but it might limit the velocity increase.
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#20 imaseoulman

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Posted 20 August 2010 - 03:07 PM

I mean, it's not cold temperatures. We're still talking about steam from a steam generator at greater than atmospheric pressure, but it is substantially below 1000K.
But from my reading, these can work with unheated gases. We don't have anything like that in our plant, but I was reading about it somewhere, using just compressed air. I'll see if I can find where I read that.
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#21 ilzot

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Posted 11 September 2010 - 12:54 PM

Ok, imaseoulman, I'll just answer your question simply; no.

I really think you can go off of what Doom says, as, from the looks of his other posts, he knows exactly what he's talking about. I've always been a firm believer in straw mods and the like, but with so many members adamantly disagreeing, I'll stand corrected.

And I think there's been enough fourth grader, slightly homosexual, 'your mom' remarks about penises. Bad jokes only go so far.
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QUOTE(Vinnie D. @ Feb 1 2010, 05:28 AM) View Post

... to be able to get a better burst or sustained fire, rather than blowing the whole load at once.



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