9 replies to this topic

[Agles]

this is just something i was thinking as i was removing an AR from a firestrike.

why not open it up and make it into more of a venturi? 

back 10 years ago, i was majorly into paintball and we would often replace firing bolts with venturi bolts. 

note the holes that would put pressure around the outside of the ball. 

 

only thing i can think is, air coming form the plunger through smaller holes would be at a higher pressure then 1 large hole. 

anyone ever try doing this in a nerf gun?

 

[jboynerf345]

My physics knowledge is primitive, so correct me if I'm wrong...

 

p=f/a (p=pressure,f=force,a= area of applied pressure (surface area)), Assuming the original blaster had it's AR fully removed, the end of the plunger tube has almost zero restriction for flowing air. As the "a" would remain constant (the END OF THE PLUNGER TUBE, not the amount of open space, if that makes sense), that leaves the problem of how much more is the output pressure of the pressure with the original PT to the venturi. As far as I can picture, The amount of time it takes for the air to compress and release from the PT, especially with an upgraded spring, is very little and at least some of the air will not have time to find the path of least resistance before the pressure becomes effective and the dart leaves the "barrel". The remaining air delayed by the is essentially useless.

 

Not to hate, but I don't think it would work that effectively. Besides, the venturi kinda reminds me of a Maverick AR. Again, I'm not that smart so correct me if I am in need, I want to learn as well.

[Aeromech]

Time to live up to my namesake.

 

This could theoretically work for paintball, and we already know this law works with homemade Nerf blasters, when the inner piston diameter of 1.36 ID PVC plunger tube decreases to 0.485 ID CPVC barrels. On the topic of using this in a Nerf mod for a blaster with AR's this MIGHT work by giving the flow a faster INITIAL velocity but as soon as the effective diameter of the pipe jumps back up to the diameter of your dart at the barrel, I feel as though this would not be very effective. The initial puff of air would have a higher velocity but a lower pressure. That being said, I always kept my dart posts in my Mavericks and found them to perform slightly better when they weren't drilled out.

 

Here's the basic equation:

Note this is for INCOMPRESSIBLE fluid and we'll get to that after these messages. But just read the equation. Basically, as the change in pressure gets larger, your velocity at the barrel gets larger. So if you decrease the constricted tube's diameter (and thereby decrease the pressure) the velocity in the constricted tube will increase. Your air will be traveling faster initially SO LONG AS THE DENSITY REMAINS CONSTANT which brings me to my next point, COMPRESSIBILITY.

 

Compressibility is a bitch. Just like a sponge can be squished and change it's density, so can a fluid, and all these easy equations go out the window. For a fluid to reach the point of compressibility, the air flowing in the pipe must be greater than 0.3. If the barrel is plugged by a dart, logically, the air flow cannot exceed the speed of the dart. In something like a suped up 4B, the FPS can reach 300 if you plug the pump, it's more like 250 for high end springers. So with the speed of sound at sea level being 1125 FPS, the mach number of a springer's barrel would be 0.22 and for a 4B, more like 0.27, so we get pretty close to compressibility being an issue, but dodged that bullet for now, so that above equation is valid. Honestly in some of these looser fit 4Bs I wouldn't be surprised if, when hooked up to the right equipment, we actually got into the realm of compressibility.

 

BUT if that restriction is made too small you begin to run into the problem of compressibility in homemades. For Nerf brand shit you won't have to worry about that.

 

Turbulent flow may also be something to consider, however, as many little holes expanding into a larger DIA pipe is bound to create some kind of nonlaminar flow. which has a whole slough of problems with it too. Long story short you want laminar flow if you want consistent performance, turbulent flow increases pressure and therefore less velocity based on that shit we just read. 

 

EDIT: As for the holes being on the outside, I suppose it has something to do with the nature of flows in pipes:

 

Basically, those green arrows represent velocity. So in general, the fluid velocity is higher at the center where there is no friction from the walls of the pipe. The velocity at the pipe walls must be zero (no-slip condition) so near the walls the velocity is less than at the center. If the flow is closer to the walls, perhaps this is an attempt to defeat the friction at the walls and make a more "flat" (consistent) and less parabolic velocity profile. Turbulent flow would also accomplish this as the velocity profile of a turbulent flow is sort of plateau shaped and very flat like the second set of green arrows. I'm not a paintballer, so I really couldn't tell you the reason they use it, but this is just my educated guess.

 

So is this method worth it? Eh, I'm skeptical because as soon as it reaches the dart the DIA will increase therefore decreasing velocity, but for the instant the dart is pushed right up against the restricted region, the velocity may have some added effect, so it may be more effective in airguns where pressure is released in one instant as opposed to a springers which build up pressure as the plunger moves forward. If anything this may just reduce deadspace and that's where the increase in (apparent) performance comes from. I'd say test it out and experiment a little, and look what other kinds of experimentation has been done, i.e. potato gunners and the like probably have some insight. In the words of Ms. Frizzle: Fuckin' go for it.

 

Resources for nerds:

Khan Academy: https://www.khanacad...and-pitot-tubes

Venturi Effect: https://en.wikipedia.../Venturi_effect

Choked Flow: https://en.wikipedia...iki/Choked_flow

Compressible Fluids: https://en.wikipedia...mpressible_flow

Edited by Aeromech, 03 January 2016 - 11:27 PM.

[Agles]

O.O ok thats a ton more math then i thought i was getting into. 

so in the end, seems the pressure being used wouldnt be enuff to make of a effect. 

 

do we know what the pressure in a nerf gun is?

in a paintball gun it would be anywhere from 200-800 psi. mattering on type of gun. 

[jwasko]

Pressures used in pneumatic nerf guns rarely go much over 100PSI.

 

I'm no physicist (Aeromech is?) but the highest air pressure in a springer is probably achieved when the piston/plunger is moving, but the dart is not. Just before the dart moves, the force being exerted on the dart would be equal to the force of static friction. The pressure would be that force divided by the area it is acting upon (the back of 0.5inch diameter dart).

 

After that, the pressure would be equal to the force of sliding friction (which is always lower than static friction).

 

The magnitude of these frictional forces depends on your darts and barrel material, but they could be measured using classic high school physics class experiments I suppose.

[Aeromech]

I'm no physicist (Aeromech is?)
 
After that, the pressure would be equal to the force of sliding friction (which is always lower than static friction).

Initially a student of aeronautical engineering then switched to mechanical for grad school. Aero. Mech. ; )

If that initial burst of higher velocity exists from these venturi tubes, as I hypothesized, it may be to overcome that initial static friction, even if the effects are negligible after that point

[piratecow]

I had been thinking along these lines.

 

Years back I saw a venturi insert in the barrel of an air cannon used for R&D. I forget what circumstances it was designed for, and I never used that attachment.

 

I figured it would be easy enough to turn a small one out of delrin and try it out. I tend to think that at this low a pressure and volume the benefits, if any, would be negligible, but I figure it's worth a quick test.

[Doom]

Compressibility is a bitch. Just like a sponge can be squished and change it's density, so can a fluid, and all these easy equations go out the window. For a fluid to reach the point of compressibility, the air flowing in the pipe must be greater than 0.3. If the barrel is plugged by a dart, logically, the air flow cannot exceed the speed of the dart. In something like a suped up 4B, the FPS can reach 300 if you plug the pump, it's more like 250 for high end springers. So with the speed of sound at sea level being 1125 FPS, the mach number of a springer's barrel would be 0.22 and for a 4B, more like 0.27, so we get pretty close to compressibility being an issue, but dodged that bullet for now, so that above equation is valid. Honestly in some of these looser fit 4Bs I wouldn't be surprised if, when hooked up to the right equipment, we actually got into the realm of compressibility.

BUT if that restriction is made too small you begin to run into the problem of compressibility in homemades. For Nerf brand shit you won't have to worry about that.

(This is somewhat technical and can be ignored by most people.)

The phrase "compressibility" is used in several different senses in thermal-fluid science, and they can be confusing. The Mach number does not need to go above 0.3 (or whatever cut-off is set) to have compressibility effects. Take a sealed piston, compress the gas, and hold it still. The Mach number is zero, yet there's a change in density.

The Mach number is about the speed of sound. You can have a flow where the velocities are well below the speed of sound, yet density changes are important. Buoyancy driven flows (e.g., from combustion) are one major example. I work in fire protection, and the variable density low-Mach Navier-Stokes equations are the starting point. Nerf is another example. The density changes are appreciable, but you generally don't need to worry about the speed of sound effects. So that equation is not applicable.

This is not to say that Mach number effects are not important in Nerf guns. The flow could easily choke in restrictions. In my old Nerf gun simulators, that was the only place I took into account Mach number effects.

To get back to the original question, I can't comment on the effect of this on performance. Seems situation dependent to me. It could act to reduce deadspace as Aeromech points out, or it could just restrict the flow. I'm skeptical of the "Venturi effect" explanation, as it doesn't matter if the velocity near the projectile is fast; the no-slip condition says the fluid will be going the speed of the projectile at its surface. So if the projectile is still, the gas will slow down, etc. It's often more correct to think in terms of pressure. Would this increase the pressure behind the projectile? Probably not, though this isn't the entire story. There's one way to know. Agles, if you think this has promise, test it out and let us know here.

[Remzak]

Without a very high output blaster this would be harmful. In a hydro cannon this could help. It may create laminar airflow and improve performance and consistency.

[Agles]

ill give this a try in a revolver. just mod 1 chamber differently for each test. 

thinking either a Strongarm or Hammershot.