For those of you Who like math and calculations....

[jeepkahn]

I know some folks on here like to build spreadsheets, extrapolate formulas, write code, and crunch data in general...

Here are some things that might be useful for the spudworld in general(If I had more formal knowledge of the subjects I would fiddle with it myself, but until winter I don't have the time to acquire the needed research...):

1. terminal velocity of a gas through a specified size of pipe/barrel.

2. pressure required to reach terminal velocity of a gas in a specified pipe/barrel.

3. minimum length of pipe to reach terminal velocity for a given pressure and pipe size.(this one because many have barrels that are too long, myself included)

4.Now that people are starting to use tapered barrel inlets, a formula for establishing the taper. i.e. a square edged orifice flows at 65% efficiency, so to flow 100% it has to start at a X*radius to prevent the efficiency reduction caused by vena contracta.

Just throwing things out there for the ones who like to sort this kind of thing and have the required math skills to do it...

[Technician1002]

I know some folks on here like to build spreadsheets, extrapolate formulas, write code, and crunch data in general...

Here are some things that might be useful for the spudworld in general(If I had more formal knowledge of the subjects I would fiddle with it myself, but until winter I don't have the time to acquire the needed research...):

1. terminal velocity of a gas through a specified size of pipe/barrel.

2. pressure required to reach terminal velocity of a gas in a specified pipe/barrel.

3. minimum length of pipe to reach terminal velocity for a given pressure and pipe size.(this one because many have barrels that are too long, myself included)


4.Now that people are starting to use tapered barrel inlets, a formula for establishing the taper. i.e. a square edged orifice flows at 65% efficiency, so to flow 100% it has to start at a X*radius to prevent the efficiency reduction caused by vena contracta.

Just throwing things out there for the ones who like to sort this kind of thing and have the required math skills to do it...

I like the way you think. Unfortunately my highest math is HS algebra. I work with engineers, but I'm just a technician. I understand many of the theory, but don't have the advanced math.


Photo attempting to match acceleration curve of the 4 inch poof ball with GGDT. Close matches only reached with high coef.

As such, I build and experiment and log results. I used the best prediction models I can find and compare results with modifications.

I would suggest writing to the author of GGDT and see if he is willing to collaborate as he seems to have the best prediction model for subsonic on the net.

I have seen exactly what you mentioned. A shorter barrel than predicted works better, terminal velocity in long barrels at lower than predicted velocity and rounded ports and shape achieving higher coef. Too bad I have no idea how to model this and predict it accurately in the transonic and supersonic region. This is exactly why I am building the 3 inch valve to narrow down to a golf ball and marshmallow barrel. GGDT is known to have issues at transonic and SS speeds. I want to collect some data to see how close or how far the model differs.

I too would love to have an accurate model for this in the higher speeds.

[btrettel]

I could probably help. It'd be important to note that I don't know everything though, but I am learning.

What do you mean by "terminal velocity" of the gas?

If you mean the speed of sound this'll help: http://en.wikipedia.org/wiki/Speed_of_s ... and_in_air

If you mean peak velocity of the projectile, I can help too. I'm not certain if GGDT can calculate that for you, but I know you can look at the velocity vs. distance plot and pick out the peak. My simulation, BAGS, also can be made to work in that way, though, I haven't released that code yet.

For what it's worth I'm in the planning stages of a simulation that should be better at transonic velocities. I think/hope solving the 1D Euler equations in the barrel will be adequate... though, I don't really know. There's no other way to find out than to try.

[Technician1002]

Terminal velocity is the point gas won't go any faster due to pipe friction. It is often way below SOS. In long barrels, the gas has friction along the length. As it travels and expands due to pressure drop the volume increases. The friction in the low pressure area near the outlet provides a back pressure near the inlet, so the speed of flow is low in the high pressure area.

More info on this can be found by studying "Capillary Tubes" used as expansion valves in refrigeration. The flow a lot less gas than just the orifice size would predict. With a larger bore, they are much less likely to plug up in small refrigerators and water coolers.

Spudding with long barrels have the same thing. GGDT gives warning barrel choking flow for this.

[tghhs]

Hmm, i have to say that this post is way over my head but i've been wiki-ing and doing a bit of reading and this is what i got out of it.

If we design our barrels to innitaly have a bigger ID for the lengh it takes the vena contracta effect to concentrate the flow to (assuming T or coxial piston valve) our desired ID size we will get better airflow.

But does this mean we will get more performance? In the case of non
o-ringed pistons will the extra pressure loss due the bigger valve to do this, cancel out the advantages of the bigger opening in the first place because of the extra surface area of equalisation?

and in o-ringed pistons will the added friction have the same effect?

also does the venturi effect increase as the difference between the larger and smaller restrictions increse, i asume so,?

so assuming everything is equal and ignoring piston friction or pressure leakage would the venturi effect speed up the gass the same amount in both cases where 1 the inital opening is bigger and get's smaller to say x and 2 where the opening is and lengh is constantly x?

Woah massive brain hurtage right now

[Technician1002]

Hmm, i have to say that this post is way over my head but i've been wiki-ing and doing a bit of reading and this is what i got out of it.

We all come to learn more. Some info is at higher levels. Take your time.

If we design our barrels to innitaly have a bigger ID for the lengh it takes the vena contracta effect to concentrate the flow to (assuming T or coxial piston valve) our desired ID size we will get better airflow.

Correct. It is why the front edge of jet engines is rounded instead of square edges. Much better flow into the engine for the same size engine.

But does this mean we will get more performance? In the case of non
o-ringed pistons will the extra pressure loss due the bigger valve to do this, cancel out the advantages of the bigger opening in the first place because of the extra surface area of equalisation?

More performance is subjective. At low flow speeds, the effect is minimal. At near supersonic, it is huge. Very heavy projectiles due to slower acceleration will have less improvement than launching lightweight stuff.

and in o-ringed pistons will the added friction have the same effect?

Huh? Venturi after the piston doesn't have much affect on the force on the piston to open it.

also does the venturi effect increase as the difference between the larger and smaller restrictions increse, i asume so,?

I'm not sure what ratio of reduction provides how much change in coef.. That one is above me.. We are all learning. I just don't know that one. Good question.

so assuming everything is equal and ignoring piston friction or pressure leakage would the venturi effect speed up the gass the same amount in both cases where 1 the inital opening is bigger and get's smaller to say x and 2 where the opening is and lengh is constantly x?

I don't fully understand the question. Can you try to re-phrase it? Would the venturi effect speed up the gass the same amount in both cases. Need to be a little clear on what the two cases are.

Woah massive brain hurtage right now

Try engineering for a living..

[btrettel]

Terminal velocity is the point gas won't go any faster due to pipe friction. It is often way below SOS. In long barrels, the gas has friction along the length. As it travels and expands due to pressure drop the volume increases. The friction in the low pressure area near the outlet provides a back pressure near the inlet, so the speed of flow is low in the high pressure area.

More info on this can be found by studying "Capillary Tubes" used as expansion valves in refrigeration. The flow a lot less gas than just the orifice size would predict. With a larger bore, they are much less likely to plug up in small refrigerators and water coolers.

Spudding with long barrels have the same thing. GGDT gives warning barrel choking flow for this.

Ah. I haven't factored that into anything I've done yet but I've been reading a bit about it. I'm not sure if it makes much of a difference; the information about light gas gun simulation I've been reading seems to neglect barrel friction's effect on the gas completely and those simulations are far more robust than ours'. Edit: Actually, friction seems to be included in some of these in some parts.

Nonetheless, this might be helpful: http://en.wikipedia.org/wiki/Fanno_flow

The barrel choking flow warning is something different (as far as I know, at least). If you have a valve that is substantially larger than the barrel, your barrel will be the main flow restriction.

As for the mentioned problems with the vena contracta... most practical fluid dynamics problems (especially with the potential for shocks) can't be solved analytically (i.e. with a formula to plug in the numbers). Some empirical data or computer simulation (what's called CFD--computational fluid dynamics--and it's a very complicated field) could find a useful relationship, but short of that I don't think much can be said.

[Ragnarok]

In GGDT, "Barrel choking flow" simply means the barrel's flow is less than that of the valve. Nothing to do with barrel friction.

Oddly though, I'm not sure why the barrel appears to have a flow coefficient of 65%, which unless I'm suffering a monumental brain-fart, applies to sharp edged orifices, not smooth pipes.

[Technician1002]

In GGDT, "Barrel choking flow" simply means the barrel's flow is less than that of the valve. Nothing to do with barrel friction.

Oddly though, I'm not sure why the barrel appears to have a flow coefficient of 65%, which unless I'm suffering a monumental brain-fart, applies to sharp edged orifices, not smooth pipes.

It's the assumed transition through the burst disk with remains of burst disk at the edges of the flow. In testing I was surprised that my valve seems to beat a burst disk in flow. All the ports have rounded edges. This was not done for flow but to remove sharp edges that could cut an o ring. I had to model my valve with a high coef to get the speeds in GGDT to begin to match my measured results.

Update, I found some scales an weighed the Poof brand foam ball. In modeling without weighing it, I came up with about 60 grams in GGDT.. The actual weight is right at 2 oz. Since 1 oz is 1 ounce = 28.3495231 Grams according to Google this is pretty close. I do have the coef shown in the GGDT model above at subsonic speeds.

[Ragnarok]

It's the assumed transition through the burst disk with remains of burst disk at the edges of the flow.

It's applied to any valve: BS piston, CS piston, burst, hammer...

It's not specific to burst discs. Anyway, I'm not talking about the valve, I'm talking about the barrel.

In testing I was surprised that my valve seems to beat a burst disk in flow.

Yes, but I seriously question the logic of modelling it as one. Quick opening though it may be, it will not be burst disk quick.
It'll be a modest amount quicker than a regular piston valve, and barely any difference compared to my TARDIS valves.

Since 1 oz is 1 ounce = 28.3495231 Grams according to Google

If we're being that specific, why not go the next couple of decimals?

1 ounce = 28.349523125 grams, exactly by the definition of the BIPM (International Bureau of Weights and Measures).

And the worrying thing is, I know these numbers off the top of my head (don't even ask about the time I memorised Pi to 100 decimal places)

[MrDEB]

WAs playing around with the GGDT program and my computer crashed!
all I was doing was changing values and see what happens.
any one else experience same?

[D_Hall]

1. terminal velocity of a gas through a specified size of pipe/barrel.

If you have a long enough barrel, ALL gas flows will end up at Mach 1 regardless of their starting velocity. Mind you, for subsonic flows to accelerate it may take an INSANELY long barrel, but the physics are well understood.

Supersonic flow: Any disturbance in the gas triggers a shock. Whammo. Sonic flow at Mach 1. Easy.

Subsonic flow: Frictional heating on the wall of the tube increases the temperature of the gas. The gas expands. It can't expand "backwards" as there's a pressure gradient. So it moves forward at an ever so slightly faster rate. This keeps happening in a continuous process until the flow is sonic. Mind you, getting this to happen may take significant pressure and tubes that are measured in many thousands of diameters in length (which generally means it only happens in a laboratorydemonstration), but the phenom of a subsonic flow going sonic has been demonstrated for generations.

Now, some of you may be thinking, "Tech mentions capillary tubes slowing down the flow, and that makes sense!" This is true provided that you're talking about a constant pressure source. But that means you've violated your initial assumption of "X lbs/sec of air at the entrance." If you increase the pressure (as tube length goes up) to maintain that "X lbs/sec of air", you'll see the effects I'm talking about.

2. pressure required to reach terminal velocity of a gas in a specified pipe/barrel.

See above. It can require insane lengths and depends on everything from flow rate to surface roughness of the barrel. I could say more, but this is not something I've dealt with in a very long time and my texts on the subject are at the office (not here at home).

3. minimum length of pipe to reach terminal velocity for a given pressure and pipe size.(this one because many have barrels that are too long, myself included)

See above.

4.Now that people are starting to use tapered barrel inlets, a formula for establishing the taper. i.e. a square edged orifice flows at 65% efficiency, so to flow 100% it has to start at a X*radius to prevent the efficiency reduction caused by vena contracta.

Ugh. Another one that's well understood and "plug and chug" in practice, but my texts are at the office.


@MrDeb, no, I don't have that problem.

[Ragnarok]

Was playing around with the GGDT program and my computer crashed!

If you put in certain things, you may well crash it (or your computer) - as with any program.

Still, GGDT is pretty robust as far as actually crashing the computer. I'm not sure it's ever crashed my machine.

It will occasionally crash out of the program. The first example that comes to mind is that I know GGDT will crash out on a divide by zero error (although you do have to work hard to achieve said error) - but that shouldn't take your computer down with it.

[jeepkahn]

Geez, i'd nearly forgotten about this post....

In retrospect, some of my nomenclature was incorrect or confusing...And I may have been drunk when I posted it, so I'll try to clarify...

1. Idiot question, Brain fart...

2. how much pressure would it take to achieve mach flow through a specified orifice (that's exhausting a chamber to atmosphere, with the chamber having infinite volume of the specified pressure).

3. length of pipe needed to accelerate a .1g projectile(zero friction) to SOS with a specified diameter of pipe and a given pressure.

4. what is the diameter required (as a factor of barrel I.D.) to avoid lateral expansion of the gases once in the barrel(velocity stacks work by converging the gas flow so that all expansion/flow is longitudinal and not lateral)


Sometimes I know what I'm after, but have a hard time expressing it in common terms...