Question for users of GGDT...

[D_Hall]

We all know GGDT gets wonky in the transonic region.

Question: Does GGDT tend to over or under estimate velocities? I suspect I know the answer, but as I've not built any such guns myself I want to hear it from you folks.

[jeepkahn]

it depends

On cannons where the barrel is smaller than the valve porting it tends to overestimate, but on guns where the barrel is larger than the valve porting(my modguns with larger than valve barrels) it underestimates... Albeit, the differences between ggdt and actual are normally within 10%-15% but that can be due to gigo....

on another note, concern heating and cooling of the gases in the barrel, when looking at the text in some of my gdt files, when I'm using highpress/heavy projectiles it has the barrel pressure rising for the first 40% of the barrel but the barrel temp is dropping... If the gases are being compressed in the barrel behind the projectile, wouldn't the temp rise??? just curious...

would this be showing where the barrel breech is choking flow???

[Gippeto]

Modeling and testing (chrony) with my copper "marble shooter", ggdt was within 20fps for both air and co2. Velocities were in the 750-850fps range measured. Ggdt was predicting higher.

Mach bb experiment ggdt'd at ~1600fps and chronied at ~1050fps at 5 feet from the muzzle.

[inonickname]

When I use it in higher energy regions it tends to heavily overestimate (not taking speed of sound in the working fluid into account). It has underestimated some lower power guns but it usually tends to overestimate more. I haven't had many problems with lower velocities.

[psycix]

My 16mm marble gun powered by a relatively large chamber with a 1/2" QEV (effectively only a 14 mm porting I believe) fired at 270-275 m/s @ 30 bar and 295 m/s on 40 bar. Measured with a chronometer.

GGDT gives around 250 m/s for 30 bar and 275 m/s for 40 bar. This is with a flow coefficient of 50%.

In my case GGDT is too low.
Am I correct when I say that GGDT was calibrated to be the most accurate on average spudguns, running on 100-150 psi with medium muzzle velocities?

[btrettel]

D_Hall, you might find this thread helpful for this purpose: http://www.spudfiles.com/forums/transon ... 18831.html

on another note, concern heating and cooling of the gases in the barrel, when looking at the text in some of my gdt files, when I'm using highpress/heavy projectiles it has the barrel pressure rising for the first 40% of the barrel but the barrel temp is dropping... If the gases are being compressed in the barrel behind the projectile, wouldn't the temp rise??? just curious...

A gas simply flowing into a region where the pressure of that region increases will not necessarily increase in temperature. If the gas slowed down suddenly, yes, that could increase the temperature due to the conservation of energy, but that's not what's going on here.

Adiabatic compression (where the temperature increases) is due to work being done on the gas. The energy can go into multiple places. For quasi-static gasses (and in general) the energy goes into thermal energy, increasing the temperature.

Similarly, if the gas does work on something else (in this case, the projectile), energy leaves the gas. And often this energy leaves in the form of thermal energy, so the temperature decreases.

With these facts in mind, there is no inconsistency.