Since this thread has now become people arguing about using DWV fittings on a pneumatic, I don't mind being the propagator of a slight thread jack.jackssmirkingrevenge wrote:I used to agree religiously with that view, however my change of heart is explained in reasonable detail here.PotatoNick wrote:Where I disagree with you is the reason for having lift of up to .50d(or any value greater than .25d) . I don't believe that much, if any, piston bounce occurs during the time a projectile is in the barrel because you have so much force constantly being applied from the pressure pushing the piston backwards I would think that it would more than cancel out any opposing force from the very low spring rate of PVC.
Ah, I see what's happening. I don't think you've quite explained it properly though. Have you ever had a sealing face get sucked off the piston and shot out the barrel during a valve test? I believe that the same effect is occurring here, and that "piston bounce" is still a very minimal factor. It would just kind of cause the effect to induce movement of the piston a little sooner.
This is pretty advanced to actually try to solve because it is outside the range of standard fluid dynamics equations since we're using a compressible fluid that's traveling at such a great speed. I think fluid dynamics equations start to significantly lose their accuracy for compressible fluids at about Mach 0.3 .
What I believe we're seeing is some form of venturi effect.
Oh and even with all this said, I still don't believe in the d/4 rule
because of boundary layers and other characteristics of fluid flow.