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Turbulent flow and muzzle relief ports

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popgun pete

Well-Known Member
Jul 30, 2008
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I was just thinking about the changes in muzzle relief port size over the years, on the original Mares "Sten" there are four, 4.5 mm diameter muzzle ports. That is for a 13 mm diameter inner barrel gun shooting an 8 mm diameter shaft. If you compare that to a much later Mares "Cyrano" with a 11 mm diameter inner barrel and shooting a 7 mm diameter shaft then you will find that the four muzzle ports on the "Cyrano" have been increased to 8 mm diameter. That is a big increase in the combined cross-sectional area of the muzzle outlets, especially when considering the smaller annular column of water surrounding the shaft in a "Cyrano" compared to a "Sten" when the guns are cocked and ready to shoot. However we know that a vacuum barrel gun based on a "Cyrano" outperforms a wet barrel version of the same gun, so why is the gun being robbed of energy when the water contained in the inner barrel should flow out without any apparent restriction? I think that the answer has two components; one is that the water running past the barrel's inner wall experiences some drag effects just as water moving past a flying shaft imposes hydrodynamic drag on the shaft, the "wetted surface area" effect. By way of contrast the water immediately adjacent to the shaft moves at the same speed as the shaft, so initially there may be a velocity gradient across the width of the annular water column as you move from its centre to the outer periphery of the column. I suspect that this soon creates turbulent flow vortices along the length of the inner barrel (it looks smooth, but has a micro texture) which may actually lower drag on the shaft, but the piston still has to drive the water out of the inner barrel and that takes energy if the water drags on the barrel wall. Turbulence at the muzzle which occurs as forward moving water rebounds from the rear face of the shocker absorber (or piston anvil) and then breaks up into separate streams to exit sideways out the relief ports and forwards around the shaft to muzzle gap creates the second component of energy loss. No matter how large you make the circular relief ports, or incline the angle that they are set on, you will still have turbulent flow inside the muzzle and hence a degree of back pressure in the gun while there is still a fluid in front of the rapidly advancing piston, be that air or water. Obviously port size changes do improve the situation or the manufacturers would never have made them. Interestingly if you calculate the cross-sectional areas for the muzzle outlets on the early "Sten" guns then you will find that they do exceed the annular area between the inner barrel and the shaft, but not by much. If relief ports are made too large then the strength of the muzzle can be compromised, unless it is made of stronger materials that allow a more open or skeletonized structure.

The vacuum barrel condition eliminates any gas or fluid trapped inside the inner barrel, so turbulence inside the inner barrel and the interior of the muzzle is not an issue. Any water inside the muzzle spaces just before loading effort is applied to the spear has no effect except to provide some lubrication to the inner barrel as it already has "accommodation room" inside the muzzle, but that will not be the case if there are subsequent leaks through the muzzle seals.
 
Hi Pete,
I follow most of what you are thinking but maybe have a different train of thought and some unanswered questions:
Why is it called a vacuum barrel? The seal of the shaft provides the boundary from ambient hydrostatic pressure to the inner barrel which in my mind is still pressurized and not at vacuum. There is a difference of fluid and pressure but not vacuum (unless I'm missing something). The whole concept of the pneumatic gun is that differential pressure providing the force required to propel the arrow.
I don't have the two guns and left some of my tools to measure the diameter at a job yesterday but looked at my second generation Scubapro's and they would seem to be the 4.5mm cross hole diameter that you mentioned. I'll double check at a later date. If the flow restriction is in the muzzle cross holes then elongated holes may serve to increase the flow as would square holes of the same "diameter" keeping the same cross sectional strength.
If the flow restriction is from turbulence, would one be able to fill the inner barrel with a dyed water or gelatin and fire it in water with a video of the muzzle to see what the turbulence looks like outside of the muzzle in comparing the two guns?
I'm a little fuzzy about the word "rebound" and think of it as back pressure or flow restriction starting at the moment of firing with the piston held by the sear at ambient water pressure inside the barrel. When the piston is released there is an immediate pressure release from the stored energy inside the inner and outer barrels and consequential driving of the piston forward and increase of pressure inside the inner barrel until the piston reaches the anvil which at that point the pressure inside the barrel is again ambient. Which leaves me puzzled about the "vacuum barrel" design and back to my original thinking about the whole concept. The seal is, in that design, in two places. One is around the shaft at the muzzle and the other is around the sliding piston. The two provide an air space and the real difference that I see is a change of fluid and not vacuum. Compressible gas versus incompressible water.
I also agree that there is a drag consideration along both the inner barrel and shaft surfaces. The drag at the shaft surface should be continuous along the entire flight of the shaft. Do you agree? While there is drag against the inner barrel it would seem to be negligent if in air as with the "vacuum barrel". I would suggest that it may be more of a component if in water but I would assume that back pressure inside the barrel be the first unknown to warrant a check.
I think that could be done by plugging three of the cross holes and putting a guage on the fourth giving a relative pressure on the inner barrel "Sten" to "vacuum barrel" fired in water.
Sorry for the long thread, but I still have the overall question, what is the real benefit? Power is so hard for me to imagine and that may be to my prejudice of experiance with my pneumatic.
 
Hi Mark, I figured that I would hear from you on this one. I wrote it mainly because I have been rather surprised by the results Tromic achieved with his "Cyrano" which should be a much better "water breather", if I can use that term, than the "Sten" in all its pre-2001 model incarnations. The vacuum barrel results appear to show a significant improvement, yet that gun has really big relief ports. I have a Scubapro Magnum 95, I measured its muzzle ports and they are 5 mm diameter or very slightly under. The muzzle is screwed on so that the front edge of the inner barrel is just visible through the ports which adds a slight restriction. I have had the gun completely apart, but for some reason the muzzle appears to be staked as I had to undo everything from the rear end.

The term "vacuum barrel" is probably misleading, it is only a relative vacuum, but the main consideration is that whatever is in there before muzzle loading effort commences does not have to be pushed out of the muzzle, it just gets squeezed back from a very low pressure or vapour state to reoccupy the same space in the muzzle. The term "dry barrel" has been used for ambient pressure air barrels, but as there is always some water, albeit in residual droplet form, this is not really a correct description either. Dry barrels have been used in spring guns as a design feature, I do not recall any pneumatic spearguns being designed around this system, although one can certainly operate any gun that way as you just have to tip the water out after loading it. "Air bubble" would probably be a more appropriate description, but that does not sound very cool.

All my inner barrel drag comments in that post pertain to the water filled barrel case, I do not think the vacuum barrel as such contains enough gas or vapour to worry about. With the vacuum barrel gun cocked ready to shoot the pressure on the front piston face will be negligible except for the ambient pressure being transmitted to it through the body of the spear shaft. That is why guns using this system are less affected by ambient pressure at depth.

If you look inside a pneumatic speargun's muzzle there is the rear face of the shock absorber at the end of the cylindrical internal muzzle space and slightly back from there the ring of relief ports. Water being pushed along the inner barrel by the piston will be flowing onto that end wall represented by the shock absorber and moving back, except for whatever can flow out via the muzzle gap around the shaft and that includes the hole through the shock absorber body. Something like a hydraulic jump, except this is not open channel flow. The water has momentum moving axially in the inner barrel, so to change direction and pour out laterally it has to be accelerated and that requires additional energy. The mass of water is not that great, it should not require a lot of energy, so I figured that something else must be going on to make the vacuum barrel gun such an improvement over the wet barrel situation in a gun with very large relief ports. The only thing that I could think of was turbulent flow, in fact I am pretty sure that water accelerating along a narrow bore pipe will not be laminar flow except for the first centimetre or so of travel.

One way that flow may be improved in a wet barrel gun would be for the shock absorber rear face to be a conical shape projecting back into the muzzle space with a piston face to match and the relief ports elongated and running right up to the edge and even slightly beyond the rear outer edge of the shock absorber in its impacted position, however such a construction would be susceptible to jams on the conical faces.
 
Bye the way Pete, I enjoy your threads and responses and urge you to write anytime. I guess if I was more up to date on the latest guns I would not have so many questions.


What if the muzzle was larger in diameter? Let's just say for the sake of discussion enlarged to the outside diameter of the outer barrel. In the area of the cross holes there could be an enlarged communicating chamber under the cross holes to more easily communicate the water flow between the holes. If the holes could be drilled at a severe rearward angle, they would counter the action of the shaft then they may be able to reduce the "kick" of the gun even further. Since the turbulence is there anyway why not at least try to get some advantage out of it? I would also think that the cross hole size could be greatly increased and still end up with a stronger muzzle.
 
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There have been pneumatic spearguns with muzzles that are the diameter of the body of the gun, there is a ring of relief ports right around the periphery of the large nose cone which incorporates the muzzle parts. A relatively recent production gun was the Russian "Neptune" which was a mid-handle layout. Unfortunately the "Neptune" had quality problems due to cost cutting on skilled labor when its ambitious design really demanded a higher standard of internal parts finish, but the company wanted to offer the gun at a competitive price. A few guns were sold through Esclapez in France as the "Murena". I don't know if the big muzzle design was an improvement or not. The gun was still visible on the "Neptun" (sic) web-site not long ago, it being manufactured at Tula which is the Russian arms capital.

http://www.neptun.tula.net/page_eng.html

Just going back to the "dry barrel" case, there is no seal on the shaft at the muzzle, the air stays in the barrel only while the gun's muzzle is pointed downwards, just like in an open mouth, old style, diving bell. When you dive down the increased ambient pressure pushes water up into the barrel as the captive air trapped in the barrel compresses. These "dry barrel" guns, which are usually spring guns, were mainly used in shooting from the surface or at shallow depths. The air trapped in the barrel and the central grip handle offset some of the gun's weight in water, spring guns are heavy, as well as improved the shooting performance of the expanding spring due to the coils not being immersed in water. If the barrel flooded you had to lift the gun clear of the water and then tipped the water out through the muzzle opening. Some models had an air pump built into the rear end, you put your thumb over the breather hole in the pump handle to act as a valve and then pushed air into the gun to displace water out of the muzzle which was kept pointing downwards. This pumping operation was carried out after the gun had been cocked as spear insertion in the water usually flooded the gun. The best of these relatively complicated and expensive guns was inferior to the pneumatic gun, so they rapidly disappeared from the speargun arsenal once the latter became more readily available.
 
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