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Bob uk
Unregistered guest
Posted From: 94.197.122.73
Posted on Friday, 29 August, 2014 - 07:50:   Edit PostDelete PostView Post/Check IP

Deep sea fish swim OK.

Myth busters swam in syrup and water in the same amount of time.

So I still don't know however when liquids are compressed the friction against the pipe wall increases and so does the shear between molecules. It make me visualize the liquid as being thicker because shear and viscosity are related.

Liquids do compress but it is ignored unless the volumes are large like 100 mile 2ft dia pipe line.

As I said this is outside my realm of hydraulic knowledge. So I won't assume anything until someone explains what I mean.

(Message approved by david_gore)
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David Gore
Moderator
Username: david_gore

Post Number: 1453
Registered: 4-2003
Posted on Friday, 29 August, 2014 - 17:41:   Edit PostDelete PostView Post/Check IP

Bob,

Fluid friction as measured by pressure drop between two or more points in a system containing a moving fluid is related to the following main factors:

1. Velocity and viscosity of the fluid moving in the system.

2. Surface roughness of the containing vessel.

3. Whether the manner of flow is laminar or turbulent - the manner of flow can be predicted by calculating the Reynold's Number for the system. Turbulent flow results in a greater pressure drop than laminar flow for a given system. A Reynold's Number of 2100 approximates the point of transition from laminar [i.e. R.N.<2100]> 2100]

Any apparent reduction in volume deemed to be compression of the fluid is most likely due to expansion of the closed vessel/pipe containing the fluid increasing its volume rather than actual fluid compression. Provided the induced stress in the containing vessel is below the elastic limit of the material used for this vessel, releasing the pressure will allow the vessel to return to its normal volume and the volume of fluid as indicated by the fluid level will appear unchanged. If the induced stress is greater than the elastic limit; there will be a permanent increase in the volume of the container and the volume of fluid will appear to have decreased when the pressure is released by a lower fluid level than that which existed when the pressure was first applied.

Any further explanation is too extensive and detailed for inclusion in this forum and anyone interested in going further should consult a text book on fluid flow and associated dynamics.
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Bob uk
Unregistered guest
Posted From: 94.197.122.82
Posted on Saturday, 30 August, 2014 - 07:28:   Edit PostDelete PostView Post/Check IP

I can visualize the pressure drop at high flow rates because a garden hose pipes do exactly that. I ignore a lot of stuff when I work on hydraulics because applying friction and hoop stress of pressure vessels just complicates stuff. That moog cnc machine took into account volumes of fluid rather than pressure and it got confusing. And one continulay had to go back to basics and apply it to what's gone wrong. The longest the machine ran without going wrong was 4 Hours everybody was so pleased that it actually worked.

A race motor bike shop near me, pump oil into tubes to force the tube to balloon. Two stroke expansion chambers. The machine measures volume rather than pressure. Because the chamber has to be a certain volume. The shape is not important which in anycase will always be smooth curves. He showed me a chamber that is banana shaped. Because the tube is curved the inside is compressed and the outside stretched and thinner the hydraulic pressure pushes the thinner steel easier which then gets thin still and so on until it's the correct volume
The tube starts of 48 thou 18 swg and ends up as thin as 8 thou. I had a go and my one looked ok but he threw it away because the volume was wrong.

If a sheet of aluminum is suspended in water and a shotgun blank fired in the water next to the sheet the pressure wave forms a parabola in the sheet.

The water can't move fast enough so a shock wave bounces the molecules off each other like snooker balls in a line or Newtonian cradle desk toys.

Question.
If I stretch say a 1" dia steel bar by hanging a ton off it. It will stretch a bit. Not a lot. Would it stretch if I hung 1. Kg only. I know if it does it would be very small.

I think it does because up to the critical point ( where the deformation becomes permanent the elastic limit.) the graph is reasonably straight until the elastic limit then the graph flattens to the critical point then the graph goes horizontal then slopes down then the sample snaps.

Or does the steel completely resists the 1 kilo. And not stretch at all. ALL the samples I have tested have been on an Avery machine which started at 50 kgf.

I used to wind the machine up until the sample was about to pop and as soon as the boss looked away I'd snap the sample and every time he'd jump and yell. I used to put chocolate bars in the swing tester. Which proved that chocolate shatters when frozen. Nutbrittle toffee explodes and goes everywhere.

(Message approved by david_gore)
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Jan Forrest
Grand Master
Username: got_one

Post Number: 638
Registered: 1-2008
Posted on Saturday, 30 August, 2014 - 20:45:   Edit PostDelete PostView Post/Check IP

"If a sheet of aluminum is suspended in water and a shotgun blank fired in the water next to the sheet the pressure wave forms a parabola in the sheet."

Just as Barnes Wallace proved, although he used a larger ... compression device and put it next to something somewhat more intractable than a thin sheet of ally!
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David Gore
Moderator
Username: david_gore

Post Number: 1455
Registered: 4-2003
Posted on Sunday, 31 August, 2014 - 08:57:   Edit PostDelete PostView Post/Check IP

"If a sheet of aluminum is suspended in water and a shotgun blank fired in the water next to the sheet the pressure wave forms a parabola in the sheet."

Only if the edges of the sheet are restrained or the surface area of the sheet is considerably greater than the area in which the shock wave is generated by the explosive charge.

This leads into the interesting field of explosive forming of metals. Due to the use of this process in defence, aerospace and other sensitive areas, the current state of the art is difficult to determine due to secrecy in order to protect technology and competitive advantages:

http://en.wikipedia.org/wiki/Explosive_forming

http://www.engineershandbook.com/MfgMethods/exforming.htm

http://thelibraryofmanufacturing.com/high_energy_rate_forming.html

I have seen fascinating architectural and artistic works made by explosive forming which would have been extremely difficult, expensive if not impossible to make by conventional means.
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Bob uk
Unregistered guest
Posted From: 94.197.122.81
Posted on Sunday, 31 August, 2014 - 09:22:   Edit PostDelete PostView Post/Check IP

Quite a bit larger. I never connected the two together exactly the same principle. Early on in the week a Lancaster bomber flew over my house followed by a Spitfire. Depth charges only have to be near a submarine to sink it. The Royal Engineers detonated a mine in Bournemouth bay, they had a two mile zone and I was 4 miles and it was very loud with a massive water spout. Explosives frighten the c**p out of me.
Another question. If an explosive is detonated in a pressure vessels that is strong enough to contain that explosion would the vessel have pressure in it after.

(Message approved by david_gore)
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David Gore
Moderator
Username: david_gore

Post Number: 1456
Registered: 4-2003
Posted on Sunday, 31 August, 2014 - 17:01:   Edit PostDelete PostView Post/Check IP

Bob,

All the explosive forming work I have actually seen was done in an open drum, pond or dam depending on the size of the work and the explosive charge. Any shock wave that is not directed onto the work is dissipated as a water spout/wave and is not utilised.

As far as an enclosed explosion in a pressure vessel goes, whether there is any residual pressure in the vessel will depend on the relative volumes of the original explosive charge, the volume of insoluble gas resulting from the explosion and the volume of any liquid residues produced by the chemical reaction responsible for the explosion. However, I suspect a successful enclosed explosion would be problematic as the whole process depends on a high pressure differential between the original material surface and the face of the mould used to generate the required finished product. The back pressure from a confined explosion would be expected to reduce this pressure differential leading to less-than-optimum deformation.
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Jan Forrest
Grand Master
Username: got_one

Post Number: 639
Registered: 1-2008
Posted on Sunday, 31 August, 2014 - 20:50:   Edit PostDelete PostView Post/Check IP

For relatively small explosive devices that cannot, in practice, be detonated in the open air where they've been found, the army Bomb Disposal Teams have a large and extremely tough spherical detonation 'safe'. It is more for the backpack/briefcase nail bomb types where significant damage could be caused to surrounding buildings if the shrapnel wasn't contained. Where possible it is only used to transport any suspected device to a safe area, but is strong enough to contain the blast if it should detonate accidentally or by a timer which cannot be disabled.

However the left-over static pressure isn't the problem: The peak impulse pressure can be hundreds of times as massive leaving behind stress micro fractures which can only multiply, spread and join up over a series of explosions. What kind of service life could be expected is a matter of conjecture.

The same must be true of a RR/B accumulator; albeit at a far lower rate and over a much longer time frame.
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Bob uk
Unregistered guest
Posted From: 94.197.122.83
Posted on Monday, 01 September, 2014 - 04:28:   Edit PostDelete PostView Post/Check IP

I think a sphere would last years. Maybe 100 years.

My idea was to explode a stick of dynamite in a pressure vessel then connect the pressure to an air motor. Totally mad. And very dangerous.

When I was small I remember half a wardrobe in the top of a tree blown there by a bomb dropped by the nasties trying to bomb RAF northolt. My dad was an NCO in REME and told me how strange things can happen with explosives. He said he once put a charge on a telegraph pole and the post jumped up and landed upright on the stump with the wires still attached so they pulled it down with a truck.

In the film flight of the Phoenix to start the engine they used explosive cartridges in a Hoffman starter, which spins a flywheel which engages with engine and the engine starts. So my idea is not entirely new.

(Message approved by david_gore)
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David Gore
Moderator
Username: david_gore

Post Number: 1457
Registered: 4-2003
Posted on Monday, 01 September, 2014 - 07:51:   Edit PostDelete PostView Post/Check IP

Early spark ignition internal combustion engines installed on farm machinery often used what are best described as blank shotgun cartridges for starting before electric starter motors became common. Have never started an engine this way although they are often seen running at old machinery displays, I presume starting cartridges are still made for use by restorers or a removeable external device with a slip clutch acting on the flywheel retaining bolt is used to start these engines.

Some early R-R Merlin aircraft engines used cartridge starters [Mark 2 type 329 Merlin was one instance]

The diesel equivalents had a compression relief valve that allowed you to "wind-up" the engine by hand until it was rotating fast enough to close the relief valve and the compression heat would then ignite the fuel. Have "fond" memories trying to start an old single cylinder Lister diesel in the dairy on cold winter mornings, dropping ether into the air intake usually did the trick to get the engine away although warming the cylinder head with a blowtorch was necessary on really cold mornings.

Life was always interesting in the "good old days"........
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Randy Roberson
Prolific User
Username: wascator

Post Number: 291
Registered: 5-2009
Posted on Monday, 01 September, 2014 - 11:57:   Edit PostDelete PostView Post/Check IP

Hummmn; for some reason I could not reply to BobUK's post.
Anyway, Bob if you load a steel rod or anything else even a little, it will deflect, perhaps a tiny amount, but it deflects nonetheless.
Yes large pipes will stretch when filled with liquid under pressure. This springiness of the pipe is many times greater than the tiny amount the liquid might compress. Even thouh we use water to test the integrity of pipelines, it will still spray out very impressively if it ruptures because of the spring in the pipe. Just not nearly as violent as, say 10 miles of 30 inch full of compressed gas at 750 psi. That is quite a lot of energy. Occasionally someone gets the bright idea to test a fabrication with nitrogen because it would be difficult to get the water out after a test. If it ruptures, it usually destroys the entire assembly, as the Chinese learned a few years ago when testing piping in a plant with nitrogen.
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Jan Forrest
Grand Master
Username: got_one

Post Number: 640
Registered: 1-2008
Posted on Monday, 01 September, 2014 - 20:57:   Edit PostDelete PostView Post/Check IP

On the question of peak pressures inside an enclosed volume of space: Firearms manufacturers have used a simple, but very rugged, device for decades if not centuries. A relatively small cylinder is welded to the side of the pressure container with a piston and mild steel 'crush ball' behind it.

When the test round is fired some of the peak pressure is bled off onto the top of the piston so that it's pushed back onto the ball, thus squashing it. Afterwards the deformation is precisely measured down to micron scales and the peak pressure read off from a pre-printed non-linear scale.

Some time back I had a small, single cylinder, 4-stroke engine which I took off an outdoor saw table. For ease of starting it had a 'floating' exhaust valve. Using the pull start the valve would stay open a fraction of an inch thus reducing the peak compression. Once started the pressure of the fuel igniting would push it closed until the cam lifted it open again.