Has An Important Property Of Fluids Been Overlooked ?

[Fletcher]

I think a simple analogy to help think about this proposal is ...

1. if we could have a symmetrical arrangement around a central pivot as I propose here except we had masses that floated in fluid when brought together, & the fluid was miraculously held behind a force field that was at right angles to the containment structure, but the masses could move freely thru the force field, then we can visualize the ascending side mass displacing fluid until equilibrium of forces was established [Archimedes volume displacement buoyancy].

N.B. the force field has the ability to adjust to mass volume displacement.

2. most, I would hope, would see that the ascending side mass's COM has been incorporated into the fluids COM at the axle, so the COM of mass on the descending side free of the fluid has the only torque.

3. after 180 degrees [12 to 6 o'cl] the switch occurs so that the descending side always has some positive torque.

4. this would be an Archimedes volume displacement Virtual Displacement of Mass IMM.

[daanopperman]

Fletcher ,
Would this mass also be taken up by the container if it was on a pivot

[Fletcher]

So what I am attempting is to create equilibrium of forces on the ascending side using Mass Induced Pressure Increase & NOT Archimedes Volume Displacement equilibrium of forces i.e. weight force = upthrust force.

Questions:

1. does applying force [depressing a piston] onto a contained fluid increases pressure ? - yes, we know that - that's how hydraulics work.

2. is equilibrium of forces achieved ? - yes, the force creates pressure in the fluid - it can't create any more pressure unless the force is increased, in which case the fluid pressure will increase in response to maintain equilibrium of forces.

3. is any work done ? - no, not according to physics because nothing has moved - force is just something that pushes or pulls something else.

4. can mass's weight force be used as a source of force ? - yes.

5. if a weight force is used to create pressure & equilibrium of forces is achieved [because pressure stabilizes] but nothing moved [no work was done] then is the mass acting as a piston to increase fluid pressure also a buoyancy device ?

Answer - I believe so !

6. where would the mass acting as a piston COM be located in relation to the pressurized fluid ?

Answer - I believe it will be felt by the system at the axle like happens in 'ordinary' Archimedes floatation [volume displacement pressure equalization] buoyancy/upthrust.

[Fletcher]

Fletcher , Would this mass also be taken up by the container if it was on a pivot

Dan .. no - the reason is [if I have interpreted your unlabelled diagram correctly & assuming those are springs & masses & the coloured part is fluid container which is centrally pivoted] because the force of gravity only acts vertically downwards here - therefore there is no sideways [lateral] component of gravity or weight force acting on the container - i.e. no ability to pressurize the fluid because the mass only acts downwards.

[daxwc]

Yes, I understand your principle and argument; it is counterintuitive which one would expect if it got overlooked before. I have been racking my brain to think if I ever seen its effects in a mechanical, hydraulic or hydrostatic application before.

Is this any different than strain or stress on a solid lever?

[Fletcher]

dax .. gases are compressible so a piston does work to compress a gas.

Solids are also compressible, if enough pressure is applied.

Liquids are non compressible [except at extreme pressures] - this is a hard point to take in - if you push on a piston which can't compress the volume of the fluid then has any work been done on the system ? - well, no work was done on the system although pressure increased - your muscles did work & expended energy but no work was done on the system.

The inevitable thought is how can pressure increase if the volume didn't reduce & the piston never moved ? - it seems to me the energy you expended in creating that force caused the fluid pressure to increase - it seems your muscle energy created increased energy density in the fluid.

In this case we are not using muscles as force to pressurize the fluid but using the weight force of a mass - the result is the same.

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

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Anyhoo .. as we all know only too well solids transmit forces very well, in the direction of that force - if I had a stick long enough I could push the moon.

Fluids do not support shearing stress so a force [as pressure which is force x unit area] applied is transmitted everywhere undiminished in that fluid as per Pascal's Principle - this should mean that the mass causing the pressure increase is in fact buoyed by that pressure differential created below in the fluid & above by the atmosphere - to my way of thinking the force isn't transmitted in one direction like a solid but in multi-directions simultaneously, just like Archimedes upthrust does to a body partially or wholly submerged.

EDIT: another way of thinking about using it, i.e. the muscle increasing energy density in the fluid example [potentializing the fluid], is to think of the piston mass's PE of POSITION potentializing the fluid as pressure [giving the entire fluid volume a greater energy density by absorbing that PE of position] - since its energy density has increased by this amount & it is symmetrical about the axle its torque has been nullified.

I steer clear of using energy terms here.

That's the crux of it !

[daxwc]

The inevitable thought is how can pressure increase if the volume didn't reduce & the piston never moved ?

"In nature, all the fluids are compressible, but we define incompressible fluids for our convenience of study."
http://www.differencebetween.com/differ ... le-fluids/



It does move just so little we take it for granted; therefore work is being done.
The PE drop of the mass you charge in first, this becomes the applied pressure. When it is finished applying its pressure, then the system feels the mass’s weight where it is.

This is similar to stressing a solid lever arm or a pivot pin. Imagine if a solid lever arm was springy; thus when loaded with mass and bowed, the stress is felt all over the arm, but its weight does not change the COM horizontally after charging the spring even though it is supplying stress else where.

[daanopperman]

Fletcher ,
Thank you for your reply , m.a. , your analysys of the drawing is completely correct . I have drawn springs on both sides with a purpose , but only the left side piston was worked upon by the spring . It does not matter if the pressure on the piston comes from gravity or from a spring , you are saying it is the pressure inside the container , not the gravity , that will spread the mass of the piston equally all over the container . I added a spring on the side of the container working on the piston with the same force applied to the piston as what gravity would have supplied in a vertical position . If I added a spring on top of the piston when in a vertical position and apply pressure to the spring , why would the container be forced down on that side of the pivot . If I remove the piston and cover the container neck with a very light cover so there is no mass , and then apply pressure to the cover via spring , if you are correct the container would not descend on that side .

Daan

[Fletcher]

The inevitable thought is how can pressure increase if the volume didn't reduce & the piston never moved ?

"In nature, all the fluids are compressible, but we define incompressible fluids for our convenience of study."

http://www.differencebetween.com/differ ... le-fluids/


It does move just so little we take it for granted; therefore work is being done.

dax .. that's being pedantic ;7) - there's always some movement even if virtually undetectable - things expand & contract just with temp changes - in the true sense work is being done I suppose because a force is applied & something moves a very small distance, even if you can't see it move - when I push on a brick wall maybe the part I'm pushing on moves a fraction also - but that wouldn't be considered work by physics even though I was tired.


About incompressible [non compressible] fluids - as the sources say all fluids are compressible if enough pressure is exerted.

The way I look at it is this - fluid molecules are closely packed with no space between them to allow a reduction in volume & increase in density [ordinarily] - if we apply pressure to a fluid its temperature increases [all else being equal] - therefore to maintain volume & density [i.e. be incompressible] the expansion rate due temp increase must exactly equal any compression factor - JMO's.

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Your last post thoughts brought me back to Tarsier's comments a couple of days ago.

The idea I have IS counter-intuitive [it could even be wrong] but proving it either way is the objective starting with thought experiments.

I racked my brains also trying to think of something more practical & I think I may have hit on something that engineers will appreciate - as you know I'm not an engineer but plenty here are - I will be interested in their opinions in particular based on this practical example that I think should be thought provoking.

I have to draw up a pic so will be back when I can.

[daxwc]

dax .. that's being pedantic

Sorry, I don’t mean to be, I was serious. It is just that a lot of energy can be put into that very small movement to compress fluid. You realise it when you break things apart or bleed under dead static pressure.

I know it seems like I am trying to tear the property down unfairly; Fletcher, I like your principle whether it turns out to be true or false. I never even thought that the applied pressure might be shifting COM, and there is some very good logic to it. It does needs proved it either way; I was just giving the reason why it could be false.


Deep down I am rooting for you and wishing it the best of luck ;)





I know you don’t wish to hear it but applied pressure is a little bit like a spring 8P

[Fletcher]

No problemo dax ..

We're only talking about a few PSI here.

I know you don’t wish to hear it but applied pressure is a little bit like a spring 8P

And so it is, only it doesn't push just downwards but in ALL directions evenly.

[Fletcher]

OK .. changing the perspective somewhat to a real engineering problem that needs a solution - but the principle is the same AFAIK.

I want to build a silo on my property - I want it big & symmetrical & up off the ground - I want it to be easily removable so the tank is separate from the stand & the legs of the stand are not attached to the plinths I want to use.

N.B. the only available space is a level sandy area but it has big rocks in the way - I'm too lazy to move the rocks & I don't want to spend money on getting machinery in to dig them out - I also don't want the expense of long thick posts deep into the sand to act as friction piles & I don't want to dig trenches between posts & put in floating cross beams to spread the load over a larger area - I want a quick solution & one where I can take down my silo & move it easily if I want.

I decide to use solid plinths under each leg - but wait, the rocks are in the way so that I can't center the legs on the solid plinths & if I don't do that the plinths will tip over in time due uneven loading & the silo & stand will lean over - that won't look so good.

I have a think about it & decide to use fluid filled plinths to spread load evenly - the legs sit on top resting on a plunger/stopper or membrane etc - the plinth containers are very sturdy & strong - I don't need to center the load on these plinths because the load will be spread evenly to the bottom surface of the plinth evenly [therefore I just have to find a clear space big enough for the plinths] & it won't tip over wherever the load is positioned on the top i.e. there will be no torque in the plinth & it will settle evenly in the sand - problem solved.

Let's have at it then ! :7) - where have I gone wrong ?

[rlortie]

You have gone wrong by the fact that fluids seek not only its own level but equalization of applied pressure. Your fluid filled plinths will not react any different than solid plinths.

This discussion reminds me of my military college class for preventive maintenance on concrete runways and parking aprons. I learned how to level (lift) 8" thick slabs of concrete up to 800 square feet with 25 pounds of air and slurry mixed cement. You would be surprised to learn how much 25 psi can lift level if given the square foot area.

Ralph

[jim_mich]

The pressure exerted by your four posts onto your fluid filled plinths will be extremely great. I'm not sure you fully understand how strong you will need to make these fluid filled plinths. Any tiny leak and they will deflate and then only the outer walls of the plinths will be supporting the silo and its contents.

First I make the assumption this is a real life problem looking for a solution and not just an illustration of the previous discussion. And I assume you have lots of sand available. And I assume you have more time than money. And I assume the rocks are not extremely high above the ground.

I propose constructing four circular fences and filling them with sand. These fences could be constructed from any solid material, say wood planks, with steel cable bands around them. Simply fill the enclosure with sand until the rocks are buried. Then build the plinths on top of the sand.

When it comes time to dismantle, simply remove the fence. Given enough time, natural wind and rain will wash the sand back flat. Or you will be left with four mounds of sand where once the ground was flat. You could always move the sand back to where it came from.

[Fletcher]

Thanks jim_mich - it looks like I put a bit too much realism into that scenario - actually I will be installing two 10,000 liter water tanks in a few weeks & I'll probably do something very similar to what you suggest, like I've done before - I won't be investing in water filled plinths ;7)


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Ralph .. thanks for the input but I tend to disagree - the fluid can't find it's own level because it can't move & the volume & density remains the same - pressure increase is always linear from what you started from.

Since fluids are isotropic I can't yet see why there should be a tilting force on the bottom of the symmetrical plinth with load because forces are in equilibrium & spread evenly thru the fluid internally within the plinth.


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I have included a couple more pics for consideration - this time I compare 2 fluid filled plinths [part hydraulic press analogues] - one is as I propose & the second has a snorkel tube attached to create the same internal pressures [this takes a tiny amount of fluid & work done to raise it].

1. will both plinths behave the same ?

2. Is this an example of the Hydrostatic Paradox in action for both ?

N.B. Hydrostatic Paradox says that the weight force of the system is the mass sum of the components - the internal pressure in the vessel is proportional to fluid depth & density, not volume - the bottom fluid pressure internally in the vessel can be converted to a force vector internally in the vessel.

Is there load spreading or will the off-set load create a torque in the plinth & why please ?


http://physics.kenyon.edu/EarlyApparatu ... radox.html