Pressure exerted on fluids

  • Thread starter hhhmortal
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  • #1
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Hi,

I'm trying to investigate how the force exerted onto a human body is affected by the presence of a surrounding liquid. Since the liquid cant compress the pressure is distributed in all directions. If a person is accelerated with this liquid, he/she + the mass of liquid will feel a force (F= ma).


If we consider the person alone he/she will feel a certain force due to acceleration, adding in the mass of the liquid will increase this force, so how does the liquid the person is immersed in, make a different to the pressure received by that person?
 

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  • #2
sophiecentaur
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G suits are a good example of this happening. Left to itself, a body in the seat of an aircraft in a tight curve will flatten into the seat and the blood will flow down into the leg and lower abdomen veins , starving your brain of blood and oxygen. The G suit is a strong envelope which can't distort. There is a small amount of fluid in the suit (between two shells so you don't get wet). When you go into the curve, the pressure of the liquid in the legs of the suit and lower parts of the suit balances the hydrostatic pressure in your veins so blood doesn't flow into them and away from your brain. If you were immersed in a tank of liquid, instead of a G suit, the same thing would occur.

The brain gets similar protection against knocks as it floats in a fluid inside your rigid skull.
 
  • #3
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G suits are a good example of this happening. Left to itself, a body in the seat of an aircraft in a tight curve will flatten into the seat and the blood will flow down into the leg and lower abdomen veins , starving your brain of blood and oxygen. The G suit is a strong envelope which can't distort. There is a small amount of fluid in the suit (between two shells so you don't get wet). When you go into the curve, the pressure of the liquid in the legs of the suit and lower parts of the suit balances the hydrostatic pressure in your veins so blood doesn't flow into them and away from your brain. If you were immersed in a tank of liquid, instead of a G suit, the same thing would occur.

The brain gets similar protection against knocks as it floats in a fluid inside your rigid skull.
Doesn't the fact that there is now more mass on the person mean there is a bigger force (assuming same acceleration of aircraft) directed on to him/her, hence how does pressure differ? Im not sure im being very clear.
 
  • #5
russ_watters
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The questions don't make a whole lot of sense to me.
 
  • #6
Doug Huffman
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Imagine G. H. Hardy's patience with S. Ramanujan who was brilliant but un-schooled.

http://en.wikipedia.org/wiki/Liquid_breathing#Space_travel
Liquid breathing has also been proposed for use in ... space travel.
[ ... ]
Space travel

Liquid immersion provides a way to reduce the physical stress of G forces. Forces applied to fluids are distributed as omnidirectional pressures. Because liquids cannot be practically compressed, they do not change density under high acceleration such as performed in aerial maneuvers or space travel. A person immersed in liquid of the same density as tissue has acceleration forces distributed around the body, rather than applied at a single point such as a seat or harness straps. This principle is used in a new type of G-suit called the Libelle G-suit, which allows aircraft pilots to remain conscious and functioning at more than 10 G acceleration by surrounding them with water in a rigid suit.

Acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15 to 20 G.[37] Extending acceleration protection beyond 20 G requires filling the lungs with fluid of density similar to water. An astronaut totally immersed in liquid, with liquid inside all body cavities, will feel little effect from extreme G forces because the forces on a liquid are distributed equally, and in all directions simultaneously. However effects will be felt because of density differences between different body tissues, so an upper acceleration limit still exists.

Liquid breathing for acceleration protection may never be practical because of the difficulty of finding a suitable breathing medium of similar density to water that is compatible with lung tissue. Perfluorocarbon fluids are twice as dense as water, hence unsuitable for this application[38]. On the other hand, although perfluorochemicals are denser than water, lung tissue floats within the PFC filled lungs, and if the lungs are not over-filled, there is no compromise in pulmonary or systemic blood flow[39]. Therefore, if the astronaut is immersed in liquid and their lungs are filled with liquid PFC, they should not experience adverse effects, in spite of the almost twofold density difference. Based on interviews with adult patients that experienced partial liquid ventilation, when they became conscious they were unaware that 20-30 ml/kg of PFC was in their lungs during recovery.
 
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Since the liquid cant compress the pressure
But how can a liquid compress pressure?
so how does the liquid the person is immersed in, make a different to the pressure received by that person?
It will increase. The liquid has to get normal reaction from somewhere and it will be the dude inside the fluid.

And it's static pressure which distributes itself, here it's dynamic pressure...so the force won't distribute.
 
  • #9
sophiecentaur
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With lungs full of air, the thorax can collapse under the hydrostatic pressure until the pressure equalises. Blood (and even soft tissue) from the head - or whatever other bits were 'above' the lungs will tend to flow downwards - pushing the lower bits of the lung upwards.
There is no difference between static and this "dynamic" pressure you mention because the timescale is much longer than any other time constants in the system.
I would imagine that, under moderate g forces, an air pressure regulator system could 'blow' air into the lungs to maintain their volume. This is, in effect, what SCUBA valves do and they can operate under quite extreme conditions. You would probably need a tube to bypass the epiglottis, though, to avoid all the problems which divers can encounter when they don't breathe out whilst coming up.
 
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