Problem understanding liquid pressure

AI Thread Summary
The discussion centers on understanding liquid pressure, particularly in moving fluids within tubes. The main confusion arises regarding the pressure measurement in a tube, specifically whether it refers to the pressure of water at the top or bottom of the cooler. It is clarified that in a horizontal tube, pressure is generally higher at the bottom due to hydrostatic effects, with the pressure difference being minimal compared to atmospheric pressure. The conversation also touches on Bernoulli's Principle, suggesting that the inlet pressure of 4 bars may not directly relate to this principle. Overall, the need for a clear system configuration and understanding of average pressure values in fluid dynamics is emphasized.
dalin mald
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liquid in mouvement, pressure , Bernoulli equation,
Good morning.

I joined this forum to improve my self and give back what i will learn here for the members of this forum.

I m confused about the notion of pressure and i will explain the point that i don't understand.
since we know that the pressure is the force over surace. and since we know that the liquide pressure is depending to the depth . all is easy now.

so the hard side i did not know is when the liquide starts to move in a long tube for example.
Can you tell me which pressure we mean by the pressure on the tube. i ork in the ship and i always hear that the inlet pressure in fresh water cooler is 4 bars for example , is it the pressure of the up side water or the bottom water on the tube of cooler ?

than what is the pressure we mean in the equation of bernuli. they say the pressure energy exists so i agree because we have a pressure on a surface but is that pressure force is the same on all particuls , why they fix it ?

i really need a help.
i looked
 
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I think I understand your question, but am not sure. Please post a diagram showing the four pressures in a liquid to liquid heat exchanger, and indicate which ones you want to know about.
 
dalin mald said:
Can you tell me which pressure we mean by the pressure on the tube. i ork in the ship and i always hear that the inlet pressure in fresh water cooler is 4 bars for example , is it the pressure of the up side water or the bottom water on the tube of cooler ?
Just to repeat: we really need to know the system configuration. It's unlikely that at a pressure of 4 bar this has anything to do with Bernoulli's Principle/hydrostatic pressure.
 
dalin mald said:
Summary:: liquid in mouvement, pressure , Bernoulli equation,

so the hard side i did not know is when the liquide starts to move in a long tube for example.
Can you tell me which pressure we mean by the pressure on the tube.
A tube placed in a horizontal position will always have a higher pressure at the bottom than at the top.

The difference will be ##\rho g D## where ##\rho## is the fluid density, ##g## is the acceleration due to gravity (9.81 m/s²) and ##D## is the tube diameter.

But if we take the example of water (##\rho## = 1000 kg/m³), the difference is in the order of 10 000 Pa/m or 100 Pa/cm. Thus, for a tube with a 15 cm diameter, the maximum pressure difference is 1500 Pa, which is usually negligible compared to, say, atmospheric pressure (101 000 Pa; about 1.5 % difference). So the pressure is an average value. Just like the velocity of the fluid is an average value as well.

If the difference would be non-negligible, then you would probably not be able to make such simplification to study a liquid in motion.
 
thank you friends for help
 
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