Pressure Gradients along an angle pipe

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Discussion Overview

The discussion revolves around the behavior of pressure gradients in a pipe connecting high and low pressure volumes, particularly when the pipe is angled. Participants explore how the orientation of the pipe affects the pressure isobars and the resulting reaction forces on the pipe walls. The conversation includes theoretical considerations and practical implications, touching on fluid dynamics and rocket thrust mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that in a straight pipe, isobars would be parallel to the pressure difference and perpendicular to the pipe, while expressing uncertainty about the configuration in an angled pipe.
  • Another participant argues that isobars are influenced by gravity and that the pressure on the opposite wall is uniform, depending on fluid height and density, challenging the idea of a reaction force varying with pipe angle.
  • A participant clarifies that gravity acts perpendicular to flow direction and that isobars reflect pressure gradients after a valve is released, indicating a need for clearer diagrams.
  • There is a suggestion that the diagrams may need to be rotated for correct orientation, and that the situation is complex, depending on valve orifice and other factors.
  • One participant introduces a bottle rocket analogy, questioning how gimbal angles affect action and reaction forces, proposing multiple scenarios regarding the direction of forces and resulting torques.
  • Another participant agrees that if the reaction force does not pass through the center of gravity, it will cause rotation, and emphasizes the importance of understanding the practical context of the question.
  • A participant mentions that the original question was prompted by a CFD result, noting interesting behavior of isobars when high and low pressure volumes are separated by an angled surface.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between pressure gradients, isobars, and reaction forces in angled pipes. There is no consensus on how these factors interact, and the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants note the complexity of the situation, which may depend on various factors such as the orifice of the valve and the specific configurations of the system being discussed. There are also indications of missing assumptions regarding the influence of gravity and fluid dynamics.

Timtam
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I have a high pressure volume coupled with a low pressure volume by a pipe and valve .
If the pipe was straight I would expect the isobars of the pressure gradient to be parallel to the pressure differences and perpendicular to the pipe , I would expect a reaction force (red arrow) on the wall directly opposite the valve acting perpendicular to the isobars (no shear component)

upload_2016-12-20_14-24-16.png

If the pipe was angled I am unsure how the pressure gradients isobars would look ?
Would they still be parallel between the pressure difference
upload_2016-12-20_14-23-1.png

or would they still be perpendicular to the pipe ?
upload_2016-12-20_14-21-58.png

Would the angle of the pipe make a difference to where the reaction force angle on the opposite wall
 
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The first two diagrams must be correct ...last one incorrect ... theses isobars are the result of gravity , no gravity , no pressure ... It's not true that the opposite wall has a 'reaction force ' depending on the pipe ... the whole of the opposite wall has the same pressure exerted on it , dependent on the height of fluid above it , density of fluid , and gravity. the pressure at any point is ...P =hrg ...(r is dencity)
 
oz93666 said:
The first two diagrams must be correct ...last one incorrect ... theses isobars are the result of gravity , no gravity , no pressure ... It's not true that the opposite wall has a 'reaction force ' depending on the pipe ... the whole of the opposite wall has the same pressure exerted on it , dependent on the height of fluid above it , density of fluid , and gravity. the pressure at any point is ...P =hrg ...(r is dencity)
Hi Oz , sorry i could have been clearer in my diagram, the gravity acts perpendicular to the flow direction between the vessels. The isobars reflect the pressure gradient between the two volumes once the valve is released. I could have probably simplified it by removing the low pressure volume and just said ambient pressure
 
I see ...so we have to rotate the diagrams through 90 degrees to get correct orientation ? and the isobars are not gravity dependent , but due to different pressures in reservoirs?

Then there is no steady state , the situation is very complex depending on the orifice in the valve and other factors ...can you describe the situation in more detail...
 
oz93666 said:
I see ...so we have to rotate the diagrams through 90 degrees to get correct orientation ? and the isobars are not gravity dependent , but due to different pressures in reservoirs?

Then there is no steady state , the situation is very complex depending on the orifice in the valve and other factors ...can you describe the situation in more detail...

Perhaps I could simplify it by asking it another way , in the thrust of a Bottle rocket (aka our high pressure volume ) fitted with a gimble, Does the gimble angle change the direction of the action force at the base , the reaction force in the nose or both

upload_2016-12-21_16-31-36.png

1. Change the direction of the action force - the Gimble angle (deflects momentum at the base of the rocket) causing a torque in the tail but the line of thrust and reaction force still points straight up to the nose of the rocket.

upload_2016-12-22_11-43-50.png


2. Change the direction of the reaction force - an equal and opposite action force the means that is now diametrically opposing the direction of thrust and as it no longer thru the center of gravity causing a torque in the nose

upload_2016-12-22_11-44-23.png


3. Both . The torque in the nose caused by the line of thrust 1. is opposite to the torque in the base by the deflection 2. so would be dampened ? If the gimble angle is severe enough the line of thrust could be below the centre of mass so cause a complimentary torque to the deflection caused by the gimble

upload_2016-12-22_12-10-25.png
 
Last edited:
yes ..with the rocket , if the extended line or reaction force is not exactly going through the CofG it will rotate the rocket .. if there is any gimbal angle at all the rocket will spin , bigger the angle greater the spin.

I'm sure if you could explain what prompted the original question , the answer could be quickly found , does this relate to a practical situation involving real tanks and plumbing , or is it purely theoretical ...
 
The question was prompted by a CFD result i am trying to understand

upload_2016-12-24_13-6-43.png
When a high pressure volume is separated from a lower pressure by an angle surface the isobars behave interestingly

The Isobars suggest that the action reaction / thrust line is (Red line) but the velocity arrows suggests most of the flow comes straight down so the action reaction line would be something like the Black line (straight down and then deflected by the surface )

Intuitively i think it would be a combination of both
 
Last edited:

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