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I Pressure Gradients along an angle pipe

  1. Dec 20, 2016 #1
    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)

    If the pipe was angled I am unsure how the pressure gradients isobars would look ?
    Would they still be parallel between the pressure difference
    or would they still be perpendicular to the pipe ?
    Would the angle of the pipe make a difference to where the reaction force angle on the opposite wall
  2. jcsd
  3. Dec 20, 2016 #2
    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)
  4. Dec 20, 2016 #3
    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
  5. Dec 20, 2016 #4
    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....
  6. Dec 21, 2016 #5
    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

    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.


    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


    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

    Last edited: Dec 21, 2016
  7. Dec 21, 2016 #6
    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 ...
  8. Dec 23, 2016 #7
    The question was prompted by a CFD result i am trying to understand


    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: Dec 23, 2016
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