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B If KE is related to pressure on the wall for flowing liquid?

  1. Jan 17, 2017 #1
    If there is no velocity and the fluid is moving still, then there is no kinetic energy, would there be no pressure exerted on the walls?

    Or perhaps there is a basal level of kinetic energy of the molecules as they are vibrating?

    Lastly, how can there still be flow if the pressure gradient is constant everywhere? (meaning constant velocity).

    Thank you physicsforum!
  2. jcsd
  3. Jan 17, 2017 #2
    Can you explain further, perhaps with an example? As a fluid mechanics guy, I have no idea what you are asking.
  4. Jan 17, 2017 #3


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    I think you are misinterpreting what pressure means. Pressure (or rather, static or thermodynamic pressure) is really a measure of the mean kinetic energy of the molecules that make up a gas or liquid. This certainly still exists when a fluid is at rest. The only way to reduce this to zero pressure is to either remove all of the molecules (i.e. create a vacuum) or slow them down to zero velocity (the fluid would change state to a solid long before this would theoretically occur).

    The bulk motion of the fluid is represented by the dynamic pressure, which is a continuum quantity and represents the kinetic energy per volume of a flowing fluid. That will certainly go to zero if the fluid slows down to zero flow. If you combine static pressure and dynamic pressure (and sometimes gravity) you get the total pressure for a flow, which pops up a lot in fluid mechanics and is the quantity that is conserved in Bernoulli's equation.

    You are misusing the term "pressure gradient". If the pressure gradient is constant, that means that the pressure is changing in space (e.g. along a length of pipe) at a constant rate, not that the pressure itself is constant everywhere. With a constant pressure gradient, there will be acceleration of the flow in the direction of decreasing pressure. If the pressure itself is constant, then there will be no acceleration (no force), and in the case of no viscosity, it will either remain still or else continue flowing however it already was. If viscosity is involved, then it is similar to friction and no pressure gradient means no flow, and the pressure gradient must be equal or greater to the viscous force in order for flow to be sustained.
  5. Jan 17, 2017 #4


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    Taking a jab, ( Perhaps a difficulty in language translation and expression )
    Are you asking about the kinetic theory of gases and the ideal gas law.?
    And whether the microscopic average kinetic energy of a of a system of molecules is ever zero, so the macroscopic value expressed as pressure is ever zero?

    Please try to explain again.

    I guess my post became moot while I was typing
  6. Jan 17, 2017 #5
    Sorry guys for not communicating my thoughts properly. Thanks Boneh3ad for clearing this issue up! I feel better about this concept. Pheww physics can get intense.

    I have two more questions that came up as these concepts are becoming more clear
    1: in the example of Bernoulli's fluid moving from a big pipe going into a smaller pipe(assuming ideal conditions) if the fluid speeds up in the pipe,(therefore having more KE, why would the pressure be decreased in this situation).

    2: In the same scenario, why is rate of flow(L/min) preserved as the fluid moves from a bigger pipe to a smaller pipe?
    Is this because the fluid is incompressible and there is a constant force that is being applied to the fluid in the large pipe?

    Thank you in advance!
  7. Jan 18, 2017 #6


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    Again, you are mixing up pressures/energies. Bernoulli's equation deals with the bulk properties of a continuous fluid. Given its assumptions, the total pressure must be constant, so if the flow speeds up, the dynamic pressure increases, and the static pressure must decrease. It's essentially a transfer of that kinetic energy from random particle motion into a more organized direction, if you must think of it that way.

    Generally speaking, mass must be conserved. That means that in a closed system operating in steady state, there can't be any more mass entering than there is leaving the system. In the case where the fluid is incompressible, this means density is effectively constant and the volumetric flow rate going in equals that going out.
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