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Bernoullis from the laws of motion

  1. Feb 18, 2015 #1
    I am trying to understand Bernoullis from Newton laws applied to molecular dynamics (as opposed to conservation laws)

    From the conservation laws I understand for a incompressible invicous (experiencing no friction effects ) Ideal fluid

    • Mass is neither created nor destroyed Mass in = Mass Out

    • Energy is neither created nor destroyed Energy in = Energy Out

    • Energy is constant along the streamline
    Energy comprised of Dynamic pressure (Kinetic Energy ) , Static pressure , Potential Energy

    E= Dynamic Pressure + Static pressure + Potential Energy

    If the datum is constant this reduces to

    E= Dynamic Pressure + Static pressure

    What I am trying to confirm is the Molecular Dynamics explanation of how these two energies are interchanged

    My explanation for a streamline relies on the interactions between single molecule width 'sheets' of fluid along the streamline

    Bernoullis  Kinetics 0.png

    As a fluid enters a reduction in pipe diameter the dynamic kinetic energy of a 'sheet' is shared via elastic collisions with the 'sheet' in front (or elastically with the walls of the pipe and back to the 'sheet' in front) .
    As there are fewer molecules in the 'sheet' in front total kinetic energy is shared between a smaller denominator thus average energy (velocity) must increase.

    Bernoullis  Kinetics 1.png

    As each sheet has a higher velocity than its preceding neighbor it is accelerated away.
    Bernoullis  Kinetics 2.png
    This acceleration means that the distance in front of the sheet is greater than the distance behind and the random kinetic motion experiences a positive bias (in mean free path / intermolecular forces ) in direction of the streamline thus also causing mean acceleration in that direction.

    Bernoullis  Kinetics 3.png

    In summary Dynamic pressure increases and reducing Static pressure reduces thru a constriction is caused by the following sequential actions

    1. Particle acceleration due to constant Dynamic Pressure energy being shared among a lower molecular denominator.
    2. Static pressure being reduced as a result of a bias in mean free path / intromolecular forces in front of sheet vs behind

    Is this correct ?
  2. jcsd
  3. Feb 23, 2015 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
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