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Control Volumes - Fluid Mechanics

  1. Sep 30, 2016 #1
    After doing extensive research, I am still confused about Control Volumes. Say that there is water flowing into a cup at a steady rate, and I am required to illustrate the momentum and mass flows, as well as the forces, how would I approach this? I have seen examples of Control Volumes but they do not reflect simple vertical flow and do not contain much information.

  2. jcsd
  3. Sep 30, 2016 #2


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    I am honestly not sure what you are asking here. What do you mean by "illustrate the momentum and mass flows"?
  4. Sep 30, 2016 #3
    So depicting (via an illustration of the control volume) the mass flow and momentum of the fluid as it enters and leaves the control volume, noting that the flow is steady and vertical. I'm not sure at all how I sketch this. I hope this clarifies things. Thanks.
  5. Sep 30, 2016 #4


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    A control volume represents a basically imaginary region containing volume and as it relates to mass flow, the entering and exiting mass flow from a control volume must always be equal regardless of the shape of the control volume size or configuration between the fluid entry and exit end points.

    (Note Edited to remove earlier erroneous statement related to momentum and inlet vs. outlet areas deleted.)
    Last edited: Sep 30, 2016
  6. Oct 1, 2016 #5
    I am a bad engineer but lets give it a try :P
    This question is not very clear, please provide some more information, is the cup stationary or moving, what is the hose dia, what is the velocity of water? Assuming the simplest case, the flow is steady,
    you have first imagine a boundary across your cup, let for example the cup is resting on a flat surface and water is entering the cup vertically,


    the total amount of forces acting in y directing would be:

    as the flow is steady C.V term would cancel out,
    Fs = surface forces.
    Fb = body forces.
    from the assumption that the cup is at rest, Fs= Ry ;reaction.
    Fb= weight of the cup plus weight of the water.

  7. Oct 2, 2016 #6


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    You don't need a control volume (a conceptual thing) if you already have a cup a real thing).
    In either case you can then proceed with normal physics, starting with mass/volume conservation.
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