Control Volumes - Fluid Mechanics

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Discussion Overview

The discussion revolves around the concept of Control Volumes in fluid mechanics, specifically in the context of illustrating momentum and mass flows in a scenario involving water flowing into a cup at a steady rate. Participants explore how to effectively represent these flows and the forces involved, with a focus on vertical flow dynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about how to illustrate momentum and mass flows in a control volume scenario involving a cup and vertical water flow.
  • Another participant seeks clarification on the initial question, questioning the meaning of "illustrate the momentum and mass flows."
  • A later reply attempts to clarify the initial query by emphasizing the need to depict mass flow and momentum as the fluid enters and exits the control volume.
  • One participant explains that a control volume represents an imaginary region and notes that the entering and exiting mass flow must be equal, regardless of the control volume's shape or configuration.
  • Another participant suggests that additional information is needed to provide a clearer response, such as whether the cup is stationary or moving, the diameter of the hose, and the velocity of the water.
  • One participant argues that a control volume may not be necessary since the cup itself is a physical object, suggesting that normal physics principles could be applied directly.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and clarity regarding the original question, with no consensus reached on how to approach the illustration of momentum and mass flows. Multiple competing views on the necessity and application of control volumes are present.

Contextual Notes

Some limitations in the discussion include missing assumptions about the system's configuration, such as the motion of the cup and specific flow parameters, which may affect the analysis.

sierra52
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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.

Thanks.
 
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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"?
 
boneh3ad said:
I am honestly not sure what you are asking here. What do you mean by "illustrate the momentum and mass flows"?
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.
 
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:
I am a bad engineer but let's 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,

Untitled.jpg
the total amount of forces acting in y directing would be:
Capture.JPG


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.
Capture.JPG


Capture.JPG
 
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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