Why do free-falling fluids contract?

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Discussion Overview

The discussion centers on the phenomenon of fluid contraction in free-falling streams, particularly focusing on a scenario involving water leaking from an elevated barrel. Participants explore the underlying forces and principles that contribute to this contraction, considering aspects of fluid dynamics and molecular interactions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that the contraction of the water stream is due to chemical forces between H2O molecules, questioning the role of pressure and kinetic energy in this process.
  • Another participant challenges the initial claim by stating that without molecular forces, the stream would not contract but rather spray, indicating a need for cohesive forces for contraction.
  • A different viewpoint posits that as the water falls and accelerates, it stretches, leading to a narrower stream, which is influenced by hydrogen bonding and aerodynamics.
  • A later reply introduces the concept of Rayleigh instability as a possible explanation for the breakup of the stream into droplets, suggesting a connection to the contraction phenomenon.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms driving fluid contraction, with some emphasizing molecular forces while others focus on the effects of acceleration and stretching. The discussion remains unresolved regarding the primary forces at play.

Contextual Notes

Participants acknowledge various assumptions, such as ignoring air resistance and treating water as incompressible, which may limit the applicability of their arguments. The discussion also reflects on the complexity of fluid dynamics without reaching a consensus on the explanations provided.

Nikitin
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Let's say you have a stream of water leaking out of an elevated barrel, and we ignore the effects of air resistance and assume water is incompressible.

So, the lower (and faster) the waterstream falls, the more it will contract. Sure, you say, that's because of the chemical forces between the H2O molecules. And that's pretty reasonable.

However, according to conservation of mass, if any fluid was leaking out of that tank, including ones where the forces between the molecules are very small, it should contract at the same pace as the water! Since the volume flow must be constant, Q=V*A, the further it falls the higher the velocity must become and thus the smaller the area of the stream must become - completely independent of what kind of fluid it is .

So what forces drive this contraction?

I assume the reason behind this phenomena is that the pressure in fluidstream decreases, but why? According to the Bernoulli equation, shouldn't the kinetic energy gained by the water particles be simply a result of their decreasing potential energy? Why would the pressure be converted into kinetic energy?
 
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Nikitin said:
Let's say you have a stream of water leaking out of an elevated barrel, and we ignore the effects of air resistance and assume water is incompressible. So, the lower (and faster) the waterstream falls, the more it will contract. Sure, you say, that's because of the chemical forces between the H2O molecules. And that's pretty reasonable.
reasonable and correct
- completely independent of what kind of fluid it is .
No, not quite. If there were no forces between the molecules the stream wouldn't contract. It would spray.
 
This is a pretty common question, overlooking what should be obvious: as the water falls, it accelerates, which makes the a fixed-volume sample of it longer and therefore narrower. Simply put: it stretches.

Hydrogen bonding and aerodynamics cause both the coherent contraction of the stream as it stretches and its eventual separation into discrete drops.
 
ahh I see! Well, it appears I was completely off track. thanks anyway, you guys just improved my intuition in fluid mechanics! :)
 
Nikitin said:
<snip>
So what forces drive this contraction?

Could you be referring to the Rayleigh instability- the reason a stream breaks up into droplets?
 

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