What Causes Jets to Entrain Mass?

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Discussion Overview

The discussion centers on the mechanisms behind mass entrainment in jets, specifically focusing on air jets and their interaction with surrounding fluids. The scope includes theoretical considerations related to fluid dynamics and the conservation of mass principle.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant suggests that the entrainment of air in jets may relate to the conservation of mass principle.
  • Another participant explains that free jets, which travel at higher velocities than the surrounding fluid, must accelerate nearby air to maintain a consistent velocity at the interface between the jet and the surrounding air, leading to entrainment.
  • Some participants question the necessity of the interface between the jet and surrounding air being at the same velocity, seeking clarification on this point.
  • It is noted that fluids are treated under continuum mechanics, implying that jets must displace air in their path and transfer energy or momentum in the process.

Areas of Agreement / Disagreement

Participants express curiosity and seek clarification on specific aspects of the discussion, indicating that multiple viewpoints and questions remain unresolved regarding the mechanics of jet entrainment.

Contextual Notes

There are unresolved questions regarding the assumptions behind the velocity interface requirement and the implications of continuum mechanics on jet behavior.

larsa
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I have done a research on google but still i can't find a reason why air is entrained in jets. An example of a jet is the fuel jet in a combustion chamber of a diesel engine. I suspect it has to do with the conservation of mass principle. Thank you in advance.
 
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Free jets emanate form an orifice and are traveling at much higher velocities then the surrounding fluid (lets say air jets in air). The interface between the jet and the surrounding air needs to be at the same velocity, so the jet has to accelerate the nearby surrounding air to satisfy this requirement. As the nearby air is accelerated, it will need to accelerate yet more nearby air, etc., and thus free jets entrain surrounding air as they expand out of the orifice and travel through the surrounding environment.
 
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Why the interface between the jet and the surrounding air needs to be at the same velocity?
 
larsa said:
Why the interface between the jet and the surrounding air needs to be at the same velocity?

Fluids are treated as continuum mechanics. The jet has to displace air in its path, right? So it is going to transfer some energy/momentum.
 
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MarkJW said:
Fluids are treated as continuum mechanics. The jet has to displace air in its path, right? So it is going to transfer some energy/momentum.
Thank you for your answer
 

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