I What Causes Jets to Entrain Mass?

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Jets entrain mass due to the conservation of mass principle, requiring the jet and surrounding air to match velocities at their interface. As a jet, such as a fuel jet in a diesel engine, travels at high speeds, it accelerates nearby air, which in turn accelerates additional surrounding air. This process creates a continuous cycle of entrainment as the jet expands from the orifice. The displacement of air by the jet involves the transfer of energy and momentum, which is fundamental in fluid dynamics. Understanding this mechanism is crucial for applications in various engineering fields.
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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