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- TL;DR Summary
- Bernoulli principle is only describing a fluid along a streamline while eductor's suction effect involves ambient air

I've google all the keywords like "eductor", "ejector", "Venturi pump". All of them are the mechanism or applications that utilize "Venturi Effect". All of them are trying to explain the "suction effect" by saying that restriction area makes the fluid's velocity higher and hence lower pressure which is the definition of Venturi Effect. So far, I can comprehend that. That can be explained by Bernoulli's principle as well. However, it is the following statement that confuses me for couple of weeks - "..., and the lower pressure generate a kind of vacuum thus suck the ambient fluid or air in."

First of all, the definition of Bernoulli's principle only describes a single fluid along a streamline. I get that the fluid will have higher speed and lower pressure at the restriction area than the other point on its own path. But how can you prove that the "lower pressure point on its path" is lower than ambient pressure so that it can suck fluid or air from outside? And even if you can prove that effect by physics, how is that related to Venturi Effect or Bernoulli's principle if you're discussing two different fluid?

Secondly, if the suction is created by low pressure result from high speed, why do I need to create a restriction area that generates Venturi Effect? Why not just generate high speed air at the first place? In other words, I guess you can also make a eductor with a straight pipe? Again, if that assumption is correct, how is an eductor/ejector related to Venturi Effect?

First of all, the definition of Bernoulli's principle only describes a single fluid along a streamline. I get that the fluid will have higher speed and lower pressure at the restriction area than the other point on its own path. But how can you prove that the "lower pressure point on its path" is lower than ambient pressure so that it can suck fluid or air from outside? And even if you can prove that effect by physics, how is that related to Venturi Effect or Bernoulli's principle if you're discussing two different fluid?

Secondly, if the suction is created by low pressure result from high speed, why do I need to create a restriction area that generates Venturi Effect? Why not just generate high speed air at the first place? In other words, I guess you can also make a eductor with a straight pipe? Again, if that assumption is correct, how is an eductor/ejector related to Venturi Effect?