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min_ht

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The particle Reynolds number makes me confused and I hope someone can help me on this please!

Normally (as I read in every books and papers) that when a bubble or drop rises in a fluid, the bubble/drop Reynolds number is calculated by:

Re = ρUD/μ

where U is particle velocity, D can be particle diameter, and ρ and μ are density and viscosity of continuous fluid

my question is why don't use ρ and μ of bubble/drop? why use them of surrounding fluid?

what is the physical meaning of this Re?

Thanks in advance.