TITLE: Reynolds Number
QUESTION [1 upvotes]: We know that the Reynolds Number is $R_e=\frac{\rho vd}{\eta}$ ($\rho$ density, $v$ velocity, $d$ diameter and $\eta$ the viscosity of fluid). 
We also know that an ideal fluid has no viscosity which means that $\eta=0$ and what we get is $R_e=\frac{\rho vd}{0}$. Does this mean that the ideal fluid doesn't have a Reynolds Number?
In my book it says that every fluid has it's own Reynolds Number.
Can anybody help me?
Thank you!

REPLY [1 votes]: "We also know that an ideal fluid has no viscosity" - this statement is incorrect. We know that there is no such a thing "ideal flow" - it's a mathematical abstraction created in order to solve the equations that are otherwise impossible or at least very difficult to solve. Therefore, the dimensionless parameter defined following the research of Reynolds cannot be applied to the non-existing matter.
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