Characteristic lengh in Reynolds number

[mastermechanic]

As we know $$Re= \frac{ρ.v.l}{μ}$$ The characteristic length (l) is said to be representative of the problem but according to what should we choose this length? I mean if we want to calculate Reynolds number of a wing of plane, should we choose the length or the width?

 

[boneh3ad]

Which do you think and why? In fact, there isn't really a correct answer here. You just have to choose based on the physics you hope to study. Really there are many possible versions of $Re$ even just on a wing depending on the phenomenon of interest.