I have a cylinder diameter d in a channel width b. Flow is U say and kinematic viscosity $\displaystyle \nu$.

I understand the Reynolds number to be based on the channel width so
$\displaystyle \mathrm{Re} = \frac{U b}{\nu}$

and the Cylindrical Reynolds number based on the cylinder diameter
$\displaystyle \mathrm{Re_p} = \frac{U d}{\nu}.$

But if I were to have two cylinders next to each other would the Reynolds be the same or now calculated with 2d ?

I'm unsure which it would be and why if anyone could help me out?

Thanks,
James