Water at $20$C flows in a porous bed of spherical particles. Find the Reynold's number for porous media $($Re$\_$pm$)$ when the particle diameter is $4$ mm$,$ superficial velocity is $15$ cm$/$s and the void fraction is $0\mathrm{.}4.$

Water at $20C$ flows in a porous bed of spherical particles. Find the Reynold's number for porous media $($Re$\_$pm$)$ when the particle diameter is $4mm,$ superficial velocity is $15c\frac{m}{s}$ and the void fraction is $0\mathrm{.}4.$