The ratio of viscous to inertial energy of the moving fluid. A measurement of flow in a smooth unpacked pipe; Re = ud/(η/ρ), where u is the average velocity (in centimenters per second), d is the pipe diameter, η is the viscosity (in grams per centimeter seconds), and ρ is the density (in grams per cubic centimeters). At low Re, viscous friction dominates and controls fluid motion, making it slow and steady. In an unpacked tube, flow becomes fully turbulent when Re exceeds 4200. In a packed bed, u is replaced with the average interstitial velocity and d with the average particle diameter. Flow becomes turbulent in a packed bed at Re values greater than approximately 10 but is not fully turbulent until Re exceeds 100–200.