Reynolds Number, Laminar or Turbulent flow

     v·D             v·4·R   
Re = ——  [1] or Re = ————  [1]
           u                                 u


v = Average velocity          [m/s]
D = Hydraulic Diameter = 4·R  [m]
R = Hydraulic Radius = D/4    [m]
u   = Kinematic viscosity       [m²/s]
u  water 20°C       = 1,00·10^-6 [m²/s]


Turbulent flow     Re > 4000  
Laminar   flow     Re < 2000