where
Re ¯ = Reynolds number
= Darcian flow velocity
d = characteristic length or in porous media, the average grain diameter
v = kinematic viscosity
Reynolds number is the ``critical velocity'' that defines the transition between laminar and turbulent flow. It is a dimensionless ratio that is the indicator of the flow mode. Equation 5 is the ratio of the inertial forces action on the fluid per unit length of flow to the viscous force acting on the same fluid.
