Dynamic similarity implies a correspondence of fluid forces in two systems. In general situation there are many classes of forces that influence the behavior of fluids. Some of these are inertial viscous, gravitational, compressibility, pressure and elastic forces. Certain dimensionless ratio are developed based on fluid properties. Velocities and dimension, which are essentially force ratio.
The more important of these are Reynolds number
For most applications in practical flow measurement the Reynolds number is taken to be sufficient criterion of dynamic similarly. The magnitude of Reynolds number not only indicates whether the flow is laminar or turbulent but also furnishes the probable shape of velocity profile. Due to the strong role it plays as an indicator of varying flow characteristics, many of the deviation from the theoretical equations are called Reynaldo number effects.