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When a liquid flows in a pipe with a small velocity, the flow is smooth streamline or laminar flow. As the velocity is gradually increased, a stage is reached when the liquid flow becomes turbulent. Whether the liquid flow is laminar or turbulent is decided by Reynolds number (NR). The expression for Reynolds number is given by :

Reynolds number,   

where                                 ? = Density of the liquid

v = Average velocity of flow

D = Diameter of the tube or pipe

? = Coefficient of viscosity of the liquid