The velocity at which the flow changes from the laminar flow to the turbulent flow, is called critical velocity. It is of two types, i.e., lower critical velocity and higher critical velocity.

The velocity at which the laminar flow stops, is known as lower critical velocity, while the velocity at which the turbulent flow starts, is known as higher critical velocity.

Weight density or specific weight is defined as the weight per unit volume of a liquid at a standard temperature and pressure. Weight density or specific weight is usually denoted by w. Weight density or specific weight is expressed in kN/m³ or N/m³ or N/mm³.

Mathematically, weight density or specific weight,
w = ρ.g

Note: For water, w = 9.81 kN/m³ = 9.81 x 10³ N/m³ = 9.81 x 10^-6N/mm³.