The flow of a liquid is said to be steady, streamline or laminar if every particle of the liquid follows exactly the path of its preceding particle and has the same velocity of its preceding particle at every point.
Let abc be the path of flow of a liquid and v1, v2 and v3 be the velocities of the liquid at the points a, b and c respectively. During a streamline flow, all the particles arriving at 'a' will have the same velocity v1 which is directed along the tangent at the point 'a'. A particle arriving at b will always have the same velocity v2. This velocity v2 may or may not be equal to v1. Similarly all the particles arriving at the point c will always have the same velocity v3. In other words, in the streamline flow of a liquid, the velocity of every particle crossing a particular point is the same.
The streamline flow is possible only as long as the velocity of the fluid does not exceed a certain value. This limiting value of velocity is called critical velocity.
When the velocity of a liquid exceeds the critical velocity, the path and velocities of the liquid become disorderly. At this stage, the flow loses all its orderliness and is called turbulent flow. Some examples of turbulent flow are :
(i) After rising a short distance, the smooth column of smoke from an incense stick breaks up into irregular and random patterns.
(ii) The flash - flood after a heavy rain.
Critical velocity of a liquid can be defined as that velocity of liquid upto which the flow is streamlined and above which its flow becomes turbulent.