Laminar or Streamline motion:
If a liquid flows in such a way that its velocity at any point in the liquid is always the same in direction as well as in magnitude, then the flow of the liquid is said to be steady or streamline flow. The path followed by the particles of the liquid in such a flow is called the streamline. The tangent at any point on this line gives the direction of the flow of the liquid at that point. Therefore a streamline represents a fixed path curved or straight line followed by an orderly procession of particles.
No two streamlines can cross each other.
When the streamlines crowd together, the velocity of flow increases. The flow is streamlined only as long as the velocity of the liquid does not exceed a particular value called the critical velocity.
Let us consider the motion of a liquid in a pipe as shown in the adjoining figure, where the particles A, B, C, D etc. have velocities \( \vec{v_1} \), \( \vec{v_2} \), \( \vec{v_3} \), \( \vec{v_4} \) etc. respectively at a given time. If with the passage of time the velocity of the particle happens to be at A is still \( \vec{v_1} \), that at B is still \( \vec{v_2} \) and so on, then the liquid is said to have a streamline flow.
Turbulent Motion:
If the velocity of the particles at a given point varies with time both in magnitude as well as in direction in an irregular manner, the flow of the liquid is called unsteady or turbulent. When the velocity of liquid exceeds its critical veocity, then the stready flow becomes turbulent motion. The path of the particles continiously change and the portion of the liquid acquare a rotatary motion forming eddies etc.