Lesson 2: Types of Liquid Flow

Liquid flow is classified into laminar and turbulent flow.

2.1 Laminar (streamline) flow

• In laminar flow the liquid moves so that adjacent layers slide smoothly and gently over one another.
• Each small portion of liquid follows a continuous path called a streamline.
• A real or imaginary flow tube can be represented by a set of streamlines.

Properties of streamlines

• They never intersect (a point cannot have two velocity directions at the same time).
• The tangent to a streamline at any point gives the direction of the instantaneous velocity of the liquid particles at that point.
• The number of streamlines crossing perpendicularly a unit area at a point is the streamline density; it indicates the flow speed. Streamlines are close together where the speed is high and far apart where it is low.

2.2 Turbulent flow

• When the speed of the liquid exceeds a certain limit, the flow is no longer smooth: small circular eddies (vortices) appear.
• The same happens with gases (smoke rising from a cigarette or incense stick starts as steady streamlines and then becomes turbulent with eddies).
• Turbulence also results from violent change of flow or from the rapid motion of an object through the liquid, or when a gas diffuses from a narrow space to a larger one or from high to low pressure.

Turbulent flow is accompanied by noise: a doctor listens with a stethoscope to detect turbulent blood flow in an artery, which indicates illness.

2.3 Steady flow

• Flow is steady if the velocity of the liquid at each point on its path does not change with time (different points may have different velocities).
• Laminar flow may be considered steady flow.

Conditions for steady flow in a pipe

1. The liquid completely fills the pipe.
2. The amount of liquid entering one end in a given time equals the amount leaving the other end in the same time (the flow rate is constant along the tube because the liquid is incompressible).
3. The velocity of the liquid at every point does not change with time.