On the outer edge of the flow pattern the flow is subcritical. This flow is thicker and moves more slowly. The boundary between the two areas is called a "hydraulic jump". That's where the flow is just critical and Froude number is equal to 1.0.

Subcritical Flow in Open Channel Flow

Open channel flow is the flow of liquid in the open space (channel) where the liquid is partially or fully exposed to the atmosphere. The flow in the open channel can be either subcritical, critical or supercritical. Let us discuss subcritical flow in open channel flow.

Definition

If the Froude number in open channel flow is less than 1.0, the flow is known as subcritical flow.

Froude Number

Froude number is a dimensionless number used in open channel flow to determine the state of the flow. It is defined as the ratio of the inertial forces to the gravitational forces.

Fr = V/√(gD)

Where,
V = velocity of the fluid
g = acceleration due to gravity
D = depth of the channel

Characteristics of Subcritical Flow

- In subcritical flow, the water particles move in a circular path.
- The velocity of the fluid is less than the velocity of the gravity wave.
- The pressure in the fluid is less than the atmospheric pressure.
- The water surface is smooth and regular.
- The depth of the water is less than the critical depth.
- The energy line is above the hydraulic grade line.
- The flow is stable and predictable.

Applications

Subcritical flow is commonly observed in rivers, canals, and other open channels.

Conclusion

Subcritical flow in open channel flow is a stable and predictable flow where the velocity of the fluid is less than the velocity of the gravity wave. The water surface is smooth and regular, and the depth of the water is less than the critical depth.

If F. N less than 1 : subcritical
F. N equal to 1 : critical
F. N greater than 1 : supercritical flow