Which of the following conditions is the chief characteristic of critical flow?

Explanation:

The chief characteristic of critical flow is Q2T/gA3 = 1

• This equation represents the condition for critical flow in open channel hydraulics.

• Here,$Q$ is the discharge, $T$ is the top width of the flow, $g$ is acceleration due to gravity, and $A$ is the cross-sectional area.

• When this condition is satisfied, the flow velocity equals the wave velocity (celerity), meaning the flow is at its critical state.

• At critical flow, the specific energy is minimum for a given discharge.

• It marks the boundary between subcritical and supercritical flow regimes.

Additional Information

• Critical flow occurs when the flow velocity is such that the Froude number equals 1. The Froude number (Fr) is a dimensionless parameter defined as the ratio of flow inertia to gravitational forces. 

• Specific Energy Concept: At critical flow, the specific energy (energy relative to the channel bottom) reaches a minimum for a given discharge, meaning the flow is at an unstable equilibrium.

• Critical flow plays a key role in the design of hydraulic structures like weirs, spillways, and channel transitions, since it governs flow control and energy dissipation.

• The state of flow before critical flow is subcritical (slow, deep flow), and after critical flow is supercritical (fast, shallow flow). Understanding this helps engineers predict flow behavior under different conditions.