ANS:B - Froude number

If the forces involved are primarily due to inertia and gravity, and frictional resistance is considered to play only a minor role, the design of the channels is typically made by comparing the Froude number. The Froude number (Fr) is a dimensionless number used to characterize the relative importance of inertial forces to gravitational forces in open-channel flow. It is defined as: Fr=V/gD​ Where:

• V is the velocity of flow,
• g is the acceleration due to gravity,
• D is the hydraulic depth of the flow.

The Froude number is important in the design of open channels because it helps engineers assess the type of flow regime present. Specifically, it helps determine whether the flow is subcritical (Fr<1), critical (Fr=1), or supercritical (Fr>1). For efficient and stable flow, certain designs and operational conditions may be preferable based on the Froude number. In situations where frictional resistance plays only a minor role, inertial and gravitational forces dominate the behavior of the flow, making the Froude number a critical parameter for channel design and analysis. Therefore, the design of the channels is made by comparing the Froude number.