In the hydraulic jump scenario, Fr1 ≈ 1.91; which is the upstream flow regime?

• A. Yes, the upstream flow is supercritical and a hydraulic jump is possible
• B. No, the upstream flow is subcritical
• C. The flow is exactly critical
• D. Not enough information to tell

Answer: (A)

Froude number tells you the flow regime in open-channel flow: if Fr is greater than 1, the flow is supercritical; if Fr is less than 1, it is subcritical; Fr equal to 1 means critical. Here the upstream Fr is about 1.91, which is well above 1, so the upstream flow is in the supercritical regime. A hydraulic jump typically forms when this fast, shallow (supercritical) flow is forced to slow down and deepen to a subcritical state downstream, so having a supercritical upstream condition makes a hydraulic jump possible. The other possibilities would require Fr below 1, exactly 1, or no information, none of which applies since Fr1 is given as 1.91.