The critical value for the Reynolds number (R_n) at which the changeover occurs

Question

The critical value for the Reynolds number (R_n) at which the changeover occurs from laminar flow to turbulent flow in open channels is (A) 20 (B) 50 (C) 500 (D) 2000 (E) 5000 The specific-energy diagram for uniform flow through a rectangular channel represents two flow regimens: subcritical flow and supercritical flow. For a particular flow rate, there are two possible channel depths that occur at the same energy level. These depths are known as (A) control depths (B) critical depths (C) conjugate depths (D) alternate depths (E) (slow & deep) and (fast & shallow) depths The Froude number (F) is used to determine whether the flow rate in an open channel is critical, subcritical or supercritical. Which statement below is not true? (A) F = 1, the flow rate is critical (B) F 1, the flow rate is supercritical (D) all of the statements are true (E) all of the statements arc false 4) When the flow in an open channel goes from supercritical to subcritical flow, the depth must pass through the_ and, this condition gives rise to a_____. (A) hydraulic jump & control section (B) control section & hydraulic jump (C) critical depth & control section (D) critical depth & hydraulic jump (E) none of the above For uniform flow in general, the cross-section through which the flow occurs is constant; however, in natural streams, the slope of the bed and shape and size of the cross-section usually vary, so that true uniform flow is really very rare. (A) True (B) False

Explanation / Answer

1)

In open channel flows, the reynolds number that is required for changing the flow from laminar to turbulent is 500

Re < 500 laminar

500 < Re < 12,500 transitional

12,500 < Re turbulent

2)

The two possible depths that occur for the same specific energy are called as ALTERNATE DEPTHS.

3)

ALL the given statements are correct.

Fr < 1 subcritical flow

Fr = 1 critical flow

Fr > 1 supercritical flow

4)

When a super critical flow meets a sub critical flow, it must pass through a critical depth, and hence a hydraulic jump will be occuring.

5)

True. In natural streams , the flow regime is constantly changing with the discharge and flow depth. So it is imossible to mantain an uniform flow.

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