Mechanical Engineering — Fluid Mechanics Questions & Answers

Q.21 Medium Fluid Mechanics

For a rectangular open channel with width 5 m carrying 12 m³/s of water, if the critical depth is 1.5 m, what is the critical velocity?

A 1.6 m/s

B 2.4 m/s

C 1.84 m/s

D 3.84 m/s

Correct Answer: C. 1.84 m/s

EXPLANATION

Critical velocity V_c = √(gD_c) = √(9.81 × 1.5) = 3.84 m/s. Alternatively Q/A = 12/(5×1.5) = 1.6 m/s. Using V_c² = g × D_h gives the correct answer.

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Q.22 Hard Fluid Mechanics

A horizontal jet of 0.02 m² cross-section with velocity 8 m/s strikes a curved vane and is deflected 60°. What is the force on the vane in the direction of jet?

A 10.24 kN

B 12.80 kN

C 6.40 kN

D 5.12 kN

Correct Answer: C. 6.40 kN

EXPLANATION

Force = ρAV²(1 - cos60°) = 1000 × 0.02 × 64 × 0.5 = 6.4 kN. The deflection angle affects the force component.

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Q.23 Hard Fluid Mechanics

In a venturi meter, if the area ratio is 4:1 and the pressure difference is 8 kPa, what is the velocity in the throat? (ρ_water = 1000 kg/m³)

A 2.06 m/s

B 4.12 m/s

C 6.18 m/s

D 8.24 m/s

Correct Answer: B. 4.12 m/s

EXPLANATION

From Bernoulli and continuity: V₂ = √(2ΔP/(ρ(A₁²/A₂² - 1))) = √(16000/(1000 × 15)) = 4.12 m/s

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Q.24 Hard Fluid Mechanics

A circular jet impinges on a 45° inclined flat plate and splits equally. If jet velocity is 15 m/s and diameter is 30 mm, what is the magnitude of the resultant force?

A 2650 N

B 2074 N

C 2648 N

D 1874 N

Correct Answer: C. 2648 N

EXPLANATION

Using momentum equation with 45° deflection, the resultant force = ρAV² × √2 = 1000 × 0.000707 × 225 × 1.414 = 2648 N

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Q.25 Hard Fluid Mechanics

A vertical pipe of diameter 40 mm has water flowing upward at 2.5 m/s. If the friction factor is 0.035 and pipe length is 50 m, what is the total head loss (friction + elevation)?

A 56.34 m

B 50.94 m

C 51.12 m

D 57.28 m

Correct Answer: C. 51.12 m

EXPLANATION

h_f = 0.035 × (50/0.04) × (6.25/19.62) = 1.12 m; total = 1.12 + 50 = 51.12 m

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Q.26 Easy Fluid Mechanics

In compressible flow, if Mach number M = 1.2, the flow is classified as:

A Subsonic

B Sonic

C Supersonic

D Hypersonic

Correct Answer: C. Supersonic

EXPLANATION

Mach number classification: M < 1 (subsonic), M = 1 (sonic), 1 < M < 5 (supersonic), M > 5 (hypersonic). M = 1.2 is supersonic.

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Q.27 Medium Fluid Mechanics

A 100 mm diameter pipe contains a 90° elbow. If water flows at 3 m/s and minor loss coefficient K = 0.9, what is the head loss?

A 0.123 m

B 0.205 m

C 0.369 m

D 0.410 m

Correct Answer: C. 0.369 m

EXPLANATION

h_minor = K(V²/2g) = 0.9 × (9/19.62) = 0.369 m

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Q.28 Medium Fluid Mechanics

Which dimensionless number is used to characterize the relative importance of gravity and inertial forces in open channel flow?

A Reynolds number

B Froude number

C Strouhal number

D Weber number

Correct Answer: B. Froude number

EXPLANATION

The Froude number Fr = V/√(gD_h) characterizes gravity effects. In open channel flow, it determines flow regime (subcritical Fr<1, supercritical Fr>1).

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Q.29 Medium Fluid Mechanics

A jet of water with velocity 20 m/s hits a flat plate perpendicularly. If the jet diameter is 50 mm and density is 1000 kg/m³, what is the force exerted on the plate?

A 3927 N

B 7854 N

C 1963 N

D 5891 N

Correct Answer: A. 3927 N

EXPLANATION

Force F = ρAV² = 1000 × (π × 0.025²) × 20² = 1000 × 0.001963 × 400 = 3927 N

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Q.30 Hard Fluid Mechanics

For a weir with discharge Q = C_w × L × H^(3/2), if the length is doubled and head is halved, the new discharge becomes:

A 0.5Q

B 0.707Q

C 1.414Q

D Q

Correct Answer: B. 0.707Q

EXPLANATION

Q_new = C_w × (2L) × (H/2)^(3/2) = C_w × 2L × (H^(3/2))/2.828 = 0.707 × C_w × L × H^(3/2) = 0.707Q

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