ASRB NET — Farm Machinery & Power / Land & Water Management Engineering / Agricultural Structures & Process Engineering · India · Agricultural Scientists Recruitment Board (ICAR)
Original practice questions written to the published syllabus · not past papersFluids at rest and in motion, pipe flow and open channels: the base of irrigation, drainage, pumping and conservation structures.
In the exams: GATE AG Section 4 (hydraulics asked in 6 of 8 papers 2019–26); ICAR AIEEA-PG Unit III; ICAR AICE subject 43 Unit 2; NCEES FE 12–18 questions (largest FE area)
ASRB NET — Land & Water Management Engineering
Similar to the ICAR PhD paper: standard results and their conditions across pipe flow, flow measurement and open-channel hydraulics.
How our questions for ASRB NET spread across the units of Fluid Mechanics & Open Channel Hydraulics. Difficulty: 4 easy, 14 medium, 17 hard.
| Specific energy and critical depth | 5 | |
| Pipes in series and parallel | 4 | |
| Hydraulic jump | 4 | |
| Dimensional analysis and similitude | 3 | |
| Properties of fluids | 2 | |
| Momentum equation | 2 | |
| Laminar and turbulent flow in pipes | 2 | |
| Orifices, mouthpieces and tank emptying | 2 | |
| Uniform flow in open channels: Manning and Chezy | 2 | |
| Most economical channel section | 2 | |
| Hydrostatics and manometry | 1 | |
| Forces on submerged surfaces and buoyancy | 1 | |
| Continuity equation | 1 | |
| Bernoulli's equation: venturimeter, orifice meter and pitot tube | 1 | |
| Hazen-Williams equation | 1 | |
| Minor losses | 1 | |
| Weirs and notches | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
A soap bubble 40 mm in diameter is made from a solution with surface tension 0.04 N/m. What is the excess of pressure inside the bubble over the outside?
D. 8.00 Pa
A soap bubble has two surfaces, so Δp = 8σ/d = 8 × 0.04/0.04 = 8.00 Pa. 4σ/d = 4.00 Pa is the result for a liquid droplet (one surface); 2σ/d is for a cylindrical jet; using the radius in 8σ/d gives 16.0 Pa.
Further reading: White, Fluid Mechanics
An inverted U-tube manometer with oil of relative density 0.8 in its upper bend is connected between two points on a water pipeline. The oil-water interfaces differ in level by 200 mm. What is the difference in piezometric head between the two points, in metres of water?
C. 0.0400 m
For a lighter manometric liquid, h = x(1 − S_light/S) = 0.2 × (1 − 0.8) = 0.0400 m of water. Using x × S_light gives 0.160 m; the inverted manometer is chosen precisely because it gives a large deflection for a small head difference. 0.360 m uses (1 + 0.8) instead of (1 − 0.8).
Further reading: Modi and Seth, Hydraulics and Fluid Mechanics
A proposed two-dimensional steady incompressible flow has the velocity component u = 2x in the x-direction. Which y-component makes the flow satisfy continuity (taking v = 0 on y = 0)?
C. v = −2y
Continuity for 2-D incompressible flow requires ∂u/∂x + ∂v/∂y = 0. ∂u/∂x = 2, so ∂v/∂y = −2 and v = −2y + f(x); with v = 0 on y = 0, f(x) = 0. v = 2y gives a divergence of 4, which is impossible for an incompressible fluid.
Further reading: White, Fluid Mechanics
A single 300 mm pipe is to be replaced by two identical pipes laid in parallel, of the same length, carrying the same total discharge with the same head loss and friction factor. What diameter should each new pipe be?
D. 227 mm
Each new pipe carries Q/2 at the same head loss: (Q/2)²/d⁵ = Q²/D⁵, so d⁵ = D⁵/4 and d = D/4^0.2 = D/2^0.4 = 0.758D = 227 mm. Keeping the same total area (D/√2 = 212 mm) is not enough, because smaller pipes lose more head per unit flow. 261 mm takes the fifth root of 2 instead of 4; 150 mm simply halves the diameter.
Further reading: Modi and Seth, Hydraulics and Fluid Mechanics
What is the coefficient of discharge of an internal (Borda, re-entrant) mouthpiece running free, where the jet does not touch the mouthpiece walls?
B. 0.50
Momentum analysis of the Borda mouthpiece, where the wall pressure around the entrance is hydrostatic, gives C_c = 0.5; with C_v ≈ 1, C_d = 0.5. Running full, C_d is about 0.707. 0.855 is the external cylindrical mouthpiece. 0.62 is the usual sharp-edged orifice value.
Further reading: Modi and Seth, Hydraulics and Fluid Mechanics
If both the bed width and the side slope of a trapezoidal channel are free to be chosen, what shape is the most economical section?
B. Half a regular hexagon, with sides at 60° to the horizontal
Optimising side slope as well as width gives z = 1/√3 (sides at 60°), with the bed and the sloping sides of equal length: half a regular hexagon. The b = 2y rectangle is the best only among rectangles.
Further reading: Chow, Open-Channel Hydraulics
A rectangular channel 4 m wide carries 8 m³/s at a depth of 0.5 m. What is the alternate depth, the depth on the other limb of the specific-energy curve with the same specific energy?
A. 1.17 m
q = 8/4 = 2 m²/s. E = y + q²/(2gy²) = 0.5 + 4/(19.62 × 0.25) = 1.32 m. Solving y + 0.2039/y² = 1.32 on the subcritical limb gives y ≈ 1.17 m. 0.742 m is the critical depth; 1.05 m is the sequent depth of a hydraulic jump, which conserves momentum, not energy.
Further reading: Subramanya, Flow in Open Channels
Subcritical flow in a rectangular channel passes over a smooth, low hump in the bed without choking. Neglecting losses, what happens to the water surface over the hump?
B. It drops
The specific energy over the hump is reduced by the hump height. On the subcritical (upper) limb of the E-y curve, lower E means lower depth, and the drop in depth exceeds the hump height, so the surface drops. In supercritical flow the depth, and the surface, would rise.
Further reading: Subramanya, Flow in Open Channels
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