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Rajasthan PSC Fluid Mechanics & Open Channel Hydraulics: practice questions

Rajasthan PSC — Assistant Agriculture Engineer (Agriculture Engineering syllabus) · India · Rajasthan Public Service Commission

Original practice questions written to the published syllabus · not past papers
10worked here, free
17written to this exam in this subject
14syllabus units

About Rajasthan PSC

Rajasthan PSC — Assistant Agriculture Engineer (Agriculture Engineering syllabus) is a government recruitment exam run by Rajasthan Public Service Commission, for candidates in India. Objective paper; the paper structure was not captured.

Source: rpsc.rajasthan.gov.in.

Checked against the exam body's published information. Formats and fees change; confirm the current edition there before you book.

Fluid Mechanics & Open Channel Hydraulics in Rajasthan PSC

Fluids at rest and in motion, pipe flow and open channels: the base of irrigation, drainage, pumping and conservation structures.

Our 17 Rajasthan PSC questions in this subject: 15 easy, 2 medium, 0 hard, across 14 units.

10 practice questions, worked

Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.

These 10 were written to Rajasthan PSC and are not shown on any other exam's page.

Question 1 · easy

As the temperature rises, how do the viscosities of water and of air change?

  1. Both rise
  2. Water's viscosity falls; air's viscosity rises
  3. Both fall
  4. Water's viscosity rises; air's viscosity falls
Answer and working

B. Water's viscosity falls; air's viscosity rises

In liquids viscosity comes mainly from intermolecular cohesion, which weakens as temperature rises. In gases it comes from molecular momentum exchange, which increases with temperature. So the two trends are opposite.

Question 2 · easy

What is the gauge pressure at a depth of 5 m in a fresh-water tank open to the atmosphere (take ρ = 1000 kg/m³, g = 9.81 m/s²)?

  1. 490.5 kPa
  2. 5.00 kPa
  3. 150.3 kPa
  4. 49.05 kPa
Answer and working

D. 49.05 kPa

p = ρgh = 1000 × 9.81 × 5 = 49050 Pa = 49.05 kPa gauge. Adding 101.3 kPa gives the absolute pressure (150.3 kPa), which was not asked. 490.5 kPa is a factor-of-ten slip in converting Pa to kPa.

Question 3 · easy

For a floating body to be in stable equilibrium, which condition must hold?

  1. The centre of buoyancy lies above the centre of gravity
  2. The centre of gravity coincides with the centre of buoyancy
  3. The metacentre lies above the centre of gravity
  4. The metacentre lies below the centre of gravity
Answer and working

C. The metacentre lies above the centre of gravity

A floating body is stable if the metacentre M is above G (positive GM), because the shifted buoyancy then produces a restoring couple. B above G is the condition for a fully submerged body, and is sufficient but not necessary for a floating one; ships and pontoons are commonly stable with G above B.

Question 4 · easy

Water flows through a pipe that reduces from 300 mm to 150 mm diameter. The velocity in the 300 mm section is 1.5 m/s. What is the velocity in the 150 mm section?

  1. 6.00 m/s
  2. 0.375 m/s
  3. 12.0 m/s
  4. 3.00 m/s
Answer and working

A. 6.00 m/s

A₁V₁ = A₂V₂, and area varies as D², so V₂ = V₁(D₁/D₂)² = 1.5 × (300/150)² = 1.5 × 4 = 6.00 m/s. Using the diameter ratio without squaring gives 3.00 m/s. 0.375 m/s inverts the area ratio; 12.0 m/s cubes the diameter ratio.

Question 5 · easy

Which of the following is NOT an assumption made in deriving Bernoulli's equation p/ρg + V²/2g + z = constant?

  1. The flow is horizontal
  2. Friction is negligible
  3. The fluid is incompressible
  4. The flow is steady
Answer and working

A. The flow is horizontal

Bernoulli's equation assumes steady, incompressible, frictionless flow along a streamline with no shaft work. Elevation is one of its terms (z), so the flow need not be horizontal.

Question 6 · easy

In fully developed laminar flow in a circular pipe, what is the ratio of the maximum (centreline) velocity to the mean velocity?

  1. 1.33
  2. 2.0
  3. 1.5
  4. 1.2
Answer and working

B. 2.0

The laminar profile is parabolic, u = u_max(1 − r²/R²); integrating over the area gives a mean of u_max/2. The ratio 1.5 applies to laminar flow between parallel plates; turbulent flow has a much flatter profile with a ratio nearer 1.2. 1.33 is 4/3, the laminar momentum correction factor, not the velocity ratio.

Question 7 · easy

What is the loss coefficient K for the exit of a pipe discharging into a large reservoir?

  1. 1.0
  2. 0.375
  3. 0.5
  4. 0.05
Answer and working

A. 1.0

At the exit the jet's entire velocity head is dissipated in the reservoir, so h = V²/2g and K = 1.0. K ≈ 0.5 is a sharp-edged entrance; about 0.05 a bell-mouthed entrance; 0.375 a sudden contraction with C_c = 0.62.

Question 8 · easy

An orifice has a coefficient of contraction of 0.64 and a coefficient of velocity of 0.97. What is its coefficient of discharge?

  1. 0.805
  2. 0.660
  3. 1.52
  4. 0.621
Answer and working

D. 0.621

C_d = C_c × C_v = 0.64 × 0.97 = 0.621. Averaging the two (0.805) or dividing has no physical meaning. 0.660 divides C_c by C_v and 1.52 divides C_v by C_c.

Question 9 · medium

A sharp-crested rectangular weir with a crest 1.5 m long operates under a head of 0.3 m. With C_d = 0.62 and no end contractions or approach velocity, what is the discharge?

  1. 451 L/s
  2. 135 L/s
  3. 1504 L/s
  4. 677 L/s
Answer and working

A. 451 L/s

Q = (2/3) C_d L √(2g) H^1.5 = (2/3) × 0.62 × 1.5 × 4.429 × 0.3^1.5 = 0.451 m³/s = 451 L/s. Using H^2.5 (the V-notch exponent) gives 135 L/s; dropping the 2/3 gives 677 L/s. 1504 L/s uses H^0.5 instead of H^1.5.

Question 10 · easy

Which dimensionless number must be matched between a model and prototype of a spillway or a hydraulic jump stilling basin?

  1. Weber number
  2. Reynolds number
  3. Froude number
  4. Mach number
Answer and working

C. Froude number

Free-surface flows over spillways and in stilling basins are gravity dominated, so Froude similarity is used. Reynolds governs viscous-dominated closed-conduit flow; Mach governs compressibility; Weber matters where surface tension is important, such as droplets.

A study plan for Fluid Mechanics & Open Channel Hydraulics in Rajasthan PSC

This plan follows where our questions actually fall, so the time goes where the marks are likely to be. It is our reading of the syllabus, not the exam body's weighting.

  1. First, the heavy units (47% of the 17 questions): Bernoulli's equation: venturimeter, orifice meter and pitot tube (2, 12%); Uniform flow in open channels: Manning and Chezy (2, 12%); Weirs and notches (2, 12%); Continuity equation (1, 6%); Dimensional analysis and similitude (1, 6%).

    Read the manual chapters for these first and do every worked question you can find on them.

  2. Then the middle band: Forces on submerged surfaces and buoyancy (1, 6%); Hydrostatics and manometry (1, 6%); Laminar and turbulent flow in pipes (1, 6%); Minor losses (1, 6%); Most economical channel section (1, 6%).

    Aim for one sitting per unit, then mixed questions across them.

  3. Then the lighter units: Orifices, mouthpieces and tank emptying (1, 6%); Pipes in series and parallel (1, 6%); Properties of fluids (1, 6%); Specific energy and critical depth (1, 6%).

    Learn the definitions, classifications and standard numbers; these units are more often recalled than worked.

  4. Bank the easy marks: Bernoulli's equation: venturimeter, orifice meter and pitot tube (2 easy of 2); Continuity equation (1 easy of 1).

    Do these early in revision so they are fast on the day.

  5. Finish timed. Sit the subject as a timed set without notes, mark it, and send each wrong answer back to its unit above.

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