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Fluid Mechanics and Hydraulics practice questions

10 free Fluid Mechanics and Hydraulics questions for the FE Environmental Exam (FE) exam, each with the correct answer and an explanation for every option.

Start the questionsFree 10-question FE quiz

What this domain covers

One of the two largest domains: fluid statics and flow in pipes, pumps and open channels.

Exam weight
12%
On a real exam
≈ 13 questions
In our bank
72 questions

Topics that come up most

  • Hydrostatic pressure and forces on surfaces
  • Continuity, Bernoulli and the energy equation
  • Head loss with Darcy-Weisbach and Hazen-Williams
  • Pump power, efficiency and system curves
  • Open-channel flow and Manning's equation

How to study it

Write out the energy equation with every term for each pipe problem before simplifying. Most questions are one head-loss or pump term away from the answer.

Sample Fluid Mechanics and Hydraulics questions

Try each one before opening the answer. Original questions written to the official exam objectives.

  1. Question 1

    Water moves at 3 m/s. Taking g = 9.81 m/s², its velocity head is about:

    • A0.15 m, using v instead of v²
    • B0.92 m, omitting the 2
    • C0.46 m
    • D4.6 m
    Show answer and explanation

    Answer: C) 0.46 m

    • AIncorrect. Square the velocity.
    • BIncorrect. The velocity head is \(v^2/(2g)\).
    • CCorrect. \(v^2/(2g) = 9/19.62\).
    • DIncorrect. That's off by a factor of 10.

  2. Question 2

    At a point in a pipeline, the pressure head is 20 m, the elevation head above the datum is 5 m, and the velocity head is 0.2 m. What is the total (energy) head at that point?

    • A25.2 m
    • B25.0 m
    • C24.8 m
    • D20.2 m
    Show answer and explanation

    Answer: A) 25.2 m

    Total head = pressure head + elevation head + velocity head = 20 + 5 + 0.2 = 25.2 m. Pressure plus elevation alone (25.0 m) is the piezometric head, which plots as the hydraulic grade line, not the energy grade line.

    • ACorrect. Total head is the sum of the pressure, elevation and velocity heads: 20 + 5 + 0.2 = 25.2 m.
    • BIncorrect. 25.0 m is the piezometric (hydraulic) head, pressure plus elevation. It leaves out the 0.2 m velocity head, which the total head must include.
    • CIncorrect. This subtracts the velocity head instead of adding it. Kinetic energy adds to the total energy per unit weight.
    • DIncorrect. This leaves out the 5 m elevation head above the datum, which is part of the total head.

  3. Question 3

    Gravity sewers are usually designed for a minimum flowing velocity of about 0.6 m/s (2 ft/s) to:

    • AKeep solids from settling (self-cleansing)
    • BPrevent freezing
    • CReduce pipe cost
    • DIncrease aeration
    Show answer and explanation

    Answer: A) Keep solids from settling (self-cleansing)

    • ACorrect. Deposits reduce capacity and cause odors.
    • BIncorrect. It's for self-cleansing.
    • CIncorrect. It's for self-cleansing.
    • DIncorrect. It's for self-cleansing.

  4. Question 4

    For pipes connected in parallel between two junctions, the:

    • AHead loss is the same in each pipe
    • BFlow is the same in each pipe
    • CVelocity is the same in each pipe
    • DTotal head loss is the sum of each pipe's loss
    Show answer and explanation

    Answer: A) Head loss is the same in each pipe

    • ACorrect. Flows divide to satisfy that condition.
    • BIncorrect. That's series.
    • CIncorrect. Velocities differ with size.
    • DIncorrect. That's series.

  5. Question 5

    As a centrifugal pump's flow rate increases, the NPSH it requires generally:

    • ADecreases
    • BIncreases
    • CStays constant
    • DBecomes zero
    Show answer and explanation

    Answer: B) Increases

    • AIncorrect. It usually rises with flow.
    • BCorrect. Cavitation risk rises at high flow.
    • CIncorrect. It varies with flow.
    • DIncorrect. It's always positive.

  6. Question 6

    A rectangular channel 4 m wide flows 1 m deep. Its hydraulic radius is about:

    • A1.5 m
    • B0.67 m
    • C4 m
    • D0.25 m
    Show answer and explanation

    Answer: B) 0.67 m

    • AIncorrect. That's inverted.
    • BCorrect. R = A/P = 4/6.
    • CIncorrect. That's the area.
    • DIncorrect. Use the wetted perimeter of 6 m.

  7. Question 7

    For a submerged vertical plane surface, the center of pressure lies:

    • AAbove the centroid
    • BBelow the centroid
    • CAt the water surface
    • DExactly at the centroid always
    Show answer and explanation

    Answer: B) Below the centroid

    • AIncorrect. Deeper water pushes harder.
    • BCorrect. Pressure increases with depth.
    • CIncorrect. It's on the submerged surface.
    • DIncorrect. It's below for vertical surfaces.

  8. Question 8

    The water surface in a tank is 30 m above a hydrant. With no flow (γ = 9.81 kN/m³), the static pressure at the hydrant is about:

    • A294 kPa
    • B30 kPa
    • C2,943 kPa
    • D98 kPa
    Show answer and explanation

    Answer: A) 294 kPa

    • ACorrect. γh = 9.81 × 30.
    • BIncorrect. That omits γ.
    • CIncorrect. That's off by a factor of 10.
    • DIncorrect. That's 10 m of water.

  9. Question 9

    Cavitation in a pump occurs when:

    • ALocal pressure falls below the liquid's vapor pressure
    • BThe discharge pressure is too high
    • CThe water is too cold
    • DThe pump runs at reduced speed only
    Show answer and explanation

    Answer: A) Local pressure falls below the liquid's vapor pressure

    • ACorrect. Vapor bubbles form and then collapse.
    • BIncorrect. That isn't the cause.
    • CIncorrect. Cold water has lower vapor pressure.
    • DIncorrect. Low speed usually reduces cavitation.

  10. Question 10

    A highway culvert flows under inlet control. Which factors primarily determine its discharge capacity?

    • AThe headwater depth and the geometry of the culvert entrance
    • BThe roughness and the length of the culvert barrel
    • CThe tailwater depth in the channel below the outlet
    • DThe barrel slope and the velocity head at the outlet
    Show answer and explanation

    Answer: A) The headwater depth and the geometry of the culvert entrance

    Under inlet control, the entrance is the bottleneck, so headwater depth and inlet geometry set the capacity. Barrel roughness, length, slope and tailwater matter only under outlet control.

    • ACorrect. Under inlet control, the entrance can't accept as much flow as the barrel can carry. Capacity is therefore set by the headwater above the inlet invert and by the inlet geometry, including barrel area and edge shape.
    • BIncorrect. Barrel friction (roughness and length) governs under outlet control, when the barrel or downstream conditions limit the flow. Under inlet control it has little effect.
    • CIncorrect. Tailwater affects capacity under outlet control. Under inlet control the flow is controlled at the entrance, so downstream depth doesn't govern.
    • DIncorrect. Barrel slope and outlet losses enter the outlet-control energy balance. Under inlet control the barrel flows partly full and doesn't limit discharge.

Fluid Mechanics and Hydraulics: FAQs

How much of the FE exam is Fluid Mechanics and Hydraulics?

Fluid Mechanics and Hydraulics is 12% of the FE exam by the official exam guide's weighting, which on a 110-question form works out to roughly 13 questions. The real exam's exact split varies by form.

What does the Fluid Mechanics and Hydraulics domain cover?

One of the two largest domains: fluid statics and flow in pipes, pumps and open channels.

How should I study Fluid Mechanics and Hydraulics for FE?

Write out the energy equation with every term for each pipe problem before simplifying. Most questions are one head-loss or pump term away from the answer.

Where can I practise more Fluid Mechanics and Hydraulics questions?

This page has 10 free Fluid Mechanics and Hydraulics questions with answers. The ExamWize FE question bank has 72 questions in this domain. Every practice exam result breaks your score down by domain, and Review Mode re-drills the questions you missed.

Other FE topics

Also: 15 free questions across all topics, the FE cheat sheet and the study guide.

Get Fluid Mechanics and Hydraulics exam-ready

The full FE bank has 72 Fluid Mechanics and Hydraulics questions. Timed practice exams match the real exam's length and domain weights, every result shows your score per domain, and Review Mode re-drills the questions you missed.

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Not affiliated with NCEES; questions are original, written from the official exam outline, and never copied from the real exam.