ASCE Hydraulics and Fluid Mechanics 1 — Questions and Answers
Question 1: Which dimensionless number is used to distinguish between laminar and turbulent flow in a pipe?
- Froude number
- Reynolds number (Correct answer)
- Mach number
- Weber number
Correct answer: Reynolds number
The Reynolds number (Re = ρVD/μ) compares inertial to viscous forces and is the standard criterion for laminar vs. turbulent pipe flow, with Re < 2300 generally indicating laminar flow.
Question 2: The Bernoulli equation assumes which of the following conditions?
- Viscous, compressible, unsteady flow along a streamline
- Inviscid, incompressible, steady flow along a streamline (Correct answer)
- Turbulent, compressible, steady flow between two cross-sections
- Viscous, incompressible, unsteady flow at a point
Correct answer: Inviscid, incompressible, steady flow along a streamline
Bernoulli's equation is derived assuming steady, inviscid (frictionless), and incompressible flow along a single streamline, making it applicable to ideal fluid scenarios.
Question 3: The hydraulic grade line (HGL) represents the sum of which two energy heads?
- Velocity head and elevation head
- Pressure head and velocity head
- Pressure head and elevation head (Correct answer)
- Elevation head and friction head
Correct answer: Pressure head and elevation head
The HGL equals pressure head (p/γ) plus elevation head (z), indicating the height to which water would rise in a piezometer tube at any point in the system.
Question 4: Which formula is most commonly used to calculate head loss due to friction in a full pipe under pressure?
- Manning's equation
- Darcy-Weisbach equation (Correct answer)
- Chezy's formula
- Hazen-Williams equation
Correct answer: Darcy-Weisbach equation
The Darcy-Weisbach equation (hf = f·L/D·V²/2g) is the most theoretically rigorous and widely accepted formula for calculating frictional head loss in pressurized pipe flow.
Question 5: In open channel flow, critical flow occurs when the Froude number equals:
- 0
- 0.5
- 1.0 (Correct answer)
- 2.0
Correct answer: 1.0
Critical flow is defined by a Froude number of exactly 1.0, where the flow velocity equals the wave celerity, representing the boundary between subcritical (Fr < 1) and supercritical (Fr > 1) flow.
Question 6: The continuity equation for steady, incompressible flow through a pipe states that:
- Pressure is constant throughout the pipe
- The product of velocity and pipe diameter is constant
- The product of cross-sectional area and velocity is constant (Correct answer)
- The sum of head losses equals the total energy
Correct answer: The product of cross-sectional area and velocity is constant
Conservation of mass for incompressible flow requires Q = A₁V₁ = A₂V₂, meaning the volumetric flow rate (area × velocity) is constant at every cross-section.
Question 7: A pump adds energy to a fluid system. Which of the following correctly represents the pump head (hp) in the energy equation?
- hp is subtracted from the upstream total head
- hp is added to the downstream total head
- hp equals the friction losses in the system
- hp is added between the upstream and downstream energy terms (Correct answer)
Correct answer: hp is added between the upstream and downstream energy terms
The modified Bernoulli equation with a pump reads: H₁ + hp = H₂ + hL, where hp is added to the upstream energy to account for the mechanical energy input between the two sections.
ASCE Hydraulics and Fluid Mechanics 1
Which dimensionless number is used to distinguish between laminar and turbulent flow in a pipe?