ETC ETC Hydraulics & Fluid Mechanics 2 — Questions and Answers
Question 1: The Reynolds number is used in fluid mechanics to determine whether flow is:
- Compressible or incompressible
- Laminar or turbulent (Correct answer)
- Pressurized or free-surface
- Steady or unsteady
Correct answer: Laminar or turbulent
The Reynolds number (Re = ρVD/μ) distinguishes laminar flow (Re < 2,300) from turbulent flow (Re > 4,000) in circular pipe systems.
Question 2: A Venturi meter measures which hydraulic quantity?
- Static pressure at a single point
- Flow rate by measuring a pressure differential at a constriction (Correct answer)
- Fluid velocity using a stagnation probe
- Pipe friction factor
Correct answer: Flow rate by measuring a pressure differential at a constriction
A Venturi meter calculates volumetric flow rate by measuring the pressure drop between the inlet and the narrow throat section, then applying Bernoulli's equation.
Question 3: The Darcy-Weisbach equation (hf = f × L/D × V²/2g) is used to calculate:
- Flow velocity in open channels
- Head loss due to pipe wall friction (Correct answer)
- Pump shaft power requirements
- Fluid density changes with temperature
Correct answer: Head loss due to pipe wall friction
The Darcy-Weisbach equation calculates major (friction) head loss in a pipe, requiring the Darcy friction factor f, pipe length L, diameter D, and flow velocity V.
Question 4: The Hydraulic Grade Line (HGL) in a pipe system represents the sum of:
- Velocity head and elevation head
- Pressure head and elevation head (Correct answer)
- Total energy head at each point
- Friction head loss along the pipe
Correct answer: Pressure head and elevation head
The HGL equals the sum of pressure head (P/γ) and elevation head (z) at each cross-section, and represents the height to which water would rise in a piezometer.
Question 5: A manometer is an instrument primarily used to measure:
- Fluid flow velocity
- Fluid temperature and density
- Pressure or differential pressure between two points (Correct answer)
- Pipe wall roughness
Correct answer: Pressure or differential pressure between two points
A manometer measures pressure or pressure differences by observing the height difference of a fluid column (commonly water or mercury) in a U-tube.
Question 6: Cavitation in hydraulic pumps occurs when:
- Fluid viscosity is excessively high
- Local fluid pressure drops below the fluid's vapor pressure (Correct answer)
- Pump speed is set too low for the system
- Discharge head exceeds the pump's design rating
Correct answer: Local fluid pressure drops below the fluid's vapor pressure
Cavitation forms vapor bubbles when local pressure falls below the vapor pressure; these bubbles collapse violently near the impeller, causing noise, vibration, and surface erosion.
Question 7: Minor losses in a piping system are caused by:
- Pipe wall friction over long straight runs
- Fittings, valves, bends, and abrupt geometry changes (Correct answer)
- Variations in fluid density along the pipe
- Temperature changes in the flowing fluid
Correct answer: Fittings, valves, bends, and abrupt geometry changes
Minor losses result from flow disruptions at fittings, elbows, tees, valves, and contractions/expansions, calculated as hm = K × V²/2g using a loss coefficient K.
ETC ETC Hydraulics & Fluid Mechanics 2
The Reynolds number is used in fluid mechanics to determine whether flow is: