ASCE Hydraulics and Fluid Mechanics 2 ā Questions and Answers
Question 1: Manning's roughness coefficient (n) in open channel flow primarily accounts for:
- Channel slope and cross-sectional shape
- Boundary resistance due to channel surface texture (Correct answer)
- Fluid viscosity and density
- Froude number effects on wave speed
Correct answer: Boundary resistance due to channel surface texture
Manning's n is an empirical coefficient that quantifies energy losses due to boundary friction from the channel's surface roughness (concrete, earth, rock, vegetation, etc.).
Question 2: The hydraulic radius (R) used in Manning's equation is defined as:
- Half the width of the channel cross-section
- The ratio of flow area to wetted perimeter (Correct answer)
- The ratio of wetted perimeter to flow area
- The depth at which the Froude number equals 1
Correct answer: The ratio of flow area to wetted perimeter
Hydraulic radius R = A/P, where A is the cross-sectional flow area and P is the wetted perimeter, representing the efficiency of the channel shape in conveying flow.
Question 3: Water hammer in a pipeline is primarily caused by:
- Excessive pipe slope causing supercritical flow
- Rapid changes in flow velocity such as sudden valve closure (Correct answer)
- High Reynolds numbers causing turbulent eddies
- Cavitation at the pump impeller
Correct answer: Rapid changes in flow velocity such as sudden valve closure
Water hammer occurs when flow velocity changes rapidly (e.g., sudden valve closure), converting kinetic energy into pressure surges that propagate as pressure waves through the pipe at acoustic velocity.
Question 4: Which of the following best describes specific energy in open channel flow?
- Total energy measured above a datum at the channel bed
- Energy per unit weight measured with respect to the channel bottom as datum (Correct answer)
- The sum of pressure head and velocity head only
- Energy dissipated per unit length of channel
Correct answer: Energy per unit weight measured with respect to the channel bottom as datum
Specific energy (E = y + VĀ²/2g) is the total energy per unit weight of fluid measured with respect to the channel bottom, combining depth (pressure term) and velocity head.
Question 5: In a pipe network analysis using the Hardy Cross method, what condition must be satisfied at each junction node?
- The sum of head losses around each loop must equal zero
- Continuity: the algebraic sum of flows at each node must equal zero (Correct answer)
- The Reynolds number must be the same in all connecting pipes
- Pressure must be equal in all pipes meeting at a junction
Correct answer: Continuity: the algebraic sum of flows at each node must equal zero
The Hardy Cross method iteratively satisfies both loop equations (sum of head losses = 0) and nodal continuity (sum of inflows = sum of outflows = 0) simultaneously to solve pipe networks.
Question 6: A venture meter measures flow rate by detecting the pressure difference between:
- Two points on the same cross-section at different depths
- The inlet section and the constricted throat section (Correct answer)
- The pipe inlet and the pipe outlet
- The pump discharge and the downstream reservoir
Correct answer: The inlet section and the constricted throat section
A venturi meter uses the pressure drop between the full-diameter inlet and the narrowed throat, where by Bernoulli's principle the velocity increases and pressure decreases, allowing flow rate calculation.
Question 7: Cavitation in hydraulic systems is most likely to occur when:
- Flow velocity is very low and pressure is very high
- Local pressure drops below the vapor pressure of the liquid (Correct answer)
- The Reynolds number falls below the laminar threshold
- Pipe diameter is too large relative to the pump capacity
Correct answer: Local pressure drops below the vapor pressure of the liquid
Cavitation occurs when local static pressure falls below the fluid's vapor pressure, causing vapor bubbles to form; when these bubbles collapse in higher-pressure zones, they cause erosion, noise, and vibration.
ASCE Hydraulics and Fluid Mechanics 2
Manning's roughness coefficient (n) in open channel flow primarily accounts for: