EIT Fluid Mechanics 5 — Questions and Answers
Question 1: The bulk modulus of elasticity of a fluid is defined as:
- K = −V(dP/dV) (Correct answer)
- K = μ(dV/dy)
- K = ρg·h
- K = P/ρ
Correct answer: K = −V(dP/dV)
Bulk modulus K = −V(dP/dV) = ρ(dP/dρ), measuring resistance to compression.
Question 2: A 0.05 m diameter pipe (f = 0.02, L = 100 m) carries water at 2 m/s. The major (friction) head loss is:
- 8.16 m (Correct answer)
- 4.08 m
- 16.32 m
- 2.04 m
Correct answer: 8.16 m
h_f = f(L/D)(V²/2g) = 0.02×(100/0.05)×(4/19.62) = 0.02×2000×0.204 = 8.16 m.
Question 3: For laminar flow between two infinite parallel plates (Couette flow with both plates stationary and a pressure gradient), the velocity profile is:
- Parabolic (Correct answer)
- Linear
- Logarithmic
- Uniform
Correct answer: Parabolic
Pressure-driven flow between stationary plates (Poiseuille flow) produces a parabolic velocity profile u ∝ (h² − y²).
Question 4: The specific speed of a pump (Ns) is used to:
- Classify pump type and predict performance at various scales (Correct answer)
- Calculate NPSH required
- Determine pipe friction losses
- Measure fluid viscosity
Correct answer: Classify pump type and predict performance at various scales
Specific speed Ns = N√Q/H^(3/4) classifies pumps (centrifugal, mixed-flow, axial) and enables similitude-based performance prediction.
Question 5: The Strouhal number is defined as St = fL/V, where f is the frequency of vortex shedding. It primarily relates to:
- Periodic vortex shedding behind bluff bodies (Correct answer)
- Pipe friction losses
- Buoyancy-driven flow
- Capillary effects
Correct answer: Periodic vortex shedding behind bluff bodies
The Strouhal number characterizes the periodic vortex shedding (Kármán vortex street) frequency behind cylinders and bluff bodies.
Question 6: For a pump operating in a piping system, the operating point is determined by:
- The intersection of the pump curve and the system curve (Correct answer)
- The maximum efficiency point of the pump alone
- The point of minimum NPSH
- The point where head equals zero
Correct answer: The intersection of the pump curve and the system curve
The operating point is where the pump head-flow curve intersects the system resistance curve (static head + friction losses).
Question 7: The net positive suction head available (NPSHA) is calculated as:
- NPSHA = (P_s − P_v)/(ρg) + V_s²/(2g) (Correct answer)
- NPSHA = P_atm/(ρg) + z_s
- NPSHA = H_pump − H_system
- NPSHA = f·L·V²/(D·2g)
Correct answer: NPSHA = (P_s − P_v)/(ρg) + V_s²/(2g)
NPSHA = (P_s/ρg + V_s²/2g) − P_v/(ρg), representing the absolute suction head minus the vapor pressure head.
EIT Fluid Mechanics 5
The bulk modulus of elasticity of a fluid is defined as: