NCEES Fluid Mechanics 4 — Questions and Answers
Question 1: A fluid with viscosity μ = 0.001 Pa·s flows between parallel plates 1 mm apart; the top plate moves at 1 m/s. What is the shear stress on the plates?
- 0.001 Pa
- 1 Pa (Correct answer)
- 0.1 Pa
- 10 Pa
Correct answer: 1 Pa
τ = μ(du/dy) = 0.001 × (1/0.001) = 0.001 × 1000 = 1 Pa.
Question 2: The specific gravity of a fluid is defined as the ratio of its density to the density of:
- Air at standard conditions
- Water at 4°C (Correct answer)
- Mercury at 20°C
- The fluid itself at a reference pressure
Correct answer: Water at 4°C
Specific gravity is the dimensionless ratio of a fluid's density to pure water's density at 4°C (1000 kg/m³).
Question 3: For flow over a flat plate, the boundary layer thickness δ grows proportionally to:
- x
- x^(1/2) (Correct answer)
- x^(1/3)
- x²
Correct answer: x^(1/2)
For laminar flow over a flat plate, the Blasius solution gives δ ∝ x^(1/2) (or δ/x = 5/√Re_x).
Question 4: Which valve type is best suited for throttling (flow control) applications?
- Gate valve
- Globe valve (Correct answer)
- Ball valve
- Check valve
Correct answer: Globe valve
Globe valves are specifically designed for throttling because their disc movement is perpendicular to flow, providing precise control.
Question 5: A 6-inch schedule 40 steel pipe has a length of 100 ft and a friction factor f = 0.02. What is the Darcy-Weisbach head loss if water flows at 5 ft/s? (D = 0.5054 ft)
- 0.77 ft
- 1.54 ft (Correct answer)
- 3.08 ft
- 6.16 ft
Correct answer: 1.54 ft
hf = f(L/D)(V²/2g) = 0.02×(100/0.5054)×(25/64.4) = 0.02×197.9×0.388 ≈ 1.54 ft.
Question 6: The continuity equation for steady, incompressible flow in a pipe states that:
- Velocity is constant throughout
- A₁V₁ = A₂V₂ (Correct answer)
- Pressure is constant throughout
- ρ₁A₁ = ρ₂A₂
Correct answer: A₁V₁ = A₂V₂
For incompressible flow, density is constant, so mass conservation reduces to A₁V₁ = A₂V₂ (volumetric flow rate is constant).
Question 7: In the Moody diagram, the fully turbulent (rough pipe) zone is characterized by:
- Friction factor decreasing with Reynolds number
- Friction factor independent of Reynolds number (Correct answer)
- Friction factor equal to 64/Re
- Friction factor proportional to velocity
Correct answer: Friction factor independent of Reynolds number
In the fully turbulent (complete turbulence) zone, the friction factor depends only on relative roughness, not Reynolds number.
NCEES Fluid Mechanics 4
A fluid with viscosity μ = 0.001 Pa·s flows between parallel plates 1 mm apart; the top plate moves at 1 m/s.
What is the shear stress on the plates?