BME Fluid Mechanics 3 — Questions and Answers
Question 1: The Moody diagram relates the Darcy friction factor to:
- Reynolds number and relative pipe roughness (Correct answer)
- Mach number and pipe length
- Prandtl number and wall temperature
- Strouhal number and frequency
Correct answer: Reynolds number and relative pipe roughness
The Moody diagram provides friction factor f as a function of Reynolds number Re and relative roughness ε/D.
Question 2: For an ideal (inviscid, incompressible) fluid, Bernoulli's equation assumes which condition along a streamline?
- Rotational flow with heat transfer
- Steady, incompressible, inviscid flow with no shaft work (Correct answer)
- Unsteady turbulent flow
- Viscous laminar flow with body forces only
Correct answer: Steady, incompressible, inviscid flow with no shaft work
Bernoulli's equation applies along a streamline for steady, incompressible, inviscid flow with no external work done.
Question 3: Which non-dimensional parameter characterizes the ratio of buoyancy forces to viscous forces in natural convection flows?
- Prandtl number
- Grashof number (Correct answer)
- Nusselt number
- Eckert number
Correct answer: Grashof number
The Grashof number (Gr = gβΔTL³/ν²) represents buoyancy to viscous force ratio in natural convection.
Question 4: A hydraulic jump occurs in open channel flow when the flow transitions from:
- Subcritical to supercritical
- Supercritical to subcritical (Correct answer)
- Laminar to turbulent
- Turbulent to laminar
Correct answer: Supercritical to subcritical
A hydraulic jump is an abrupt transition from supercritical (Fr > 1) to subcritical (Fr < 1) flow with significant energy loss.
Question 5: The stream function ψ is defined such that lines of constant ψ represent:
- Lines of constant pressure
- Streamlines of the flow (Correct answer)
- Lines of constant vorticity
- Equipotential lines
Correct answer: Streamlines of the flow
The stream function ψ is defined so that ∂ψ/∂y = u and ∂ψ/∂x = −v, making iso-ψ lines identical to streamlines.
Question 6: Minor losses in pipe systems (bends, fittings, valves) are commonly expressed as:
- h_m = K(V²/2g) where K is a loss coefficient (Correct answer)
- h_m = f(L/D)(V²/2g)
- h_m = ρgΔz
- h_m = ΔP/ρ
Correct answer: h_m = K(V²/2g) where K is a loss coefficient
Minor losses are expressed as h_m = K·V²/(2g), where K is the dimensionless loss coefficient specific to each fitting.
Question 7: In a centrifugal pump, the specific speed Ns is used to:
- Predict cavitation onset
- Classify pump geometry and select the optimal impeller type (Correct answer)
- Calculate NPSH available
- Determine pipe friction losses
Correct answer: Classify pump geometry and select the optimal impeller type
Specific speed classifies pump geometry: low Ns favors radial-flow (centrifugal) designs, high Ns favors axial-flow designs.
BME Fluid Mechanics 3
The Moody diagram relates the Darcy friction factor to: