Mechanical Aptitude Mechanical Aptitude Fluid Dynamics Principles 4 — Questions and Answers
Question 1: Two pipes of equal length carry the same fluid at the same pressure difference. Pipe A has a diameter of 1 inch and Pipe B has a diameter of 2 inches. Which carries more flow?
- Pipe A carries more flow
- They carry equal flow
- Pipe B carries more flow (Correct answer)
- It depends on the fluid type
Correct answer: Pipe B carries more flow
By the Hagen-Poiseuille equation, flow rate is proportional to the fourth power of radius, so doubling diameter increases flow by a factor of 16.
Question 2: What does the Reynolds number indicate about fluid flow?
- The speed of sound in the fluid
- Whether flow is laminar or turbulent (Correct answer)
- The density of the fluid
- The pressure drop across a pipe
Correct answer: Whether flow is laminar or turbulent
The Reynolds number predicts flow regime: below ~2,300 is laminar, above ~4,000 is fully turbulent, and in between is transitional.
Question 3: A ship floats because it displaces a volume of water whose weight equals the ship's total weight. This is a direct application of:
- Bernoulli's principle
- Pascal's law
- Archimedes' principle (Correct answer)
- The continuity equation
Correct answer: Archimedes' principle
Archimedes' principle states that a floating object displaces fluid equal in weight to the object's own weight.
Question 4: In a fluid system, a manometer is used to measure:
- Flow velocity
- Fluid density
- Pressure differences (Correct answer)
- Pipe diameter
Correct answer: Pressure differences
A manometer measures pressure by comparing fluid column heights, giving direct readings of pressure or pressure differences.
Question 5: When selecting a centrifugal pump for a system, what does the 'head' rating refer to?
- The physical height of the pump above the floor
- The energy added to the fluid per unit weight, expressed as a height (Correct answer)
- The maximum pipe diameter the pump can handle
- The flow rate at maximum efficiency
Correct answer: The energy added to the fluid per unit weight, expressed as a height
Pump head is the energy per unit weight of fluid, equivalent to the height the pump could raise the fluid against gravity.
Question 6: A fluid is described as 'incompressible.' What does this mean in practical terms for most hydraulic calculations?
- The fluid cannot be heated
- The fluid's density remains essentially constant under pressure changes (Correct answer)
- The fluid cannot flow through small orifices
- The fluid has zero viscosity
Correct answer: The fluid's density remains essentially constant under pressure changes
Incompressible fluids like water maintain constant density regardless of pressure changes, which greatly simplifies hydraulic calculations.
Question 7: What happens to the pressure at the bottom of a tank if you double the depth of water but keep the tank's base area the same?
- Pressure at the bottom doubles (Correct answer)
- Pressure at the bottom stays the same
- Pressure at the bottom halves
- Pressure at the bottom quadruples
Correct answer: Pressure at the bottom doubles
Hydrostatic pressure P = ρgh, so doubling depth (h) directly doubles the pressure at the bottom.
Mechanical Aptitude Mechanical Aptitude Fluid Dynamics Principles 4
Two pipes of equal length carry the same fluid at the same pressure difference.
Pipe A has a diameter of 1 inch and Pipe B has a diameter of 2 inches.
Which carries more flow?