NAPT Fluid Mechanics and Hydraulics 2 — Questions and Answers
Question 1: Laminar flow occurs when the Reynolds number is approximately:
- Greater than 4,000
- Between 2,300 and 4,000
- Less than 2,300 (Correct answer)
- Exactly 1,000
Correct answer: Less than 2,300
Laminar flow is characterized by smooth, parallel streamlines and occurs when the Reynolds number is below approximately 2,300, indicating viscous forces dominate over inertial forces.
Question 2: Absolute pressure is related to gauge pressure by:
- Absolute = Gauge − Atmospheric
- Absolute = Gauge × Atmospheric
- Absolute = Gauge + Atmospheric (Correct answer)
- Absolute = Atmospheric − Gauge
Correct answer: Absolute = Gauge + Atmospheric
Absolute pressure is the total pressure including atmospheric pressure, so Absolute = Gauge + Atmospheric (≈ 101,325 Pa at sea level).
Question 3: Cavitation in a hydraulic system occurs when:
- Fluid temperature drops below freezing
- Local fluid pressure drops below the vapor pressure of the fluid (Correct answer)
- Flow velocity decreases below a critical threshold
- System pressure exceeds the pump rating
Correct answer: Local fluid pressure drops below the vapor pressure of the fluid
Cavitation occurs when local pressure in a flowing fluid drops below its vapor pressure, causing vapor bubbles to form; when these collapse, they can cause severe damage to pump impellers and pipes.
Question 4: The specific gravity of a fluid is defined as:
- The ratio of its density to the density of water at 4°C (Correct answer)
- Its density in kg/m³
- Its weight per unit volume in Newtons
- The ratio of its pressure to atmospheric pressure
Correct answer: The ratio of its density to the density of water at 4°C
Specific gravity is a dimensionless ratio of a substance's density to the density of water (1,000 kg/m³ at 4°C), so water has a specific gravity of exactly 1.
Question 5: Head pressure in fluid systems refers to:
- The pressure at the pump inlet only
- The equivalent height of fluid column that produces a given pressure (Correct answer)
- The maximum rated pressure of a hydraulic component
- The dynamic pressure due to fluid velocity
Correct answer: The equivalent height of fluid column that produces a given pressure
Head pressure expresses fluid pressure as an equivalent column height (h = P/ρg), allowing pressure to be visualized as a vertical distance the fluid could be raised.
Question 6: Which type of pump uses meshing gears to move fluid and is commonly found in hydraulic systems?
- Centrifugal pump
- Gear pump (Correct answer)
- Axial flow pump
- Diaphragm pump
Correct answer: Gear pump
Gear pumps trap fluid in the spaces between gear teeth and the pump housing, then carry it from inlet to outlet; they are positive-displacement pumps widely used in hydraulic systems for their reliability and consistent flow.
Question 7: The Reynolds number is calculated using which set of variables?
- Pressure, density, and viscosity
- Velocity, pipe diameter, density, and dynamic viscosity (Correct answer)
- Flow rate, pipe length, and fluid temperature
- Surface tension, gravity, and fluid depth
Correct answer: Velocity, pipe diameter, density, and dynamic viscosity
Reynolds number Re = ρVD/μ, where ρ is fluid density, V is flow velocity, D is pipe diameter, and μ is dynamic viscosity; it indicates whether flow will be laminar or turbulent.
NAPT Fluid Mechanics and Hydraulics 2
Laminar flow occurs when the Reynolds number is approximately: