Mechanical Aptitude Mechanical Aptitude Fluid Dynamics Principles 2 — Questions and Answers
Question 1: A pipe narrows from 4 inches in diameter to 2 inches in diameter. How does the fluid velocity change at the narrower section?
- It decreases to half
- It remains the same
- It quadruples (Correct answer)
- It doubles
Correct answer: It quadruples
By the continuity equation, when diameter halves the cross-sectional area quarters, so velocity must quadruple to maintain the same flow rate.
Question 2: Which factor does NOT directly affect the buoyant force acting on a submerged object?
- Density of the fluid
- Volume of the object
- Weight of displaced fluid
- Material composition of the object (Correct answer)
Correct answer: Material composition of the object
Buoyant force equals the weight of displaced fluid, which depends on the object's volume and fluid density, not the object's material composition.
Question 3: Water flows through a horizontal pipe. At a section where the pipe widens, what happens to the pressure?
- Pressure decreases
- Pressure increases (Correct answer)
- Pressure stays the same
- Pressure fluctuates randomly
Correct answer: Pressure increases
According to Bernoulli's principle, where velocity decreases (wider pipe) pressure increases, since total energy is conserved.
Question 4: A hydraulic jack uses a small piston of area 2 in² and a large piston of area 20 in². If you apply 50 lb of force on the small piston, what force is produced at the large piston?
- 5 lb
- 50 lb
- 500 lb (Correct answer)
- 1000 lb
Correct answer: 500 lb
Pascal's law states pressure is equal throughout: F_out = F_in × (A_out/A_in) = 50 × (20/2) = 500 lb.
Question 5: What is the primary reason turbulent flow creates more resistance than laminar flow in a pipe?
- Turbulent flow moves faster on average
- Turbulent flow creates chaotic mixing that dissipates more energy (Correct answer)
- Turbulent flow only occurs in wider pipes
- Turbulent flow reduces friction between fluid layers
Correct answer: Turbulent flow creates chaotic mixing that dissipates more energy
Turbulent flow causes random, chaotic motion and eddies that convert kinetic energy into heat, significantly increasing energy loss and resistance.
Question 6: A fluid with higher viscosity will generally have which of the following characteristics compared to a low-viscosity fluid?
- It flows more easily through pipes
- It has a lower Reynolds number at the same velocity (Correct answer)
- It creates less resistance when stirred
- It always flows turbulently
Correct answer: It has a lower Reynolds number at the same velocity
Reynolds number = ρvD/μ, so higher viscosity (μ) produces a lower Reynolds number, making laminar flow more likely.
Question 7: In a closed hydraulic system, if a valve is suddenly shut, the resulting pressure surge is called:
- Cavitation
- Water hammer (Correct answer)
- Hydraulic jump
- Venturi effect
Correct answer: Water hammer
Water hammer is the pressure wave created when flowing fluid is suddenly stopped, often heard as a banging in pipes.
Mechanical Aptitude Mechanical Aptitude Fluid Dynamics Principles 2
A pipe narrows from 4 inches in diameter to 2 inches in diameter.
How does the fluid velocity change at the narrower section?