Free Fluid Flow MCQs with Answers

1. Flow is described as laminar when

• A. the fluid particles move in irregular, mixing paths
• B. each fluid particle follows the same smooth path as the one before it
• C. the fluid is at rest
• D. the fluid is compressible

Explanation: In laminar or streamline flow the layers slide over one another without mixing, so the velocity at any fixed point stays constant in time and streamlines never cross. Above a critical speed the motion breaks up into eddies and becomes turbulent, which dissipates far more energy. The Reynolds number is what predicts which regime a given flow is in.

2. Upthrust on a body immersed in a fluid is equal to

• A. the weight of the body
• B. the weight of the fluid displaced by the body
• C. the volume of the body
• D. the density of the fluid

Explanation: This is Archimedes's principle, and the upthrust arises because pressure increases with depth, so the upward force on the bottom of the body exceeds the downward force on the top. A body floats when the upthrust at partial immersion already equals its weight. The principle also explains why a body appears lighter when weighed in water.

3. The flow of a fluid becomes turbulent when

• A. the speed falls below a critical value
• B. the speed exceeds a critical value and the Reynolds number becomes large
• C. the pipe is perfectly smooth
• D. the fluid is very viscous

Explanation: Turbulence sets in when inertial effects dominate viscous ones, which is what a large Reynolds number expresses, and it appears as eddies and rapid mixing. A high viscosity favours laminar flow rather than turbulence, since it damps the disturbances out. Turbulent flow carries far more resistance, which matters in pipelines, aircraft design and blood vessels alike.

4. A fluid is flowing through a tube. To undergo transition from laminar to turbulent flow its velocity must be:

• A. Slightly less than critical velocity
• B. Equal to critical velocity
• C. Greater than critical velocity
• D. Increasing gradually but less than critical velocity

Explanation: Below the critical velocity the flow stays in orderly layers, and only once that threshold is passed does the motion break up into eddies and become turbulent. The critical velocity is set by the Reynolds number, which combines density, viscosity, speed and pipe diameter. Turbulent flow dissipates far more energy, which matters in pipelines and in blood vessels alike.