Mechanical Aptitude Fluid Dynamics Principles Questions and Answers 2 — Questions and Answers
Question 1: According to Bernoulli's principle, what happens to the pressure of a fluid as its velocity increases?
- Pressure increases
- Pressure decreases (Correct answer)
- Pressure remains constant
- Pressure fluctuates randomly
Correct answer: Pressure decreases
Bernoulli's principle states that an increase in fluid velocity corresponds to a decrease in pressure. This is because the total energy (kinetic + potential + pressure) along a streamline remains constant.
Bernoulli's principle is derived from the conservation of energy for flowing fluids. Along a streamline, the sum of pressure energy, kinetic energy (½ρv²), and potential energy (ρgh) remains constant. When fluid speeds up (increased kinetic energy), something must decrease to maintain the balance — that something is pressure. This principle explains airplane lift, the curve of a baseball, and why shower curtains blow inward when water is running.
Question 2: What does viscosity measure in a fluid?
- Density
- Resistance to flow (Correct answer)
- Temperature
- Compressibility
Correct answer: Resistance to flow
Viscosity is a measure of a fluid's resistance to flow or deformation. High-viscosity fluids like honey flow slowly, while low-viscosity fluids like water flow easily.
Viscosity quantifies the internal friction within a fluid — the resistance of adjacent fluid layers sliding past each other. Dynamic viscosity (measured in Pascal-seconds or poise) describes this resistance directly. Kinematic viscosity (dynamic viscosity divided by density) is also commonly used. Temperature significantly affects viscosity: heating a fluid generally decreases its viscosity (oil thins when heated), while cooling increases it. Understanding viscosity is critical for lubrication systems, pipeline design, and hydraulic equipment.
Question 3: What type of flow is characterized by smooth, parallel layers of fluid with no mixing between them?
- Turbulent flow
- Laminar flow (Correct answer)
- Chaotic flow
- Vortex flow
Correct answer: Laminar flow
Laminar flow consists of smooth, orderly layers of fluid moving in parallel without mixing. It occurs at low velocities and in fluids with high viscosity. Turbulent flow, by contrast, involves chaotic mixing and eddies.
Laminar flow (from Latin 'lamina' meaning layer) occurs when fluid particles follow smooth, predictable paths in parallel layers. The Reynolds number (Re) determines whether flow will be laminar or turbulent: Re < 2,000 typically indicates laminar flow, while Re > 4,000 indicates turbulent flow. Between these values is a transitional zone. Laminar flow produces less friction and energy loss in pipes, which is why it is preferred in precision applications like medical IV drips and microfluidic devices.
Question 4: What principle explains why a heavier-than-water steel ship floats?
- Bernoulli's principle
- Archimedes' principle (Correct answer)
- Pascal's principle
- Newton's third law
Correct answer: Archimedes' principle
Archimedes' principle states that a body submerged in fluid experiences an upward buoyant force equal to the weight of fluid it displaces. A steel ship floats because its hull shape displaces enough water to create a buoyant force exceeding the ship's weight.
Archimedes' principle says the buoyant force on an object equals the weight of the fluid displaced. Although steel is about 8 times denser than water, a ship's hull encloses a large volume of air, making the ship's average density less than water. The hull displaces a volume of water whose weight exceeds the ship's total weight, producing net upward force. If the ship takes on water (reducing the air volume), its average density increases and it sinks — which is exactly what happens when a hull is breached.
Question 5: In a venturi tube, where is the fluid velocity the highest?
- At the widest section
- At the narrowest section (throat) (Correct answer)
- At the inlet
- At the outlet
Correct answer: At the narrowest section (throat)
The continuity equation (A₁V₁ = A₂V₂) requires that fluid velocity increases as the cross-sectional area decreases. In a venturi tube, the throat (narrowest section) has the highest velocity and lowest pressure.
The venturi effect is a direct consequence of the continuity equation for incompressible fluids, which states that the product of cross-sectional area and velocity must remain constant (A₁V₁ = A₂V₂). As fluid enters the converging section of a venturi tube and the area decreases, velocity must increase proportionally. At the throat (minimum area), velocity reaches its maximum. By Bernoulli's principle, this maximum velocity corresponds to minimum pressure, which is why venturi tubes are used in carburetors, aspirators, and flow measurement devices.
Question 6: What does the Reynolds number help predict about fluid flow?
- Fluid temperature
- Whether flow is laminar or turbulent (Correct answer)
- Fluid density
- Flow direction
Correct answer: Whether flow is laminar or turbulent
The Reynolds number is a dimensionless quantity that predicts whether fluid flow will be laminar or turbulent. It depends on fluid velocity, characteristic length, and kinematic viscosity.
The Reynolds number (Re = ρvL/μ or Re = vL/ν) compares inertial forces to viscous forces in a fluid. When viscous forces dominate (low Re), flow is smooth and laminar. When inertial forces dominate (high Re), flow becomes chaotic and turbulent. For pipe flow, Re < 2,000 is laminar, Re > 4,000 is turbulent, and between is transitional. Osborne Reynolds demonstrated this in 1883 by injecting dye into flowing water, showing the transition from a smooth dye stream to chaotic mixing as flow rate increased.
Mechanical Aptitude Fluid Dynamics Principles Questions and Answers 2
According to Bernoulli's principle, what happens to the pressure of a fluid as its velocity increases?