Free Fluid Dynamics MCQs with Answers

• A. 1.25 m
• B. 12.5 m
• C. 0.125 m
• D. 125 m

Explanation: Velocity head = v²/(2g). With v = 5 m/s and g = 10 m/s²: v²/(2g) = 25/(20) = 1.25 m.

• A. 100 m/s
• B. 10 m/s
• C. 1 m/s
• D. 10√10 m/s

Explanation: Using Bernoulli (pressure drop → kinetic energy): ΔP = ½ ρ v².With ΔP = 0.5×10⁵ Pa and ρ ≈ 1000 kg/m³: v = √(2ΔP/ρ) = √(2×5×10⁴/1000) =…

• A. 20 m/s
• B. 10 m/s
• C. 30 m/s
• D. None

Explanation: Gauge pressure causing flow is (3 atm − 1 atm) = 2 atm ≈ 2×1.013×10⁵ PA. v = √(2P/ρ) ≈ √(2×2.026×10⁵/1000) ≈ √405 ≈ 20.1 m/s.

• A. t
• B. 4t
• C. 2t
• D. t/4

Explanation: For a tank draining through a small hole, Torricelli gives outflow speed v∝√h.

• A. 2.5 cm
• B. 5 cm
• C. 10 cm
• D. 0.25 cm

Explanation: At steady max height outflow = inflow. Outflow rate via Torricelli: Q = A √(2 g h). Solving h (Q/(A))²/(2 g).

• A. 0.1 m
• B. 0.05 m
• C. 0.005 m
• D. 0.0005 m

Explanation: For Couette flow F = η A v / d. Convert η = 0.01 poise = 0.001 Pa·s. A = 0.1×0.1 = 0.01 m², so d = η A v / F = (0.001×0.01×0.1)/0.002 =…

• A. Inversely proportional to 'r' but directly proportional to velocity 'v'
• B. Directly proportional to both radius 'r' and velocity 'v'
• C. Inversely proportional to both radius 'r' and velocity 'v'
• D. Directly proportional to 'r' but inversely proportional to 'v'

Explanation: Stokes' law for low Reynolds number gives drag F = 6 π η r V, which is directly proportional to radius r and to velocity V.

• A. 112.5 kPa
• B. 121.8 kPa
• C. 122.5 kPa
• D. 127.5 kPa

Explanation: A₁v₁ = A₂v₂, (D₁/D₂)² = 1/4, so v₂ = v₁ / 4 = 2 / 4 = 0.5 m/s. Bernoulli's: P₁ + ½ρv₁² = P₂ + ½ρv₂².

• A. Zero
• B. Maximum
• C. Equal to critical velocity
• D. May have any value

Explanation: The no-slip condition requires that the fluid relative velocity at the solid boundary equals the wall velocity.

• A. 4:9
• B. 9:4
• C. 8:27
• D. 1:1

Explanation: v ∝ 1/A and A ∝ r². With r1:r2 = 3:2 ⇒ A1:A2 = 9:4 ⇒ v1:v1 = A2: A1= 4:9.

• A. Maximum speed and least pressure
• B. Maximum pressure and least speed
• C. Both pressure and speed maximum
• D. Both pressure and speed least

Explanation: By continuity, narrow section → higher speed; Bernoulli's principle then implies higher kinetic energy corresponds to lower static…

• A. The speed of a particle always remains the same along its streamline
• B. The velocity of a particle always remains constant throughout the flow
• C. The kinetic energies of all the particles arriving at a given point are the same
• D. The moments of all the particles arriving at a given point are the same

Explanation: In steady flow the velocity field at each point is fixed in time; thus any particle passing through a particular point experiences the…

• A. Dynamic lift of an airplane
• B. Viscosity meter
• C. Capillary rise
• D. Hydraulic press

Explanation: Bernoulli's principle relates pressure and velocity and explains pressure differences over an airplane wing that produce lift; viscosity…

• A. Gravitational head, pressure head, and velocity head
• B. Gravity, gravitational head, and velocity head
• C. Pressure head, gravitational head, and velocity head
• D. Gravity pressure and velocity head

Explanation: In Bernoulli form P/(ρ g) is the pressure head, h is the gravitational (elevation) head, and v²/(2g) is the velocity head.

• A. Remains unchanged
• B. Decreases
• C. Increases
• D. Increases or decreases depending on the external pressure

Explanation: For liquids like water, increasing temperature reduces intermolecular cohesion, lowering viscosity.

• A. Greater than the coefficient of viscosity for cold air
• B. Smaller than the coefficient of viscosity for cold air
• C. Same as the coefficient of viscosity for cold air
• D. Increases or decreases depending on the external pressure

Explanation: For gases, viscosity increases with temperature because molecular momentum transfer rises with thermal speed; hence hot air has a larger…

• A. High viscosity
• B. Low viscosity
• C. Moderate viscosity
• D. High density

Explanation: High viscosity maintains a durable fluid film between moving surfaces under load, preventing metal contact and reducing wear.

• A. Glycerin
• B. Water
• C. Kerosene
• D. All of them at the same time

Explanation: Glycerin has the highest viscosity among the three, so it dissipates flow energy fastest and returns to rest quickest.

• A. 3 m/s
• B. 6 m/s
• C. 12 m/s
• D. 8 m/s

Explanation: Halving diameter reduces area by factor 4, so velocity increases by factor 4: v2 = 3×4 = 12 m/s.

• A. Area decreases
• B. Area increases
• C. Velocity remains same
• D. Area remains same

Explanation: As water falls, it accelerates under gravity increasing velocity; by continuity (A v = constant) the cross-sectional area of the stream…