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Thermodynamics, hydraulics, machine design

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Topics in Mechanical Engineering

All Thermodynamics 100 Fluid Mechanics 79

Q.41 Easy Fluid Mechanics

A fluid undergoes viscous deformation with velocity gradient du/dy = 50 s⁻¹. For Newtonian fluid with μ = 0.1 Pa·s, the shear stress is:

A 5 Pa

B 500 Pa

C 5000 Pa

D 0.002 Pa

Correct Answer: A. 5 Pa

EXPLANATION

τ = μ(du/dy) = 0.1 × 50 = 5 Pa

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Q.42 Easy Fluid Mechanics

For steady flow of an incompressible fluid through a variable area duct, which equation relates velocities at different sections?

A Bernoulli equation

B Continuity equation (A₁v₁ = A₂v₂)

C Energy equation

D Momentum equation

Correct Answer: B. Continuity equation (A₁v₁ = A₂v₂)

EXPLANATION

Continuity equation for incompressible flow states Q = constant, so A₁v₁ = A₂v₂

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Q.43 Hard Fluid Mechanics

In a centrifugal pump, the impeller diameter is doubled while keeping speed constant. The head capacity changes by factor of:

A 2

B 4

C 8

D √2

Correct Answer: B. 4

EXPLANATION

Head H ∝ D²N² at constant N, so head increases by 2² = 4 times

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Q.44 Hard Fluid Mechanics

The equivalent length method for minor losses in pipes replaces a fitting with an equivalent length of pipe. For a 90° elbow with diameter 50 mm, typical equivalent length is:

A 0.5 m

B 1.0 m

C 2.0-3.0 m

D 5.0 m

Correct Answer: C. 2.0-3.0 m

EXPLANATION

Equivalent length L_e = K × (d/f) ≈ 0.9 × (0.05/0.02) ≈ 2.25 m for typical elbow

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Q.45 Medium Fluid Mechanics

For water flow in a pump system, cavitation occurs when:

A Pressure exceeds saturation pressure

B Pressure falls below vapor pressure

C Velocity becomes very high

D Temperature increases significantly

Correct Answer: B. Pressure falls below vapor pressure

EXPLANATION

Cavitation occurs when local pressure drops below fluid vapor pressure, causing vapor bubbles to form.

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Q.46 Medium Fluid Mechanics

The Mach number M = 0.3 indicates:

A Compressible flow effects are significant

B Incompressible flow assumption is valid

C Supersonic flow regime

D Shock waves will form

Correct Answer: B. Incompressible flow assumption is valid

EXPLANATION

M < 0.3 indicates incompressible flow approximation is valid; compressibility effects are negligible.

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Q.47 Hard Fluid Mechanics

For a submerged curved vane deflecting a jet, the component of force in the direction of jet is:

A ρQ²/A

B ρQv(1 - cos θ)

C ρQv cos θ

D ρQv sin θ

Correct Answer: B. ρQv(1 - cos θ)

EXPLANATION

Force component = ρQv(1 - cos θ) where θ is deflection angle from momentum equation.

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Q.48 Medium Fluid Mechanics

A pipeline carrying oil (ν = 2 × 10⁻⁴ m²/s) has diameter 0.5 m and velocity 2 m/s. The flow regime is:

A Laminar (Re < 2300)

B Transitional (2300 < Re < 4000)

C Turbulent (Re > 4000)

D Cannot be determined

Correct Answer: C. Turbulent (Re > 4000)

EXPLANATION

Re = vd/ν = (2 × 0.5)/(2 × 10⁻⁴) = 5000 > 4000, so turbulent

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Q.49 Hard Fluid Mechanics

In open channel flow, the critical depth occurs when:

A Froude number = 0

B Froude number = 1

C Reynolds number = 500

D Velocity = maximum

Correct Answer: B. Froude number = 1

EXPLANATION

Critical depth in open channel flow corresponds to Fr = 1, minimum specific energy condition.

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Q.50 Medium Fluid Mechanics

The Froude number characterizes the relative importance of:

A Inertial forces to viscous forces

B Inertial forces to gravitational forces

C Surface tension to inertial forces

D Pressure forces to elastic forces

Correct Answer: B. Inertial forces to gravitational forces

EXPLANATION

Fr = v/√(gL) compares inertial forces (ρv²) with gravitational forces (ρgL).

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