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Civil EngineeringWBPSC AE · SSC JE · RRB JE

Fluid Mechanics— Important Topics, Formulas & MCQs

Fluid Mechanics & Hydraulics deals with fluids at rest and in motion — pressure, continuity, energy, momentum, pipe and open-channel flow. It is a heavily numerical subject with strong conceptual links to Hydrology and Irrigation.

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Important topics in Fluid Mechanics

Fluid properties — viscosity, surface tension, compressibility
Hydrostatics — pressure, manometry, forces on surfaces
Continuity, Bernoulli & momentum equations
Flow measurement — venturi, orifice, notches
Laminar & turbulent flow, Reynolds number
Flow through pipes — major & minor losses
Boundary layer theory
Open channel flow — uniform, critical, GVF
Dimensional analysis & model studies
Hydraulic machines — turbines & pumps
Must-know

Key concepts & formulas — Fluid Mechanics

Bernoulli's equation

p/γ + V²/2g + z = constant — conservation of energy along a streamline (ideal fluid).

Reynolds number

Re = ρVD/μ = inertia/viscous force. Pipe flow: laminar Re < 2000, turbulent > 4000.

Continuity equation

A₁V₁ = A₂V₂ — based on conservation of mass.

Critical flow

For given discharge, critical depth depends only on channel geometry (Q²/g = A³/T), not slope or roughness.

Exam relevance

6–8 questions in AE/JE papers; the conceptual base for Hydrology and Irrigation questions too.

Study tip

Bernoulli and continuity solve a surprising share of MCQs — set them up carefully with correct datum and units.

Worked example

Sample Fluid Mechanics MCQ

Water flows at 2 m/s through a pipe of diameter 0.1 m. If the diameter reduces to 0.05 m, the velocity becomes:

A. 1 m/s
B. 4 m/s
C. 8 m/s
D. 2 m/s
Answer: C. Continuity A₁V₁ = A₂V₂; area ∝ d². (0.1/0.05)² = 4, so V₂ = 2 × 4 = 8 m/s.

Common mistakes to avoid

Reynolds number is the ratio of inertia to viscous force (not gravity).
Critical depth depends only on channel geometry and discharge — not on slope or roughness.
Continuity is conservation of mass; Bernoulli is conservation of energy.
In continuity, area scales with d² — halving the diameter quarters the area.
Keep going

Related Civil Engineering subjects

Strength of Materials
RCC
Steel Structures
Soil Mechanics
Foundation Engineering
Surveying
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