Questions
6–10 questions in major PSU papers
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Fluid Mechanics and Hydraulics is a cornerstone of Civil and Mechanical engineering curricula, focusing on the behavior of fluids at rest and in motion. For PSU exams, this topic is critical as it features frequent numerical applications involving pipe networks, channel design, and turbomachinery performance. Mastering the core energy and momentum equations is essential for securing high marks.
Bernoulli's Theorem & Energy Principles
Bernoulli’s equation is the energy conservation principle for ideal fluids, stating that the sum of pressure, kinetic, and potential energy per unit weight is constant along a streamline. In PSU exams, you must account for head losses when applying this to real-world engineering problems.
- P/ρg + v²/2g + z = Constant
- Assumptions: Ideal, incompressible, steady, and irrotational flow
- Euler's equation is the precursor (neglecting gravity)
- Continuity equation: A1V1 = A2V2
Flow Through Pipes
This section deals with frictional losses and energy dissipation in closed conduits. The Darcy-Weisbach equation is the primary tool for calculating head loss, frequently tested in the context of minor vs. major losses.
- Darcy-Weisbach: hf = 4fLV²/2gD
- Chezy's formula: v = C√(mi)
- Major loss: Friction loss (hf)
- Minor losses: Entry, exit, bends, and expansion/contraction
- Flow in series vs. parallel pipes
Open Channel Flow
Open channel hydraulics concerns flow with a free surface, where gravity is the driving force. Manning’s formula is the industry standard for determining uniform flow velocity in channels.
- Manning's Equation: v = (1/n) * R^(2/3) * i^(1/2)
- Hydraulic Mean Depth (R) = Area / Wetted Perimeter
- Most economical section for rectangular channel: b = 2y
- Froude Number (Fr) dictates flow regime (Fr < 1 subcritical, Fr > 1 supercritical)
Hydraulic Turbines & Pumps
This area covers the energy conversion between fluid pressure/kinetic energy and mechanical work. Efficiency calculations and specific speed parameters are high-probability numerical targets for competitive exams.
- Specific Speed (Ns) = N√P / H^(5/4) for turbines
- Efficiency = Power output / Power input
- Impulse turbines (Pelton wheel) vs. Reaction turbines (Francis, Kaplan)
- Centrifugal pump cavitation occurs when pressure falls below vapor pressure
Formula Sheet
P/ρg + v²/2g + z = Constant
hf = 4fLV²/2gD
v = (1/n) * R^(2/3) * i^(1/2)
Ns = N√P / H^(5/4)
Fr = v / √(gD)
A1V1 = A2V2
P = ρgQH
R = A/P_wetted
Exam Tip
Always verify if the question asks for 'major head loss' only or 'total head loss' before finalizing your numerical calculation.
Common Mistakes
- Confusing the Darcy-Weisbach friction factor 'f' (where hf=4fLV²/2gD) with the Fanning friction factor.
- Neglecting units consistency, especially when calculating specific speed or using Manning's 'n'.
- Forgetting to include major losses alongside minor losses when calculating total head in a system.
More Revision Notes
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