Fluid Mechanics and Hydraulic Machines Notes for SSC JE Mechanical

Fluid mechanics studies the behaviour of liquids and gases at rest and in motion, and hydraulic machines are the pumps and turbines that use water. It is an important part of the Mechanical Engineering paper in SSC JE, and the questions are based on a few standard formulas and facts. These fluid mechanics and hydraulic machines notes for SSC JE explain the properties, the pressure, the flow equations and the machines in simple tables with practice questions.

Properties of Fluids

• The viscosity of a liquid falls as the temperature rises, but the viscosity of a gas rises with temperature.
• A Newtonian fluid follows Newton's law of viscosity, such as water and air.
• An ideal fluid has no viscosity and is incompressible.

Pressure and Its Measurement

Hydrostatic Force and Buoyancy

• Total force on a submerged plane surface: F = ρ g A h, where h is the depth of the centroid of the surface.
• The centre of pressure lies below the centroid.
• Archimedes' principle: the buoyant force on a body is equal to the weight of the fluid displaced.
• The point through which the buoyant force acts is the centre of buoyancy.
• A floating body is stable if its metacentre (M) lies above its centre of gravity (G).

Fluid Flow Equations

Flow Measuring Devices

Types of Flow

• Reynolds number Re = ρ V d / μ, the ratio of inertia force to viscous force.
• Head loss in pipes by the Darcy-Weisbach equation: h_f = f L V² / (2 g d).
• For laminar flow in a pipe, f = 64 / Re, and the maximum velocity is twice the average velocity.

Notches and Weirs

Turbines

Pumps

• A centrifugal pump uses the centrifugal force of a rotating impeller to lift water; it needs priming before the start.
• A reciprocating pump uses a piston and gives a high head with a small discharge.
• Cavitation is the formation and collapse of vapour bubbles when the pressure falls below the vapour pressure; it damages the pump and the turbine blades.
• NPSH (Net Positive Suction Head) is used to avoid cavitation in pumps.
• Hydraulic ram, hydraulic jack and hydraulic press work on Pascal's law.

Quick Tips

• Learn the Reynolds number limits 2000 and 4000.
• Remember that the metacentre must be above the centre of gravity for stability.
• Remember the turbines with head: Pelton (high), Francis (medium) and Kaplan (low).
• Always keep the units of Bernoulli's equation in the same system.

Quick Revision Points

• Pressure at depth h is ρ g h.
• Continuity equation: A₁V₁ = A₂V₂.
• Bernoulli's equation expresses the conservation of energy.
• Laminar flow has Re below 2000.
• Pelton wheel is an impulse turbine for high heads.

Practice Questions With Answers

1. What is the unit of dynamic viscosity?
   Answer: Pa s (N s/m²)
2. What is the relation between absolute and gauge pressure?
   Answer: Absolute = Gauge + Atmospheric
3. State the continuity equation.
   Answer: A₁V₁ = A₂V₂
4. For which Reynolds number is the pipe flow laminar?
   Answer: Less than 2000
5. Which device measures the velocity of flow at a point?
   Answer: Pitot tube
6. Which turbine is used for a high head?
   Answer: Pelton wheel
7. Which turbine is used for a low head and large discharge?
   Answer: Kaplan turbine
8. What is the condition of stability for a floating body?
   Answer: Metacentre above the centre of gravity
9. What is the friction factor for laminar flow in a pipe?
   Answer: 64 / Re
10. What is the phenomenon of the formation of vapour bubbles in a pump called?
    Answer: Cavitation

How to Practise This Chapter

Solve two Bernoulli and flow numericals and revise the turbine table daily, and keep a formula sheet. Then solve questions from our practice papers and the previous year papers page. Read the related chapters, Thermodynamics and Strength of Materials, and the next chapter, Production Engineering. See all topics on the SSC JE Mechanical Engineering notes page.

Frequently Asked Questions

What is fluid mechanics?

Fluid mechanics is the study of the behaviour of liquids and gases at rest and in motion.

What is viscosity?

Viscosity is the resistance of a fluid to flow. Dynamic viscosity is the force needed to move one layer over another, with the unit Pa s.

What is Bernoulli's equation?

It says that in a steady flow of an ideal fluid the sum of the pressure head, the velocity head and the potential head is constant along a streamline.

What is Pascal's law?

Pascal's law says that the pressure applied to a fluid at rest is transmitted equally in all directions.

What is Archimedes' principle?

It says that the buoyant force on a body in a fluid equals the weight of the fluid displaced by the body.

What is Reynolds number?

It is the ratio of the inertia force to the viscous force, and it tells whether the flow is laminar or turbulent.

What is a venturimeter?

A venturimeter is a device to measure the discharge in a pipe by using the pressure difference between the inlet and the throat.

What is the difference between impulse and reaction turbines?

In an impulse turbine, like the Pelton wheel, the water jet strikes the buckets at atmospheric pressure. In a reaction turbine, the water changes pressure in the runner, as in Francis and Kaplan.

What is cavitation?

Cavitation is the formation and sudden collapse of vapour bubbles in a flowing liquid when the pressure falls below the vapour pressure.

What is priming of a centrifugal pump?

Priming is filling the pump and the suction pipe with water before starting, so that there is no air in the pump.

Is fluid mechanics important for SSC JE Mechanical?

Yes. It is one of the main parts of the Mechanical paper and gives direct formula-based questions.

How many questions come from fluid mechanics in SSC JE?

Usually three to six questions come from fluid mechanics and machinery, but the number changes from year to year.