Below are some fundamental questions with answers in Fluid Mechanics. These will be useful for viva, written exams and interviews
1. Define fluids.
Fluid is a substance which can flow. It has no fixed shape or size, but it acquires the shape of the container it is kept in.
2. What are the properties of ideal/perfect fluid?
An ideal fluid has the following properties
i) incompressible (can't be compressed)
ii) viscosity is zero
iii) Shear force is zero
3. What are the properties of real fluid?
A real fluid has the following properties
i) compressible
ii) viscous in nature
iii) Shear force exists
4. Do ideal fluid exist in reality? or, Why ideal fluid is generally used in calculation/ analysis?
Ideal fluids don't actually exist, but in order to simplify the fluid flow problem, it is often considered ideal.
5. Explain Density
Density or mass density is defined because the ratio of the mass of the fluid to its volume. Thus mass per unit volume of a fluid is named density. It is denoted by the symbol (ρ) (pronunciation: row).
Density(ρ) = Mass of the fluid (kg) / Volume of the fluid (m3)
6. Explain Specific weight or weight density
Specific weight or weight density of a fluid is that the ratio between the load of a fluid to its volume. So, weight density is weight per unit volume of a fluid and is denoted by the symbol (W).
(W) = Weight of the fluid / Volume of fluid
= (Mass x Acceleration due to gravity) / Volume of fluid
W = pg
7. Explain the Specific volume
Specific volume of a fluid is defined as the volume of the fluid occupied by a unit Mass.
So, volume per unit mass of a fluid is known as specific volume.
Specific volume = Volume / Mass = m3 /kg = l/p
It is inverse of density.
8. Explain the Specific gravity
Specific gravity is defined as the ratio of weight density of a fluid to the weight density of a standard fluid. For liquid, standard fluid is water and for gases, it is air.
Specific gravity = Weight density of any liquid or gas / Weight density of standard liquid or gas
It is actually the comparison of the weight density of any fluid w.r.t a standard fluid.
9.What is viscosity?
It is defined as the property of a liquid due to which it offers resistance to the movement of one layer of liquid over another adjacent layer.
It actually describes the internal friction between different layers of a moving fluid.
So, higher the viscosity , more is the resistance to motion because of higher internal friction.
Ex: caster oil is more viscous than water, but both are fluids.
10. What is kinematic viscosity?
It is defined as the ratio between dynamic viscosity and mass density. (m²/sec)
11. Define Relative or Specific viscosity.
It is the ratio of dynamic viscosity of fluid to dynamic viscosity of water at 20°C.
It's actually a comparative value of viscosity w.r.t. a standard fluid, e.g., water. But as viscosity is highly sensitive to temperature change, 20 deg Celsius is taken as ideal.
12. State Newton's law of viscosity and explain with examples.
Newton's law states that the shear stress on a fluid element layer is directly proportional to the rate of shear strain. The constant of proportionality is called co-efficient of viscosity.
r = μ du / dy
13. What is the importance of viscosity on fluid motion and its effect on temperature.
Viscosity is the property of a fluid which offers resistance to the movement of one layer of fluid over another layer adjacent to it. The viscosity is an important property which affects the fluid motion.
The viscosity of liquid decreases with increase in temperature and for gas it increases with increase in temperature.
14. Explain the Newtonian fluid
The fluid which obeys the Newton's law of viscosity is called Newtonian fluid.
i.e., the shear stress is directly proportional to the rate of shear strain, r = μ du / dy
15. Explain the Non-Newtonian fluid
The fluids which does not obey the Newton's law of viscosity is called non-Newtonian fluid.
i.e., the shear stress is not directly proportional to the ratio of shear strain,
16. Define compressibility.
Compressibility is the reciprocal of bulk modulus of elasticity, "k"
modulus of elasticity, "k" is defined as the ratio of compressive stress to volume strain.
k = Increase of pressure / Volume strain
Compressibility = 1 / k = Volume of strain / Increase of pressure
17. Define surface tension.
Surface tension is defined as the tensile force acting on the surface of a liquid in contact with a gas or on the surface between two immiscible liquids in such a way that, the contact surface behaves like a stretched membrane.
18. Define Capillarity.
Capillary is the property of a fluid responsible for the rise or fall of liquid surface relative to the adjacent general level of liquid.
19. What is cohesion and adhesion in fluids?
Cohesion occurs due to the force of attraction between the molecules of the same liquid. Adhesion occurs due to the force of attraction between the molecules of two different liquids or between the molecules of the liquid and molecules of the solid boundary surface.
20. State momentum of momentum equation?
It states that the resulting torque acting on a rotating fluid is equal to the rate of change of moment of momentum.
21. What is momentum equation
It is based on the law of conservation of momentum or in other words the momentum principle
It states that, the net force acting on a fluid mass is equal to the change in momentum of flow per unit time in the same direction.
22. What is Euler's equation of motion
Euler's equation of motion considers, forces due to gravity and pressure.
This is derived by considering the motion of a fluid element along a stream line.
23. What is venturimeter?
A Venturimeter is a device for measuring the rate of fluid flow through a pipe. It consists of three parts.
a. A short convergent tube
b. Throat
c. Long Divergent tube.
It is based on the principle of Bernoalli's equation.
24. What is an orifice meter?
Orifice meter is the device used for measuring the rate of flow of a fluid through a pipe. it is cheaper as compared to the venturimeter. It also works on the same priniciple as that of venturimeter. It consists of a flat circular plate which has a circular sharp edged hole called the orifice.
25. What is a pitot tube?
The Pitot tube is a device for measuring the velocity of flow at any point in a pipe or a channel. It is based on the principle that if the velocity of flow at a point becomes zero, the pressure there is increased due to the conversion of kinetic energy into pressure energy.
26.What are the types of fluid flow?
Steady & unsteady fluid flow
Uniform & Non-uniform flow
One dimensional, two-dimensional & three-dimensional flows
Rotational & Irrotational flow
27. State the application of Bernouillie’s equation ?
It has the application on the following measuring devices.
1.Orifice meter.
2.Venturimeter.
3.Pitot tube.
28. What are properties of Fluids?
The properties of fluid are:
Density or Mass Density
Weight Density or Specific Weight
Viscosity
Specific Volume and
Specific Gravity
Types of Fluids:
29. What are different types of fluids?
There are five different types of fluid has been classified:
i) Ideal fluid
ii) Real fluid
iii) Newtonian fluid
iv) Non-Newtonian fluid, and
v) Ideal plastic fluid
30. What are the different methods for fluid study?
There are two approach to study fluid.
i) Lagrangian Approach: In this approach, a single fluid particle is taken and its behavior is analyzed at different sections (with respect to space).
ii) Eulerian Approach: In this approach, a certain section is taken in space and the behavior of fluid particles passing through this section is analyzed at a different instant. We mainly use this approach in Fluid Mechanics [With respect to Time].
31. What are the different types of fluid flow?
There are Six different types of fluid flow:
Steady and Unsteady
Uniform and Non-Uniform
Laminar and Turbulent
Compressible and In-compressible
Rotational and Ir-rotational and
One, Two, and Three -dimensional Fluid Flow
32.
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