Fluid mechanics Basic | Interview Question and Answers
Fluid mechanics Basic | Interview, Viva , oral Question and Answers
1. Define density or mass density.
Density of a fluid is defined as the ratio of the mass of a fluid to its volume.
Density, ρ = mass/volume (Kg/m3 )
ρwater = 1000 Kg/m3
2. Define specific weight or weight density.
Specific weight or weight density of a fluid is defined as the ratio between the weight of a fluid to its volume.
Specific weight, γ = weight/volume (N/m3 )
γ = ρg
γwater = 9810 N/m3
3. Define specific volume.
Specific volume of a fluid is defined as the volume of fluid occupied by an unit wt or unit mass of a fluid.
Specific volume vs = volume/ wt = 1/γ = 1/ρg —– for liquids
Specific volume vs = volume/ mass = 1/ρ —– for gases
4. Define dynamic viscosity.
Viscosity is defined as the property of fluid which offers resistance to the movement of one layer of fluid over another adjacent layer of the fluid.
1 N-s/m2 = 1 Pa-s = 10 Poise
5. Define Kinematic viscosity.
It is defined as the ratio between the dynamic viscosity and density of fluid.
ν = μ/ρ (m2 /s)
1 m2/s = 10000 Stokes (or) 1 stoke = 10-4 m2/s
6. Types of fluids.
Ideal fluid, Real fluid, Newtonian fluid, Non-Newtonian fluid, Ideal Plastic fluid.
7. Define Compressibility.
It is defined as the ratio of volumetric strain to compressive stress.
8. Define Surface Tension.
Surface tension is defined as the tensile force acting on the surface of the liquid in contact with a gas or on the surface between two immiscible liquids such that the contact surface behaves like a membrane under tension.
Surface Tension, σ = Force/Length (N/m)
9. Define Capillarity.
Capillarity is defined as a phenomenon of rise or fall of a liquid surface in a small tube relative to the adjacent general level of liquid when the tube is held vertically in the liquid. The rise of liquid surface is known as capillary rise while the fall of liquid surface is known as capillary depression.
10. Define Vapour Pressure.
When vaporization takes place, the molecules start accumulating over the free liquid surface exerting pressure on the liquid surface. This pressure is known as Vapour pressure of the liquid.
11. Define Control Volume.
A control volume may be defined as an identified volume fixed in space. The boundaries around the control volume are referred to as control surfaces. An open system is also referred to as a control volume.
12. List the types of fluid flow.
- Steady and unsteady flow
- Uniform and non-uniform flow
- Laminar and Turbulent flow
- Compressible and incompressible flow
- Rotational and ir-rotational flow
- One, Two and Three dimensional flow
15. Define Steady and Unsteady flow.
Fluid flow is said to be steady if at any point in the flowing fluid various characteristics such as velocity, density, pressure,etc do not change with time.
∂V/∂t = 0 ∂p/∂t = 0 ∂ρ/∂t = 0
Fluid flow is said to be unsteady if at any point flowing fluid any one or all characteristics which describe the behaviour of the fluid in motion change with time.
∂V/∂t ≠ 0 ∂p/∂t ≠ 0 ∂ρ/∂t ≠ 0
16. Define Uniform and Non-uniform flow.
When the velocity of flow of fluid does not change both in direction and magnitude from point to point in the flowing fluid for any given instant of time, the flow is said to be uniform.
∂V/∂s = 0 ∂p/∂s = 0 ∂ρ/∂s = 0
If the velocity of flow of fluid changes from point to point in the flowing fluid at any instant, the flow is said to be non-uniform flow.
∂V/∂s ≠ 0 ∂p/∂s ≠ 0 ∂ρ/∂s ≠ 0
17. Compare Laminar and Turbulent flow.
Laminar and Turbulent flow
A flow is said to be laminar if Reynolds number is less than 2000 for pipe flow. Laminar flow is possible only at low velocities and high viscous fluids. In laminar type of flow, fluid particles move in laminas or layers gliding smoothly over the adjacent layer.
In Turbulent flow, the flow is possible at both velocities and low viscous fluid. The flow is said to be turbulent if Reynolds number is greater than 4000 for pipe flow. In Turbulent type of flow fluid, particles move in a zig – zag manner.
18. Define Compressible and incompressible flow
The compressible flow is that type of flow in which the density of the fluid changes from point to point i.e. the density is not constant for the fluid. It is expressed in kg/sec.
ρ ≠ constant
The incompressible flow is that type of flow in which the density is constant for the fluid flow. Liquids are generally incompressible. It is expressed in m3 /s.
ρ = constant
19. Define Rotational and Ir-rotational flow.
Rotational flow is that type of flow in which the fluid particles while flowing along stream lines and also rotate about their own axis.
If the fluid particles are flowing along stream lines and do not rotate about their own axis that type of flow is called as ir-rotational flow.
22. State the assumptions used in deriving Bernoulli’s equation
- Flow is steady;
- Flow is laminar;
- Flow is irrotational;
- Flow is incompressible;
- Fluid is ideal.
24. List the instruments works on the basis of Bernoulli’s equation.
- Venturi meter;
- Orifice meter;
- Pitot tube.
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