Interview Questions On Fluid Mechanics
1. How Cavitation can be eliminated in a Pump? | Interview Questions on Fluid Mechanics
Cavitation means bubbles are forming in the liquid.
· To avoid Cavitation, we have to increase the Pump size to One or Two Inch;
· To increase the pressure of the Suction Head, or
· Decrease the Pump Speed.
2. Which pump is more efficient Centrifugal pump or Reciprocating pump?
Because flow rate is higher compared to reciprocating pump. Flow is smooth and it requires less space to install. Lower initial cost and lower maintenance cost.
3. Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?
The centrifugal has varying flow depending on pressure or head, whereas the Positive Displacement pump has more or less constant flow regardless of pressure.
Likewise viscosity is constant for positive displacement pump where centrifugal pump have up and down value because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency. When there is a viscosity change in supply there is also greater loss in the system. This means change in pump flow affected by the pressure change.
One more example is, positive displacement pump has more or less constant efficiency, where centrifugal pump has varying efficiency rate.
4. Why the centrifugal pump is called high discharge pump? | Interview Questions on Fluid Mechanics
Centrifugal pump is a kinetic device. The centrifugal pump uses the centrifugal force to push out the fluid. So the liquid entering the pump receives kinetic energy from the rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity, transferring mechanical (rotational) energy to the liquid. So it discharges the liquid in high rate. It is given in the following formulae:
Centrifugal force F= (M*V2)/R.
5. Why cavitation will occur in Centrifugal Pumps and not in Displacement pumps?
The formation of cavities (or bubbles) is induced by flow separation, or non-uniform flow velocities, inside a pump casing. In centrifugal pumps the eye of the pump impeller is smaller than the flow area of pipe. This decrease in flow area of pump results in increase in flow rate. So pressure drop happened between pump suction and the vanes of the impeller. Here air bubbles or cavities are formed because of liquid vapour due to increase in temperature in impeller. This air bubbles are transmitted to pump which forms cavitation.
6. Surface tension is due to | Interview Questions on Fluid Mechanics
- Viscous forces
- The difference between adhesive and cohesive forces
Surface tension is due to cohesion between liquid particles at the surface, where as capillarity is due to both cohesion and adhesion. The property of cohesion enables a liquid to resist tensile stress, while adhesion enables it to stick to another body.
7. Which property of mercury is the main reason for use in barometers? | Interview Questions on Fluid Mechanics
- High density
- Negligible capillary effect
- Very low vapour pressure
- Low compressibility
Very Low Vapour Pressure
8. When can a piezometer be not used for pressure measurement in pipes? | Interview Questions on Fluid Mechanics
- The pressure difference is low
- The velocity is high
- The fluid in the pipe is a gas
- The fluid in the pipe is highly viscous
The fluid in the pipe is a gas
9. The reading of the pressure gauges fitted on a vessel is 25 bar. The atmospheric pressure is 1.03 bar and the value of g is 9.81 m/s2. The absolute pressure in the vessel is
- 23.97 bar
- 25.00 bar
- 26.03 bar
- 34.84 bar
Absolute pressure = Atmospheric pressure + Gauge pressure
= 25 + 1.03 = 26.03 bar
10. Resultant pressure of the liquid in case of an immersed body acts through which one of the following? | Interview Questions on Fluid Mechanics
- Centre of gravity
- Centre of pressure
- Centre of buoyancy
Centre of pressure
11. Bodies in floatation to be in stable equilibrium, the necessary and sufficient condition is that the centre of gravity is located below the
- Centre of pressure
- Centre of gravity
- Centre of buoyancy
12. What is buoyant force? | Interview Questions on Fluid Mechanics
- Lateral surface acting on a submerged body
- Resultant force acting on a submerged body
- Resultant hydrostatic force on a body due to fluid surrounding it
- None of the above
Resultant hydrostatic force on a body due to fluid surrounding it.
When a body is either wholly or partially immersed in a fluid, a lift is generated due to the net vertical component of hydrostatic pressure forces experienced by the body. This lift is called the buoyant force and the phenomenon is called buoyancy.
13. For floating bodies, how is the metacentric radius defined? | Interview Questions on Fluid Mechanics
- The distance between centre of gravity and the metacentre
- Second moment of area of plane of flotation about centroidal axis perpendicular to plane of rotation / immersed volume
- The distance between centre of gravity and the centre of buoyancy
- Moment of inertia of the body about its axis of rotation / immersed volume
The distance between centre of gravity and the metacentre
Metacentric radius or Metacentric height is the distance between centre of gravity and the metacentre
14. The metacentric height of a passenger ship is kept lower than that of a naval or a cargo ship because | Interview Questions on Fluid Mechanics
- Apparent weight will increase
- Otherwise it will be in neutral equilibrium
- It will decrease the frequency of rolling
- Otherwise it will sink and be totally immersed
It will decrease the frequency of rolling
15. A bucket of water hangs with a spring balance. If an iron piece is suspended into water from another support without touching the sides of the bucket, the spring balance will show
- An increased reading
- A decreased reading
- No change in reading
- Increased or decreased reading depending on the depth of immersion
No change in reading
16. What are the forces that influence the problem of fluid static? | Interview Questions on Fluid Mechanics
- Gravity and viscous forces
- Gravity and pressure forces
- Viscous and surface tension forces
- Gravity and surface tension forces
Gravity and pressure forces influence the problem of fluid statics
17. The Euler’s equation of motion is a statement of
- Energy balance
- Conservation of momentum for an inviscid fluid
- Conservation of momentum for an incompressible flow
- Conservation of momentum for real fluid
Conservation of momentum for an inviscid fluid
18. A tank containing water has two orifices of the same size at depths of 40 cm and 90 cm below the free surface of water. The ratio of discharge through these orifices is | Interview Questions on Fluid Mechanics
The ratio of discharges will be proportional to jet velocities, and will be proportional to their square root of water heads at the same size orifices. As a result, Q1 / Q2 = A1V1 / A2V2 ∝ √h1/h2
Q1/Q2 = √40/90 = 2/3.