Fluid Mechanics And Machinery Set 2

Mechanical Engineering

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This set of Fluid Mechanics and Machinery Multiple Choice Questions & Answers (MCQs) focuses on Fluid Mechanics And Machinery Set 2

Q1 | A circular pipe of radius 7 cm is used for water flow transmission. This pipe is moulded into another pipe with a square cross-section keeping the length same. (Ignore the thickness of the pipe). Calculate the hydraulic diameter of the moulded pipe. (Take π = 22/7).
  • 11 cm
  • 7 cm
  • 3.5 cm
  • 22 cm
Answer: 11 cm
Explaination:
the perimeter of the circular cross section and the square cross section will remain the same. perimeter = 44 cm. side of square = 11 cm. hydraulic diameter dh of
Q2 | The Reynolds number is found out for a flow in a circular pipe. This circular pipe is moulded into a square pipe, keeping length of the pipe same. Ignore the thickness of the pipe. The Reynolds number changes by
  • 57% decrease
  • 57% increase
  • 43% decrease
  • 43% increase
Answer: 57% increase
Explaination:
the reynolds number directly depends upon the hydraulic diameter of the pipe. suppose the diameter of the pipe is d, the hydraulic diameter of square pipe is 1.57d. hence, 57% increase.
Q3 | The flow through a circular pipe is laminar. Now, the fluid through the pipe is replaced with a more viscous fluid and passed through the pipe again with the same velocity. What can we say about the nature of this flow?
  • the flow will become turbulent
  • the flow will be a transition flow
  • the flow will remain laminar
  • the reynolds number of the earlier flow is required to answer this question
Answer: the flow will remain laminar
Explaination:
a flow through a circular pipe is said to be laminar when the reynolds
Q4 | What can be the maximum diameter of the pipe for the water flow of velocity 1 m/s (ν = 10-6) to be laminar in nature? Assume Lower critical Reynolds number to be 2100.
  • 2.1 mm
  • 21 mm
  • 21 cm
  • 0.21 mm
Answer: 2.1 mm
Explaination:
if the reynolds number of the flow is below its lower critical reynolds number, the flow is clearly laminar. the maximum diameter can be found for re = 2100. the diameter comes out to be 2.1 mm.
Q5 | Which of the following flows have the highest critical Reynolds number (lower)?
  • flow in a pipe
  • flow between parallel plates
  • flow in an open channel
  • flow around spherical body
Answer: flow in a pipe
Explaination:
the approximate lower critical reynolds number for flow in a pipe, flow between parallel plates, flow in an open channel and flow around the spherical body are 2000, 1000, 500 and 1 respectively.
Q6 | The flow separation occurs when the fluid travels away from the  
  • surface
  • fluid body
  • adverse pressure gradient
  • inter-molecular spaces
Answer: adverse pressure gradient
Explaination:
adverse pressure gradient takes place when the static pressure increases. it increases the direction of the flow. adverse pressure gradient plays an important role in flow separation. thus, option c is correct.
Q7 | The swirl caused due to eddies are called as
  • vortices
  • vertices
  • volume
  • velocity
Answer: vortices
Explaination:
vortices are a region in a fluid. it takes place when the flow revolves around an axis line. vortices can be straight or curved. they form shapes like smoke rings and whirlpools.
Q8 | Eddy viscosity is a turbulent transfer of 
  • fluid
  • heat
  • momentum
  • pressure
Answer: momentum
Explaination:
eddy viscosity is a turbulent transfer of momentum by eddies. it gives rise to an internal fluid friction. it is in analogous to the action of molecular viscosity in laminar fluid flow. eddy viscosity takes place on a large scale.
Q9 | Which among the following is a device that converts a laminar flow into a turbulent flow?
  • dead weight gauge
  • vacuum gauge
  • turbulator
  • ionization gauge
Answer: turbulator
Explaination:
turbulator is a device that converts a laminar flow into a turbulent flow. the turbulent flow can be desired parts of an aircraft or also in industrial applications.
Q10 | Boundary layer separation does not undergo detachment.
  • true
  • false
Answer: false
Explaination:
boundary layer separation undergoes detachment from the surface into a broader wake. it occurs mainly when the portion of the boundary layer is closest to the wall. it leads to reverse in the flow direction.
Q11 | With the boundary layer separation, displacement thickness 
  • increases
  • decreases
  • remains same
  • independent
Answer: increases
Explaination:
with the boundary layer separation, displacement thickness increases sharply. this helps to modify the outside potential flow and its pressure field. thus, option ‘a’ is the correct choice.
Q12 | automatic control scheme during the fluid flow?
  • rotameters
  • pulley plates
  • rotary piston
  • pilot static tube
Answer: pilot static tube
Explaination:
pilot static tube is a system that uses an automatic control scheme to detect pressure. it has several holes connected to one side of the device. these outside holes are called as a pressure transducer, which
Q13 | What is D’Alembert’s Paradox?
  • resistance= 0
  • drag force= 0
  • temperature = 0
  • pressure gradient= 0
Answer: drag force= 0
Explaination:
d’alembert’s paradox states that for an incompressible and inviscid flow potential flow, the drag force is equal to zero. the fluid is moving at a constant velocity with respect to its relative fluid.
Q14 | The steady- state flow must satisfy
  • kirchhoff’s law
  • newtons law
  • rutherford’s experiment
  • kepler’s law
Answer: kirchhoff’s law
Explaination:
the steady state flow must satisfy kirchhoff’s first and second law. the first law states that the total flow into the junction equals the total flow away from the junction. second law is called as the law of conservation of mass. it states that between two junctions, the head loss is independent of the path followed.
Q15 | depend on the friction factor?
  • pipe diameter
  • fluid density
  • viscosity
  • weight
Answer: weight
Explaination:
the friction factor(f) depends on the velocity of flow, fluid density, pipe diameter and the viscosity of the pipe.
Q16 | How do we calculate losses for a larger range of Reynolds number?
  • moody chart
  • bar chart
  • scatter chart
  • column histogram
Answer: moody chart
Explaination:
moody chart is a graph of frictional factor(f) vs reynolds numbers. it gives various values corresponding to the
Q17 | Darcy- Weisbach equation gives relation between 
  • pressure and temperature
  • mass, volume and pressure
  • head loss and pressure loss
  • pressure loss only
Answer: head loss and pressure loss
Explaination:
darcy-weisbach equation relates the head loss and pressure loss due to friction along a given pipe with a specified length. it contains a dimensionless friction factor called the darcy friction factor. the equation was named after henry darcy and julius weisbach.
Q18 | Which among the following is formula for friction factor of circular pipes?
  • 16/re
  • 64/re
  • re/16
  • re/64
Answer: 64/re
Explaination:
circular pipes have a diameter treated in a round manner. for a fluid flow which is laminar head loss is directly proportional to the fluid velocity. thus, friction factor is inversely proportional to its velocity. therefore, the correct option is ‘64/re’.
Q19 | Loss of head due to friction is  
  • directly proportional to hydraulic radius
  • inversely proportional to velocity
  • inversely proportional to hydraulic radius
  • directly proportional to gravitational constant
Answer: inversely proportional to hydraulic radius
Explaination:
hydraulic radius is one of the properties of a fluid flow in a channel. it controls the water discharge. it also determines the amount of work the channel
Q20 | The formula for hydraulic diameter is 
  • 4a/p
  • 4ap
  • 4av
  • 4v
Answer: 4a/p
Explaination:
hydraulic diameter handles the flow in non-circular channels and tubes. the most suitable term to calculate the hydraulic diameter for a round tube is dh= 4a/p. where ‘a’ is the cross-sectional area and ‘p’ is the wetted perimeter.
Q21 | What are the reasons for minor head loses in a pipe?
  • friction
  • heat
  • valves and bends
  • temperature
Answer: valves and bends
Explaination:
minor losses play an important role in calculating the flow, pressure and energy of the piping system. fluid that moves through the pipe carries momentum and energy due to the forces acting on them.
Q22 | What happens to the head loss when the flow rate is doubled?
  • doubles
  • same
  • triples
  • four times
Answer: four times
Explaination:
if the flow rate is doubled, the head loss increases by a factor of four. since, the head loss is directly proportional to the square of the flow rate. option (d) is the correct option.
Q23 | Relative roughness is 
  • ϵ/d
  • ϵ*d
  • ϵ/dm
  • ϵgd
Answer: ϵ/d
Explaination:
relative roughness is defined as the quantity used to measure the roughness
Q24 | pipes can take up 
  • pipes of different diameters
  • pipes of the same diameters only.
  • single pipe only
  • short pipes only
Answer: pipes of different diameters
Explaination:
when pipes of different diameters are connected at its ends to form a pipe, this pipe so developed is called as pipes in series. they might not have to be of the same diameters. but, having the same diameters are better as it avoids the losses so developed.
Q25 | What is the total loss developed in a series of pipes?
  • sum of losses in each pipe only
  • sum of local losses only
  • sum of local losses plus the losses in each pipe
  • zero
Answer: sum of local losses plus the losses in each pipe
Explaination:
when the pipes of different diameters are connected in series from end to end to form a pipe line. the total loss so developed is equal to the sum of local losses
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