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# Fluid Mechanics MCQ – Viscosity – 2

Two horizontal plates placed 250mm have an oil of viscosity 20 poises. Calculate the shear stress in oil if upper plate is moved with velocity of 1250mm/s.

a) 20 N/m2

b) 2 N/m2

c) 10 N/m2

d) None of the mentioned

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Answer:c

Explanation: Shear Stress = Viscosity * Velocity Gradient

= 20/10* 1.25/0.25

= 10 N/m2.

The kinematic viscosity of oil of specific gravity .8 is .0005 .This oil is used for lubrication of shaft of diameter .4 m and rotates at 190 rpm. Calculate the power lost in the bearing for a sleeve length of 90mm. The thickness of the oil film is 1.5mm.

a) 477.65 Watts

b) 955.31 Watts

c) 238.83 Watts

d) None of the mentioned

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Answer: a

Explanation: Power lost= torque * angular velocity

= force* radius* angular velocity

= shear stress * area* radius* angular velocity

Shear Stress = viscosity* velocity gradient

Power lost= 0.0005*0.8*1000* 2*3.142*190/60*0.2*3.142*0.23 * 190/60

= 477.65 Watts.

Find the kinematic viscosity of oil having density 1962 g/m3. the force experienced for area of 20 m2 is 4.904 kN and velocity of gradient at that point is 0.2/s.

a) 0.625

b) 1.25

c) 2.5

d) None of the mentioned

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Answer: a

Explanation: kinematic viscosity = dynamic viscosity / density

= (shear stress*density)/velocity gradient

= (4904* 1962)/(20*0.2)

= .625.

The velocity distribution for fluid flow over a flat plate is given by u=2y-6y2 in which u is the velocity in metre per second at a distance of y metre above the plate. Determine the shear stress at y=0.15m.Take dynamic viscosity of fluid as 8.6 poise.

a) 0.172 N/m2

b) 0.344 N/m2

c) 0.086 N/m2

d) None of the mentioned

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Answer:a

Explanation: for y = 0.15m, velocity gradient = 0.2

viscosity= shear stress/velocity gradient

shear stress = 0.86*0.2 = 0.172N/m2.

In which types of fluids it is observed that momentum transfer dominates cohesive forces with increase in temperature and hence viscosity increases

a) Gases

b) Liquids

c) Solids

d) None of the mentioned

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Answer:a

Explanation: It is the characteristic property of gases which show increase in viscosity with increase in temperature.

What is the characteristic variation shown by the thixotropic fluids in their shear stress vs. rate of shear strain graph?

a) shear stress increases with increase in rate of shear strain

b) shear stress decreases with increase in rate of shear strain

c) shear stress shows variation only after a definite shear stress is reached

d) shear stress has decreasing constant and then variation relationship with rate of shear strain

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Answer: c

Explanation: Thixotropic fluid show a Non-Newtonian variation for shear stress vs. rate of shear strain graph after a characteristic limiting value of shear stress is reached.

What happens to viscosity in the case of incompressible fluids as temperature is increased?

a) It remains constant

b) It increases

c) It decreases

d) None of the mentioned

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Answer: c

Explanation: In case of incompressible fluids, cohesive forces govern the viscosity. As temperature increases the cohesive forces between fluid molecules decreases due to increase in molecular agitation. Hence, as a result, viscosity decreases.

If a fluid, which has a constant specific gravity, is taken to a planet where acceleration due to gravity is 3 times compared to its value on earth, what will happen to its kinematic viscosity.

a) It increases

b) It decreases

c) It remains constant

d) None of the above

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Answer: c

Explanation: Kinematic viscosity depends on density and dynamic viscosity. Both, density and dynamic viscosity, are independent of acceleration due to gravity. Therefore, kinematic viscosity is independent of acceleration due to gravity.

In liquids in order to measure the viscosity of fluid experimentally we consider the variation of shear stress with respect to what property?

a) strain

b) shear strain

c) rate of shear strain

d) none of the mentioned

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Answer: c

Explanation: By definition, viscosity is shear stress per unit ‘rate of shear strain’.

For a compressible fluid the kinematic viscosity is affected by temperature and pressure variation.

a) True

b) False

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Answer: a

Explanation: Viscosity shows variation for change in temperature and pressure for compressible fluids. Hence, kinematic viscosity is affected by temperature and pressure variation.