# Food Processing Unit Operations Questions and Answers – Fluid Flow Theory – Viscosity

This set of Food Processing Unit Operations Multiple Choice Questions & Answers (MCQs) focuses on “Fluid Flow Theory – Viscosity”.

1. Viscous force is of the same character as _______ forces in solids.
a) shear
b) impact
c) gravitational
d) electromagnetic

Explanation: Viscous force is the force between object and the fluid moving, in a direction to oppose the fluid flow past the object. It is similar to the shear stress forced upon a solid and they arise from forces which exist between the molecules.

2. The ratio between shear stress and shear rate in a fluid is called __________
a) bulk modulus
b) viscosity
c) specific density
d) young’s modulus

Explanation: The ratio between shear stress and shear rate in a fluid is called viscosity. Bulk modulus can be defined as the relative change in volume of the object produced by tensile stress acting homogenously over its surface. Specific density is the ratio of density of the substance to density of a given reference material. Young’s modulus is defined as the ratio of tensile stress to tensile strain of a given material.

3. Viscosity is based on _________
a) law of conservation of mass only
b) law of conservation of energy only
c) Newton’s law
d) third law of thermodynamics only

Explanation: Viscosity is based on Newton’s law of viscosity, which states that the shear stress between adjacent fluid layers is proportional to the velocity gradients between the two layers.

4. Are these statements about the pseudoplastic fluids true?
Statement 1:The viscosity of these fluids decreases as the shear rate decreases.
Statement 2: Tomato puree is an example of pseudoplastic fluid.
a) True, False
b) True, True
c) False, True
d) False, False

Explanation: The viscosity of these fluids decreases as the shear rate increases. Blood, tomato puree are the examples of the pseudo plastic fluids. Another example is that, suspension of the corn starch in water with 1:1 concentration. Then this corn starch behaves like water when no force is applied. Still it is solidified as force is applied.

5. Shear thinning is also called as __________
a) thixotropy
b) pseudoplastic
c) non-newton fluid
d) relative shear stress

Explanation: Shear thinning is also called asthixotropy. It is a non-Newtonian fluid, where there is a decrease in viscosity under low shear strain and there is no flow at all until a yield stress is reached after which the flow occurs excluding time-dependent effects. Ketchup is an example of shear thinning.

6. The viscosity of gases ____________as temperature increases.
a) increases
b) decreases
c) remains same
d) fluctuation

Explanation: As the temperature rises, the viscosity of gases also increases. This is due to increase in the frequency of the intermolecular collisions. The relationship between viscosity and temperature is velocity is directly proportional to the square root of the temperature.

7. The viscosity of liquid ____________as temperature increases.
a) increases
b) decreases
c) remains same
d) fluctuation

Explanation: The viscosity of liquid decreases as temperature increases. As temperature rises, there is an increase in the average speed of the molecules in the liquid and the contact time in between the molecules is also decreased.

8. What are examples of the rheopexy fluids?
a) Gypsum pastes, printer inks
b) Gypsum pastes, ketchup
c) Printer inks, ketchup
d) Only ketchup

Explanation: Rheopexy is an exceptional property that is seen in some of the non-Newtonian fluids. In these fluids increase in the viscosity depends on the time. When a fluid has a longer shear force, the viscosity is also higher. Some of the examples are Gypsum pastes, printer inks and lubricants.

9. Laminar flow and turbulent flow depends on __________
a) Reynolds number
b) Prandtl number
c) Nusselt number
d) Archimedes number

Explanation: The Reynold number (R) is a dimensionless quantity that is used to determine whether the fluid is following laminar flow (R<2300), transition flow (23004000).

10. What type of fluid is alcohol?
a) Thixotropy
b) Rheopexy fluids
c) Non- Newtonian fluid
d) Newtonian fluid

Explanation: Newtonian fluid depends on the Newton’s law of viscosity. The law states that shear stress between the neighboring fluid layers is directly proportional to velocity gradients between the two layers. Alcohol, water, air are some of the examples of Newtonian fluid.

Sanfoundry Global Education & Learning Series – Food Processing Unit Operations.

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