# Aerodynamics Questions and Answers – Infinitesimal Fluid Element

This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Infinitesimal Fluid Element”.

1. What is a suitable flow model for a fluid?
a) Finite control volume approach
b) Infinitesimal fluid element approach
c) Molecular approach
d) All the three models can be used based on the need

Explanation: The finite control volume considers a finite volume which may be either fixed in space or moving along a fluid. The infinitesimal small element considers a finite element which may be either fixed or moving along with the flow. The molecular approach is a microscopic approach and can be applied to atoms and molecules.

2. In the equation, ∇.V=1/ v*D (v)/Dt, the term ∇.V is _________
a) change in velocity
b) divergence of velocity
c) volume
d) velocity as a function of time

Explanation: Divergence of velocity is physically the time rate of change of the volume of a moving fluid element per unit volume where v is small volume and because of this small volume, ∇.V is same throughout the volume. Moving control volume is used for the physical interpretation of the divergence of velocity.

3. Length, Scale and Volume fall under which of the following flow similarity?
a) Geometric Similarity
b) Kinematic Similarity
c) Dynamic Similarity
d) Dimensionless number

Explanation: Geometric similarity occurs when the model and prototype have same dimensional ratio in each direction. These ratios are called as scale ratio. For ex, Length scale ratio Lr = Lm/Lp where, m and p represent model and prototype.

4. Interia force falls under which flow similarity?
a) Geometric Similarity
b) Kinematic Similarity
c) Dynamic Similarity
d) Dimensionless number

Explanation: For a dynamic similarity, the model and prototype must be geometrically and kinematically similar along with the condition of total sum of forces acting at a point on model and prototype must be equal. Inertia force satisfies the condition and hence it falls under dynamic similarity.

5. Which of the following can attribute the flow of jet of oil in an unbroken stream?
a) Capillarity
b) Surface tension
c) Temperature
d) Density

Explanation: In a jet of oil, the molecules at the surface and the free layer differs because of which the oil resists the external force. The cohesive nature of the oil molecules helps to increase the surface area of the oil.

6. Which of the following is an example of Newtonian fluid?
a) Water
b) Plastic
c) Colloids
d) Gel

Explanation: The fluids which obey Newton’s law of viscosity are Newtonian fluids. It states that the applied shear stress is directly proportional to the amount of deformation.
τ = μdv/dy
Where, μ—–viscosity of the fluid
τ—-shear stress

7. In which of the following does the shock wave form?
a) subsonic flow
b) transonic flow
c) supersonic flow
d) incompressible flow

Explanation: Supersonic flows are characterized by the presence of shock waves across which the flow properties and streamlines change discontinuously.

8. Reynold’s number is the ratio of ____________
a) Inertia force to viscous force
b) Viscous force to inertia force
c) Inertia force to gravity force
d) Pressure force to inertia force

Explanation: Reynold’s number is given by Re = VL ρ / μ, which signifies the inertia to viscous force occurring in a flow. ρ / μ is called as kinematic viscosity of the fluid. Larger Reynold’s number, greater will be the inertial effects.

9. Centre of pressure is defined as the point on the body where the resultant forces acts.
a) True
b) False

Explanation: Pressure force acts on various parts of an aircraft. When these forces are resolved into a single component they act at a point and that a point is center of pressure. Also, it is a point where all the hydrostatic forces (forces due to fluids) act.

10. The aircraft fly in the atmosphere due to which of the following principle?
a) Archimedes principle
b) Bernoulli’s principle
c) Gravitational force
d) Newton’s law of viscosity

Explanation: Bernoulli’s principle states that the sum of all the forms of energy (potential energy, kinetic energy, internal energy) remains constant. Only the weight of the body and the pressure forces act on the body.

Sanfoundry Global Education & Learning Series – Aerodynamics.

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