Aerodynamics Questions and Answers – Continuity Equation

 This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Continuity Equation”.

1. Continuity equation is related to _______
a) Mass conservation
b) Energy conservation
c) Momentum conservation
d) Velocity change

Answer: a
Explanation: Continuity equation is related to mass conservation. It states that the total mass entering a body is equal to the total mass leaving a body.
Mass entering the body = mass leaving the body.

2. The quantity specifying the flow or motion is termed as _________
a) Density
b) Flux
c) Field
d) Electrostatic force

Answer: b
Explanation: Flux is also referred to as flux density. It is the amount of quantity (q) which is flowing per unit volume(v). The quantity should have the ability to move or flow.

3. In electromagnetic theory, continuity equation relates _______
a) Volume conservation
b) Mass conservation
c) Charge conservation
d) Energy conservation

Answer: c
Explanation: In electromagnetic theory, charge conservation is a result of Maxwell’s equation which states that divergence of current density is equal to the negative rate of change of charge density.
∇.J=- ρ/t
Where J-current density and ρ-charge density.

4. For an incompressible flow, the mass continuity equation changes to ________
a) energy equation
b) momentum equation
c) volume continuity equation
d) remains same

Answer: c
Explanation: ∇.u=0 – In this equation, u is flow velocity and it states that the divergence of flow velocity is zero. Since the flow is incompressible, the density remains constant and thus the volume cannot be changed.

5. Which of the flowing is an example of incompressible flow?
a) gas
b) sponge
c) water
d) gel

Answer: c
Explanation: Incompressible flow refers to the flow in which the density is constant. The water molecular is relatively closer and hence, the intermolecular distance is very small. So, the water is incompressible.

6. The differential form of continuity equation is __________
a) ∇.u=constant
b) Dv/Dt=0
c) ρ/t + ∇.(ρV)=0
d) ρ=0

Answer: c
Explanation: ρ-density V-velocity vector t-time
The divergence represents the inflow and outflow whereas the time derivative represents the accumulation of mass inside a body. It states that the amount of mass entering the body is equal to the amount of mass leaving the body.

7. The equation which results in the change in pressure with change in the vertical height is called as __________
a) energy equation
b) momentum equation
c) continuity equation
d) hydrostatic equation

Answer: d
Explanation: dp = -g ρ dy represents the hydrostatic equation where p is pressure acting on a body, g is acceleration due to gravity, dy is changed in vertical height. The net force on the element acts only in the vertical direction. The pressure force on the front and back faces are equal and opposite and hence cancel.

8. When Reynold’s number limits to infinity, inviscid flow is approached.
a) True
b) False

Answer: a
Explanation: For inviscid flows, the influence of friction, thermal conduction, diffusion is limited to a very thin region adjacent to the body surface called the boundary layer and remainder of the flow outside this thin region is essentially inviscid.

9. Mass can neither be created nor be destroyed is the principle of_______
a) Energy equation
b) Momentum equation
c) Continuity equation
d) Bernoulli’s principle

Answer: c
Explanation: Continuity equation is based on the conservation of mass. According to this law, for any system, the amount of mass always remains constant unless and until the mass is added or removed from the system. This means that the quantity of matter does not change.

10. If the system is in steady state, it is in an equilibrium state.
a) True
b) False

Answer: b
Explanation: If the system is in steady state, it does not mean the system is in an equilibrium state. Whereas, if the system is in an equilibrium state, it is in a steady state. A system is in equilibrium if all the thermodynamic properties are fixed. A system is said to be in the steady state if the dissipation of energy takes place within the system.

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

Answer: d
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

Answer: b
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

Answer: a
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

Answer: c
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

Answer: b
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

Answer: a
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
dv/dy -velocity gradient.

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

Answer: c
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

Answer: a
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

Answer: a
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

Answer: b
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.

Aerodynamics Questions and Answers – Finite Control Volume Approach

 This set of Basic Aerodynamics Questions and Answers focuses on “Finite Control Volume Approach”.

1. Which of the following is most difficult to analyze?
a) solid
b) plasma
c) fluids
d) cannot be determined

Answer: c
Explanation: Because fluid (liquid and gas) is a “squishy” substance. While the solid body is geometrically well defined and hence its motion can be defined easily. Whereas, in fluids, it is difficult to decide the qualitatively model the motion of the fluid.

2. By the time the vehicle reaches the surface of the earth, the potential and kinetic energy become zero.
a) True
b) False

Answer: a
Explanation: During re-entry, near the outer edge of the atmosphere, the vehicle has a large amount of kinetic energy due to its high velocity and a large amount of potential energy due to its altitude but as it reaches the surface of earth it becomes zero because the kinetic and potential energy is utilized in heating the body and heating the airflow around the body.

3. Which of the following has similar characteristics?
a) solid and liquid
b) liquid and gas
c) solid and gas
d) cannot be determined

Answer: b
Explanation: When force is applied to liquid and gas, there will be an increasing deformation, proportional to the rate of change of deformation. Also, the molecules in liquid and gas are loosely packed. When filled in a container, they take the shape of that container.

4. Which of the following is a point property?
a) Pressure
b) Mass
c) Density
d) Pressure and density

Answer: d
Explanation: Both pressure and density vary from one point to another. Pressure and density vary with the altitude and are directly proportional to each other. Pressure and density does depend on a path as they change from one point to other. If one parameter is changed, it affects pressure and density.

5. Which of the following flow is best suited for space shuttle?
a) continuum flow
b) free molecular flow
c) turbulent flow
d) laminar flow

Answer: b
Explanation: In free molecular flow, the gas molecules are spaced so far apart that collisions with the body surface occur only infrequently and the body surface can feel distinctly each molecular impact which reduces the vibration in the space vehicle.

6. Which of the following flow is practically impossible in nature?
a) viscous flow
b) inviscid flow
c) laminar flow
d) turbulent flow

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

Answer: c
Explanation: Supersonic flows are characterized by the presence of shock waves across which the flow properties and streamlines change discontinuously. In supersonic flow, the gas can be compressed using shock waves.

8. Continuity equation is one of the most fundamental equations in fluid dynamics.
a) True
b) False

Answer: a
Explanation: Continuity equation relates aerodynamic phenomena over a finite region of space without being concerned about the details of precisely what is happening at a given distinct point in the flow.

9. Surface forces includes _________
a) pressure and shear stress
b) point force
c) body force
d) gravity

Answer: a
Explanation: Pressure force and shear stress acts on the complete surface of the body which means that the surface force acts on the surface and not on a point or the body. Surface forces may act either on external or on an internal surface of the body.

10. System and surrounding are both inter-related things.
a) True
b) False

Answer: a
Explanation: Heat and work are forms of energy. During the transfer of heat and work, the system and surrounding lose its energy. Boundary separates system and surrounding. A system is included in the surrounding.