# 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

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

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

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

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

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

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

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

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

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

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.

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