# Aerodynamics Questions and Answers – Definition of Compressibility

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

1. What is the most compressible substance of the following?
a) Salt
b) Liquid oxygen
c) Water
d) Laughing gas

Explanation: Gases are the most compressible substances. Liquids have much lesser compressibility and solids are the least compressible. Hence, of the given options, laughing gas is the most compressible substance.

2. In physical sense, compressibility is the fractional change in volume of the fluid element per unit change in_____
a) Mass
b) Temperature
c) Pressure
d) Density

Explanation: The compressibility is defined as the fractional change in the volume of the fluid element pet unit change in pressure. It is a negative quality since the volume decreases as pressure increases and vice versa.

3. Which of these is not true when a gas is compressed in a bicycle pump?
a) Temperature increases
b) Pressure increases
c) Volume increases
d) Heat is exchanged

Explanation: When a gas is compressed, the temperature and pressure increases. This is observed in real life also. The volume of the gas decreases due to compression. Heat is transferred in or out of the system also.

4. The subscript ‘T’ in the formula for compressibility symbolize that temperature of the gas is kept constant in the process.
a) True
b) False

Explanation: If we employ some heat transfer mechanism such that the temperature of the gas is kept constant during compression, it is called isothermal compressibility. It is symbolized by τT where T stands for isothermal compression.

5. The isothermal compressibility for a gas is same as isothermal compressibility.
a) True
b) False

Explanation: The isothermal compression of a gas means that no heat is transferred from the fluid element and there is no friction involved. Thus, isothermal compressibility has additional condition of no fiction along with thermal compressibility.
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6. τT and τS are precise thermodynamic properties of the gas and the values can be obtained easily.
a) True always
b) False always

Explanation: The isothermal and isentropic compressibility for a fluid are precise thermodynamic properties of the fluid. The values have already been found and compiled in various handbooks for easy reference.

7. In terms of compressibility, the density change when pressure of the fluid increases is ______
a) Density increases
b) Density decreases
c) Density is constant
d) Insufficient data

Explanation: The formula for compressibility when expressed in terms of density gives a direct relation between pressure change and density change. i.e. dρ = ρ τ dp. Hence, when the pressure increases density of the fluid increases.

8. For the flow over an airfoil, which of the following is the incorrect statement if the fluid is liquid?
a) High pressure change
b) Incompressible fluid
c) Compressibility is very small
d) Density is constant

Explanation: The pressure change can be both high or low and we can’t ascertain it with the given information. Moreover, for liquids the compressibility is negligible and so we can assume that density change is also small. Hence, the flow is compressible.

9. Gaseous flow over an airfoil is always compressible because of very high compressibility value of gases.
a) Always true
b) Always false
c) True only for high speeds
d) True for low speeds

Explanation: Gases have high compressibility values which states that for a given pressure change, the density change can be higher giving a compressible flow. But low speed flows are an exception since the pressure change across the field is small. This dominates the high compressibility value giving an incompressible flow.

10. The best estimate of whether the flow is compressible or not comes from ______
a) Pressure
b) Mach number
c) Compressibility values
d) Experiments

Explanation: Compressibility values gives a good estimate of the nature of fluid i.e. whether it is compressible or not. But it has exceptions like low speed flow over airfoil etc. The Mach number is the best parameter to judge the compressibility of the fluid.

11. Mach number is defined as the ratio of local flow velocity to the _______
a) Speed of sound in vacuum
b) Local speed of sound
c) Speed of light
d) Speed of sound at 25°C

Explanation: The Mach number is not related to the speed of light. It is defined as the ratio of local speed of flow to the local speed of sound. The local speed varies with temperature and pressure in that region.

12. Select the incompressible flow out of the following.
a) High speed gaseous flow
b) M=5
c) M=3
d) Liquid flow

Explanation: The high speed gaseous flow is a compressible flow due to high compressibility of gases. Liquid flow is incompressible due to low compressibility of liquids. And M=5 or M=3 is compressible flow since the incompressible flow has M<0.3 only.

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