Aerodynamics - Compressible Flow – Preliminary Aspects Quiz(MCQ)

1 .

The equation of state for a perfect gas is p = δRT, where R is______ (for air at standard conditions).

A)

287 J/kg.K

B)

287 kJ/kg.K

C)

8314 kJ/kg

D)

8.314 kJ/kgK

Correct Answer : 287 J/kg.K

Explaination : The universal gas constant R in the equation of gas has different values for different gases and at different conditions. For air at standard conditions, the correct value is 287 J/kg.K (check units).

2 .

Which is the wrong result for a calorically perfect gas?

A)

e=c_vT

B)

h=c_pT

C)

For T < 1000 K, specific heats are constant

D)

c_p and c_p are functions of T

Correct Answer : c_p and c_p are functions of T

Explaination : The calorically perfect gases have constant specific heats at low temperatures (T < 1000 K). c_p and c_p are nothing else but specific heat constants at constant pressure and temperature respectively. Internal energy and enthalpy can be given in terms of the specific heats i.e. e=c_v T and h=c_pT.

3 .

Specific heat for gas is almost constant in the case of_______

A)

Air in a desert

B)

Space vehicle

C)

High-temperature flow

D)

Chemically reacting flow at high- speeds

Correct Answer : Air in a desert

Explanation : Instances of very high temperature, chemically reacting flow at high-speeds do not have constant specific heats. Space vehicles fall in this category. The air in any desert would not have temperatures higher than 1000K and thus fall in the category of calorically perfect gas (i.e. constant specific heat).

4 .

Which is not a state variable/function in the given options according to thermodynamics?

A)

Enthalpy

B)

Temperature

C)

Internal energy

D)

Functions dependent on path

Correct Answer : Functions dependent on path

Explanation : By definition, state functions do not depend on the path and only depend on the initial and final state of the gas. Temperature, enthalpy and internal energy are all path independent i.e. state functions.

5 .

For air, at 35°C what is the total enthalpy? (c_p=1005 J/kgK)

A)

Zero

B)

8977 J

C)

309.54 J

D)

309.54 kJ

Correct Answer : 8977 J

Explaination : The specific enthalpy (h) is given as h=c_pT where T is in Kelvin. This is the enthalpy per unit mass. For total enthalpy of the air, we need to multiply h by the molecular mass of air i.e. H=29h which gives total enthalpy equal to 8977 J, irrespective of the process.

6 .

The first law of thermodynamics is proved by experiments. Select the correct choice for it.

A)

de is an exact differential

B)

dq is an exact differential

C)

dw is an exact differential

D)

The relation has been proved theoretically

Correct Answer : de is an exact differential

Explanation : The first law in thermodynamics is an empirical relation and has been verified by experiments. It has never been violated. Exact differential depends only on final and initial points. de is an exact differential while dq and dw are not exact differentials.

7 .

For a reversible process, which is not the correct statement?

A)

dw = -pdv

B)

dq – pdv = de

C)

No presence of viscosity

D)

Mass diffusion occurs

Correct Answer : Mass diffusion occurs

Explanation : Reversible process has no dissipative effects i.e. viscosity and mass diffusion are absent. Also, work in a reversible process is given as dw = -pdv. Therefore, the first equation of thermodynamics can be written as dq – pdv = de.

8 .

Which is the process in which no heat exchange is there between system and surroundings and in which no dissipative phenomena occur?

A)

Work

B)

Isentropic process

C)

Adiabatic process

D)

Reversible process

Correct Answer : Isentropic process

Explanation : A reversible process is one in which no dissipative phenomena occur. An adiabatic process is one in which no heat exchange is there between the system and the surroundings. A process that is both adiabatic and reversible is called an isentropic process.

9 .

Identify the incorrect source for the corresponding variables of the first law.

A)

Heating: de

B)

Absorbed radiation: dq

C)

Squeezing of volume: dw

D)

Displacement of system boundary: dq

Correct Answer : Displacement of system boundary: dq

Explanation : The change in dq or dw brings a change in de. Displacement of system boundary or squeezing of system volume are sources of dw. While heating and absorbing of radiation by mass in the system are sources of dq, therefore de.

10 .

The equation of the second law of thermodynamics says that_______

A)

Entropy is a state process

B)

Total change in entropy is always zero

C)

We cannot find entropy for an irreversible process

D)

Entropy change is defined as the heat added in an irreversible process

Correct Answer : Entropy is a state process

Explanation : According to the second law, entropy is a state process. We can find the entropy change by the heat added in a reversible process. Using the total head added, we can find the heat added in the irreversible process also (since entropy is a state process).

11 .

Which is the correct result according to the second law of thermodynamics?

A)

Total change in entropy is always zero

B)

Total change in entropy for an irreversible process is negative

C)

Total change in entropy in a reversible process is always zero

D)

Total change in entropy in an irreversible process is always zero

Correct Answer : Total change in entropy in a reversible process is always zero

Explanation : In an irreversible process, entropy is generated. This means the change in entropy for an irreversible process is positive. Also, for a reversible process, change in entropy is zero since the entropy is conserved in the universe. Thus, the change in total entropy for an irreversible process is greater or equal to zero while that for the reversible process is zero.

12 .

For a block of ice on a plate, second law of thermodynamics says that_______

A)

Ice will melt

B)

Ice will get cooler

C)

The plate will get hotter

D)

Cannot give conclusive direction

Correct Answer : Ice will melt

Explanation : The second law of thermodynamics states which direction the process will take place, unlike the first law which just tells whether the process is possible or not. From entropy considerations, ice will melt in this case. This is visible from real life observation as well. Total entropy has to increase or stay the same.

13 .

First law when expressed in terms of entropy takes the form_____

A)

T ds = dh – p dv

B)

T ds = de + p dv

C)

T ds = de – p dv

D)

T ds = dh + p dv

Correct Answer : T ds = de + p dv

Explanation : The first law of thermodynamics when expressed in terms of entropy gives us two alternate equations. These equations are T ds = de + p dv and T ds = dh – v dp. We have used the relation of heat and entropy in the first law equation to get these equations.

14 .

For a calorically perfect gas, entropy is not________

A)

State function

B)

Zero always

C)

Conserved in a reversible process

D)

A function of two thermodynamic variables

Correct Answer : Zero always

Explanation : The enthalpy is a state function. This is true for all type of gases. It is conserved for a reversible process (since no generation of entropy takes place due to dissipation). From the formula, it is visible that entropy always is a function of two thermodynamic variables (p, T) or (v, T), etc. It is not zero always.

15 .

Select the incompressible flow out of the following.

A)

M=3

B)

M=5

C)

Liquid flow

D)

High speed gaseous flow

Correct Answer : 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.

16 .

Mach number is defined as the ratio of local flow velocity to the _______

A)

Speed of light

B)

Speed of sound at 25°C

C)

Speed of sound in vacuum

D)

Local speed of sound

Correct Answer : Local speed of sound

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.

17 .

The best estimate of whether the flow is compressible or not comes from ______

A)

Pressure

B)

Experiments

C)

Mach number

D)

Compressibility values

Correct Answer : Mach number

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.

18 .

Gaseous flow over an airfoil is always compressible because of very high compressibility value of gases.

A)

Always true

B)

Always false

C)

True for low speeds

D)

True only for high speeds

Correct Answer : True only for high 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.

19 .

What is the most compressible substance of the following?

A)

Salt

B)

Water

C)

Liquid oxygen

D)

Laughing gas

Correct Answer : 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.

20 .

In physical sense, compressibility is the fractional change in volume of the fluid element per unit change in_____

A)

Mass

B)

Pressure

C)

Density

D)

Temperature

Correct Answer : Pressure

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.

21 .

Which of these is not true when a gas is compressed in a bicycle pump?

A)

Volume increases

B)

Heat is exchanged

C)

Pressure increases

D)

Temperature increases

Correct Answer : Volume increases

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.

22 .

In terms of compressibility, the density change when pressure of the fluid increases is ______

A)

Insufficient data

B)

Density increases

C)

Density decreases

D)

Density is constant

Correct Answer : Density increases

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.

23 .

For the flow over an airfoil, which of the following is the incorrect statement if the fluid is liquid?

A)

Incompressible fluid

B)

Density is constant

C)

High pressure change

D)

Compressibility is very small

Correct Answer : High pressure change

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.

24 .

The pressure which is a result of the random motion of the gas molecules is ______

A)

Static Pressure

B)

Total Pressure

C)

Dynamic Pressure

D)

Stagnation Pressure

Correct Answer : Static Pressure

Explanation : Gases are the most randomly moving molecules. The pressure due to this random motion of gas molecules is the static pressure. It is as if we are riding along with the gas at the local flow velocity.

25 .

Which of these is not an assumption made when we derive the equation h + V²/2 = constant?

A)

Steady flow

B)

Inviscid flow

C)

Liquid flow

D)

Adiabatic flow

Correct Answer : Liquid flow

Explaination : For an inviscid, adiabatic and steady flow we can show that h + V²/2 = constant. This is valid along any streamline. It does not take into account the flow being a liquid flow and is valid for gaseous flows also.

26 .

Select the incorrect statement if all the streamlines of the steady, inviscid, adiabatic flow originate from a common uniform freestream.

A)

h0 is not equal to the freestream value

B)

h0 is constant along the entire flow

C)

h + V² = h0 is energy equation per unit mass

D)

Stagnation enthalpy is same for each streamline

Correct Answer : h0 is not equal to the freestream value

Explaination : h0 is the stagnation enthalpy for the steady, inviscid, adiabatic flow which is equal to the static enthalpy plus kinetic energy, all per unit mass. If all the streamlines originate from a common freestream, stagnation enthalpy is equal to the freestream value and is constant along the flow i.e. same for all streamlines.

27 .

For a steady, inviscid, adiabatic flow of a gas, the total temperate is constant along the flow.

A)

True always

B)

False always

C)

False only for calorically perfect gas

D)

True only for calorically perfect gas

Correct Answer : True only for calorically perfect gas

Explanation : The total enthalpy of a steady, inviscid, adiabatic gaseous flow is constant. For a calorically perfect gas the total enthalpy can be written in terms of total temperature times the specific heat at constant pressure (which is also a constant). Thus, the total temperature is also a constant.

28 .

The total enthalpy in a general flow at point 1 is H1 and at point 2, which is downstream, is H2. Then __________

A)

H1>H2 always

B)

H2>H1 always

C)

H1=H2 for general flow

D)

H1=H2 for adiabatic flow

Correct Answer : H1=H2 for adiabatic flow

Explaination : The total enthalpy at two points along the flow may or may not be equal in general. The total enthalpies are equal in case of an adiabatic flow between the two points in consideration.