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# Aerodynamics – Definition of Total Stagnation Conditions

1 - Question

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

a) Total Pressure

b) Stagnation Pressure

c) Dynamic Pressure

d) Static Pressure

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Answer: dExplanation: 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.

2 - Question

The stagnation pressure can be visualized as the pressure we feel when we ride along the gas molecules at the local flow velocity.

a) False

b) True

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Answer: aExplanation: The gas molecules have random motion in the flow. If we visualize ourselves as riding at the local flow speed, we feel the static pressure. Contrary to this, the pressure felt at the point of zero velocity is the stagnation pressure.

3 - Question

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

a) Inviscid flow

b) Steady flow

c) Adiabatic flow

d) Liquid flow

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Answer: dExplanation: 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.

4 - Question

The constant h + V2/2 for an inviscid, adiabatic and a steady flow is equal to the stagnation enthalpy for the fluid.

a) True

b) False

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Answer: aExplanation: By definition, stagnation enthalpy is the enthalpy of the fluid when the velocity is zero. Thus, putting V = 0 in the equation gives the stagnation enthalpy as the constant because the equation holds for any point along the streamline.

5 - Question

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

a) Stagnation enthalpy is same for each streamline

b) h0 is constant along the entire flow

c) h0 is not equal to the freestream value

d) h + V2 = h0 is energy equation per unit mass

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Answer: cExplanation: 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.

6 - Question

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

a) True always

b) False always

c) True only for calorically perfect gas

d) False only for calorically perfect gas

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Answer: cExplanation: 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.

7 - Question

The total pressure and total temperature for a flow can only be talked about if the fluid particles are brought to rest by some means.

a) False

b) True

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Answer: aExplanation: The stagnation quantities (total pressure, total temperature, total density etc) are defined quantities and exist in the flow at each point as if we are actually stopping the fluid particle. In reality, we don’t have to bring the fluid particles to rest to calculate them.

8 - Question

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

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Answer: dExplanation: 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.

9 - Question

The stagnation temperature To in case of an isentropic flow is the same as defined for the adiabatic flow.

a) True

b) False

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Answer: aExplanation: Isentropic flow is a flow which is both adiabatic and reversible. In this case the resulting pressure and density are defined as the stagnation pressure and stagnation density while the stagnation temperature is the same as that of an adiabatic flow.

10 - Question

The total pressure and total density are present there only for an isentropic flow.

a) True

b) False

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Answer: bExplanation: At any point along the flow where static density and pressure are defined, we can assign the values of stagnation density and pressure. The definition of these involves the isentropic compression to zero velocity but the flow need not be isentropic. These concepts are valid for all general flows, including non isentropic flow at any point.

11 - Question

For an isentropic flow, consider two points 1 and 2 along the flow. Select the incorrect statement out of the following.

a) v1 = v2

b) ρo,1 = ρo,2

c) po,1 = po,2

d) ho,1 = ho,2

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Answer: aExplanation: In isentropic flow, between any two points, the stagnation quantities are same. They are constant along the entire isentropic flow. Thus, ρo,1 = ρo,2; po,1 = po,2 and ho,1 = ho,2 while the velocities are not equal necessarily. So, V1 is not equal to V2.

12 - Question

The quantity at a point in a subsonic flow where the element is speeded up to sonic velocity adiabatically is defined as_____

a) Static local temperature, T

b) Stagnation local temperature, T°

c) T*

d) Standard temperature

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Answer: cExplanation: The T* is defined as the temperature at a point where the local static temperature is T, in the sonic flow when the fluid element is speeded up to sonic velocity adiabatically. Also, T* is the temperature when the supersonic fluid element is slowed down to sonic velocity adiabatically.