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Compressible Flow > Chapter 7 > Flashcards

Chapter 7 Flashcards

1
Q

Define a perfect gas.

A

A gas in which the intermolecular forces are neglected.

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2
Q

The energy of a given molecule is the sum of what?

A

It’s transnational, rotational, vibrational, and electronic energies.

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3
Q

The sum of the energies of all the molecules in a given volume of gas is defined as the ____.

A

Internal energy of the gas.

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4
Q

The internal energy per unit mass is defined by what variable?

A

Entropy – e

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5
Q

For a perfect gas, both e and h are functions of ____. only.

A

Temperature

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6
Q

When temperatures are below _____K the specific heats are usually ____.

A

1000K

constant.

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7
Q

What is a “Calorically Perfect” gas?

A

As gas in which the two specific heats (cp and cv) are constant.

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8
Q

Define an Adiabatic Process.

A

One in which no heat is added to or taken away from the system.

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9
Q

Define a Reversible Process.

A

One in which no dissipative phenomena occur, this is, where the effects of viscosity, thermal conductivity, and mass diffusion are absent.

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10
Q

Defin Isentropic process.

A

One that is both adiabatic and reversible.

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11
Q

What are the three main dissipative phenomena?

A
  1. Viscosity
  2. Thermal Conductivity
  3. Mass diffusion
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12
Q

Explain why many practical flows can be modeled as isentropic?

A

The viscous boundary layer adjacent to the surface is thin compared with the entire flow field, and hence large regions of the flow can be assumed isentropic.

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13
Q

Entropy is a function of what two variables?

A

Temperature and pressure.

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14
Q

True or False

Entropy increases with increasing temperature.

A

True.

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15
Q

True or False

Entropy decreases with increasing temperature.

A

False, it decreases with decreasing temperature.

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16
Q

How does pressure affect entropy?

A

As pressure increases, entropy decreases and VV.

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17
Q

Incompressible flows obey ____ laws.

A

Mechanical Laws

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18
Q

What are the primary dependant variables for compressible flow?

A

p, V, rho, e, and T

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19
Q

Is Bernoulli’s equation valid for compressible flow?

A

No

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20
Q

What is static pressure?

A

The measure of the purely random motion of molecules in a gas.

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21
Q

What is stagnation pressure?

A

The pressure existing at a point (or points) in the flow where V=0.

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22
Q

What is the total temperature?

Note: Define it conceptually

A

If a fluid element is adiabatically slowed to zero velocity the temperature of the fluid element will change. At the point of zero velocity, the temp of the fluid element is the total temperature.

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23
Q

What is total enthalpy?

Note: Define it conceptually

A

If a fluid element is adiabatically slowed to zero velocity the temperature of the fluid element will change. At the point of zero velocity, the temp of the fluid element is the total temperature. The total enthalpy is the total temperature multiplied by cp.

24
Q

True or False

Total enthalpy is constant along a streamline.

A

True

25
Q

Total pressure and total density are related to what process?

A

An Isentropic process/flow.

26
Q

Conceptually define specific heat.

A

The energy required to raise the temperature of a unit mass of a substance by one unit degree.

TD Textbook

27
Q

What is the difference between cp and cv?

A

cv represents the energy required to raise the temperature of the unit mass of a substance by one degree as the volume is maintained constant. The energy required to do the same thing as the pressure is maintained constant is the specific heat at a constant pressure (cp).

TD Textbook

28
Q

How are the magnitudes of cp and cv related?

A

cp > cv

29
Q

True or False
The specific heat at a constant volume (cv) is related to the changes in internal energy, and the specific heat at constant pressure (cp) to the changes in enthalpy.

A

True

TD Textbook

30
Q

What is the variable cv named?

A

Specific heat at a constant volume.

31
Q

What is the variable cp named?

A

Specific heat at a constant pressure.

32
Q

What is a shockwave?

A

A shock wave is an extremely thin region, typically on the order of 10^−5 cm, across which the flow properties can change drastically.

33
Q

True or False

Flow across a shockwave is adiabatic.

A

True

34
Q

The pressure, density, temperature, and entropy_____ across a shockwave.

A

Increase

35
Q

Mach number and velocity ____ across a shockwave.

A

Decrease

36
Q

True or False

Subsonic flows have a smoothly varying streamline pattern.

A

True

37
Q

True or False
In subsonic flow, the streamline far ahead of the body is forewarned about the presence of the body and begins to adjust accordingly.

A

True.

38
Q

True or False
In supersonic flows, the flow upstream of the body does not know about the presence of the body until it encounters the leading edge shockwave.

A

True

39
Q

True or False

Shockwaves are usually at normal angles to the flow.

A

False, they are usually at oblique angles to the flow.

40
Q

Shockwaves can be considered as an ______ process.

A

Explosive decompression.

41
Q

True or False

Total entropy is constant across a shockwave.

A

True

42
Q

Flow ahead of a shockwave (in both the normal and oblique case) must be _____.

A

Supersonic (M>1).

43
Q

True or False

In some cases behind an oblique shockwave, the flow can be sufficiently decelerated such that the flow is subsonic.

A

True

44
Q

For normal shockwaves, the downstream flow is always _____.

A

Subsonic.

45
Q

When a fluid is not flowing, it is at Stagnation State and its thermodynamic properties are _____.

A

Stagnation Properties.

46
Q

Describe a stagnation reference state.

A

For a flowing fluid, A “Stagnation State” is defined as a reference estate that would exist if the fluid were decelerated to zero speed and zero potential.

47
Q

To obtain a unique stagnation state, the deceleration process must be ______.

A

Adiabatic and reversible = “isentropic.”

48
Q

What instrument is usually employed to measure total temperature and total pressure?

A

A pitot-tube.

49
Q

When a fluid is not flowing, its thermodynamic properties are _______.

A

Stagnation or total properties.

50
Q

What is a stagnation reference state?

A

A reference state that would exist if the fluid were decelerated to zero speed and zero potential.

51
Q

To obtain a unique stagnation state, the deceleration process must be ____ and _____.

A

Adiabatic and reversible.

52
Q

True or False

A process does not have to be reversible for Stagnation Enthalpy and Stagnation Temperature.

A

True

53
Q

True or False

A

Total Enthalpy and Total temperature are constant for steady and adiabatic flow.

54
Q

For non-adiabatic flow, the temperature and Enthalpy are ____ constant.

A

Not

55
Q

Irreverstibilites cause entropy to _____ and total pressure to _____.

A

Increase

Decrease

Compressible Flow (7 decks)

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