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Compressible Flow > Chapter 8 > Flashcards

Chapter 8 Flashcards

1
Q

Why does temperature increase across a shockwave even when an adiabatic flow is being considered?

A

Because KE is converted to internal energy across the shockwave.

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2
Q

True or False

Flow-through a shockwave is adiabatic.

A

True

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3
Q

The physical mechanism of sound propagation in a gas is based on ___________.

A

molecular motion.

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4
Q

The speed of sound in a calorically perfect gas is a function of what property?

A

Temperature only.

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5
Q

True or False

The lower the compressibility, the higher the speed of sound.

A

True

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6
Q

In a scenario where the shockwave is stationary and flow moves through a stationary control volume containing the shockwave; is the flow steady?

A

Yes, the flow, in this case, can be considered steady.

See the submarine example in FM textbook chapter 5 for a good explanation.

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7
Q

True or False

Flow-through a stationary normal shock wave is isentropic.

A

True

Pg. 569

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8
Q

The speed of sound in a theoretically incompressible flow is _____.

A

Infinite

Pg. 571

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9
Q

The speed of sound in a calorically perfect gas is a function of ____ only.

A

Temperature

Pg. 571

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10
Q

The critical and total speed of sound is constant along a streamline so long as what conditions are met?

A

If the flow is both adiabatic and inviscid.

Ref: Pg. 578

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11
Q

What is the general rule of thumb as far as treating density as a variable in a flow field?

A

If the flow is less than Mach 0.3 the flow can be treated as incompressible. Higher than that, the flow should be treated as compressible and always above Mach 1.0.

Ref: Pg 584

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12
Q

What is a Mach wave?

A

An infinitely weak normal shock wave.

Ref: pg 589

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13
Q

Does the Mach number in front of a Mach wave differ from the Mach number behind it?

A

No, a Mach wave does not change the flow properties as they go across it.

If M1 = 1 then M2 = 1.

Ref: Pg 589

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14
Q

For a normal shockwave, if M1>1 then M2 is …?

A

Less than Mach 1.

Ref: Pg 589

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15
Q

For a higher upstream Mach number (M1) the wave will be weaker or stronger?

A

Stronger

Ref: Pg 589

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16
Q

If the upstream Mach number is equal to one, the entropy change across the wave will be ____.

A

Zero
s2-s1=0 –> s2=s1

Ref: Pg 590

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17
Q

Why does entropy increase across a shockwave?

A

Within the shock wave itself, large gradients in velocity and temperature occur. In other words, the mechanisms of friction and thermal conduction are very strong. These are both dissipative, irreversible mechanisms that always acts to increase entropy.

Ref: Pg 591

18
Q

True or False

The total temperature varies across a stationary normal shockwave.

A

False, total temperature is constant across a stationary normal shock wave.

Ref: Pg. 592

19
Q

How does total pressure change across a normal shockwave?

A

Total pressure decreases across the shock.

Ref: Pg 593

20
Q

How does the ratio of total pressure across a shockwave change with increasing Mach number?

A

It decreases

Ref: Pg 593

21
Q

How does the ratio of pressure across a shockwave change with increasing Mach number?

A

Increases exponentially

Ref: Pg 593

22
Q

How does the ratio of temperature across a shockwave change with increasing Mach number?

A

Increases

Ref: Pg 593

23
Q

How does the ratio of density across a shockwave change with increasing Mach number?

A

Increases logarithmically.

Ref: Pg 593

24
Q

If a pitot tube is immersed in sub-sonic flow, why can isentropic conditions be safely assumed?

A

Because of the nature of how the pitot tube works. Flow is brought from a high velocity to zero velocity without a source of heating. Therefore the flow can be considered isentropic and the isentropic relations can be used.

Ref: Pg 603

25
Q

For supersonic flow, is the shockwave generated by the pitot tube attached?

A

No, it is a bow shock wave.

Ref: Pg 605

26
Q

Define steady flow.

A

None of the flow properties are varying with time.

Ref: Lecture Slides

27
Q

Define “one-dimensional” flow.

A

The velocity vector has only one component over the portion of the flow field considered.

Ref: Lecture Slides

28
Q

What is the definition of a wave?

A

A wave is a disturbance that propagates energy through a medium without net mass transport.

Ref: Lecture Slides

29
Q

Explain how a wave propagates energy.

A

Molecules near a disturbance gain excess translational KE and transfer the energy surplus to neighboring molecules and in doing so creates a traveling wave.

Ref: Lecture Slides

30
Q

What is a “sound wave.”

A

The propagation of very small pressure variations by molecular collisions.

31
Q

When you change the reference frame when observing wave phenomena, do the static properties change?

A

No.

Ref: Lecture Slides

32
Q

When will the zone of silence ahead of a disturbance be at a 90-degree angle to the Mach cone?

A

When the speed of the disturbance is equal to the speed at which information about the disturbance can propagate.

Ref: Lecture Slides

33
Q

For a translating disturbance, when is the Mach cone at an oblique angle to the incoming flow?

A

When the speed of the disturbance exceeds the speed at which information about the disturbance can propagate.

Ref: Lecture Slides

34
Q

True or False

In sub-sonic flow, information about a disturbance can propagate upstream.

A

True

Ref: Lecture Slides

35
Q

Describe how a shock wave is formed.

A

In supersonic flow, disturbances caused by the body cannot propagate upstream fast enough. Therefore compression waves will coalesce and reinforce each other forming a single large discontinuity known as a shockwave.

Ref: Lecture Slides

36
Q

State ALL assumptions used in deriving the normal shock wave equations.

A
  1. Steady
  2. 1D
  3. Inviscid
  4. Adiabatic
  5. Calorically perfect gas

Ref: Lecture Slides

37
Q

Is a shock wave a “Thermodyamicly reversible” phenomenon? Why or why not?

A

No, because entropy increases across a shockwave.

Ref: Lecture Slides

38
Q

What two total properties are always constant across a normal shockwave?

A

Total enthalpy and total temperature.

Ref: Lecture Slides

39
Q

The total pressure change across a normal shockwave depends on what thermodynamic parameter?

A

The change in entropy.

Ref: Lecture Slides

40
Q

In supersonic flow, a pitot tube measures ____.

A

The total pressure behind the shockwave which exists upstream of the tube.

Ref: Lecture Slides

41
Q

For a moving normal shockwave, the upstream velocity can be set equal to ____.

A

Vs which is the absolute shockwave velocity observed by an inertial reference point.

Ref: Lecture Slides

42
Q

For a moving normal shockwave, the downstream velocity can be set equal to ____.

A

The absolute shock wave velocity (Vs) minus the absolute gas velocity behind the wave (Vb).

V2 = Vs – Vb

Ref: Lecture Slides

Compressible Flow (7 decks)

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