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Incompressible Flow > Chapter 1 > Flashcards

Chapter 1 Flashcards

1
Q

Why are most re-entry vehicles blunt bodied instead of sharp?

A

To increase the amount of heat transferred to the air rather than to the body.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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2
Q

Define a fluid.

A

A substance that experiences a continuous deformation upon application of shear stress. The shear stress will usually be proportional to the rate of change of the deformation.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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3
Q

What are the three areas of fluid dynamics?

A
  1. Hydrodynamics (flow of liquids)
  2. Gas dynamics (flow of gases)
  3. Aerodynamics (flow of air)

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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4
Q

Define pressure with respect to fluid dynamics.

A

Pressure is the normal force per unit area exerted on a surface due to the time rate of change of momentum of the fluid molecules imparting on (or crossing) that surface.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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5
Q

True or False

Pressure can be considered a point property and can vary from one point to another in the fluid.

A

True

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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6
Q

True or False

Density can not be considered a point property.

A

False. Density is considered a point property that can vary throughout a flow.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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7
Q

The temperature of a gas is directly proportional to the ______.

A

Average kinetic energy of the molecules of the fluid.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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8
Q

True or False

Temperature can be considered a point property in a moving flow of fluid.

A

True

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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9
Q

True or False

Velocity is both considered a point property as well as a vector quantity.

A

True

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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10
Q

Define “Streamlines.”

A

The lines that are tangent to the velocity vectors throughout a steady flow field.

Ref: Munsen, Fundamentals of Fluid Mechanics, Ch 3.1

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11
Q

True or False

Streamlines are applicable to both steady and unsteady flow fields.

A

False. Streamlines are only considered in steady flow situations.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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12
Q

What direction does shear stress act with respect to streamlines?

A

Tangentially.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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13
Q

The value of the shear stress (Tao) at a point on streamlines is proportional to ______.

A

The spatial rate of change of velocity normal to the streamline at that point. The constant of proportionality is defined as the viscosity coefficient (Mu)

Given Equation:
Tao = (Mu)*(dV/dy)

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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14
Q

Is the viscosity coefficient a constant value?

A

No, it is dependant upon the temperature of the fluid.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.4

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15
Q

What are the two basic sources of forces and moments experienced by a body in a fluid flow?

A
  1. A pressure distribution of the body’s surface.
  2. A shear stress distribution of the body’s surface.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.5

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16
Q

The net effect of the shear and pressure distributions over a body results in what two resultants?

A
  1. The resultant aerodynamic force (R)
  2. The resultant aerodynamic moment (M)

Ref: Anderson, Introduction to Aerodynamics, Ch 1.5

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17
Q

The lift component is ____ to the relative wind.

A

Perpendicular

Ref: Anderson, Introduction to Aerodynamics, Ch 1.5

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18
Q

The drag component is ____ to the relative wind.

A

Parallel.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.5

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19
Q

Define the “chord” of an airfoil.

A

The linear distance from the leading edge to the trailing edge of a body.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.5

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20
Q

Define the “Normal Force (N).”

A

The component of R perpendicular to the chord.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.5

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21
Q

Define the “Axia Force (A).”

A

The component of R parallel to the chord.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.5

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22
Q

What is the angle between L & N as well as D & A?

A

The angle of attack (alpha).

Ref: Anderson, Introduction to Aerodynamics, Ch 1.5

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23
Q

Aerodynamic moments are positive if they ____.

A

Tend to increase the angle of attack.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.6

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24
Q

Aerodynamic moments are negative if they ____.

A

Tend to decrease the angle of attack.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.6

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25
Q

Define the “center of pressure.”

A

The location where the resultant of a distributed load effectively acts on the body.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.6

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26
Q

If moments were taken about the center o pressure, the integrated effect of the distributed loads would add to ____.

A

Zero.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.6

27
Q

At the center of pressure, the aerodynamic moment is ____.

A

Zero.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.6

28
Q

What three conditions must hold if two different flow fields over two different bodies are to be classified as “dynamically similar?”

A
  1. The streamline patterns are geometrically similar.
  2. The distributions of V/V_ininity, p/p_infinity, T/T_infinity, etc throughout the flow field are the same when plotted against common non-dimensional coordinates.
  3. The force coefficients are the same.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.8

29
Q

True or False
If the non-dimensional pressure and shear stress distributions over different bodies are the same, then the non-dimensional force coefficients will be the same.

A

True

Ref: Anderson, Introduction to Aerodynamics, Ch 1.8

30
Q

With respect to dimensional analysis, two flows will be dynamically similar if what conditions hold?

A
  1. The bodies and any other solid boundaries are geometrically similar for both flows.
  2. The similarity parameters are the same for both flows.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.8

31
Q

For an aircraft of a given shape, at a given Mach and Reynolds number, Cl and Cd are functions of _____.

A

The angle of attack.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.8

32
Q

The stalling velocity is determined in part by what aerodynamic coefficient?

A

The lift coefficient.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.8

33
Q

The maximum velocity of an arbitrary aircraft is determined, at least in part, by what aerodynamic coefficient?

A

The drag coefficient.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.8

34
Q

For a given airfoil in subsonic flow, the required values of the lift coefficient (Cl) will _____ as the upstream velocity (V_infinity) ________.

A

Decrease
Increase

Ref: Anderson, Introduction to Aerodynamics, Ch 1.8

35
Q

Generally speaking for an aircraft in subsonic flight, if the speed is high the aircraft has a _____ angle of attack. Does the inverse statement hold?

A

Low
Yes

Ref: Anderson, Introduction to Aerodynamics, Ch 1.8

36
Q

What is the most commonly used value for measuring a given subsonic airfoil’s efficiency?

A

The lift to drag ratio.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.8

37
Q

As the upstream velocity increases from a low value to a high value, how does the L/D ratio behave?

A

L/D will first increase to a maximum at the stall angle of attack and then decrease as the drag coefficient becomes greater than the lift coefficient.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.8

38
Q

For a stationary fluid element, what two forces act?

A

Gravity and a pressure force.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.9

39
Q

For a static fluid element, are there unbalanced forces on the sides of the elements?

A

No, all forces cancel on the element side faces and the only differential force present is between the top and the bottom faces.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.9

40
Q

What is the definition of the “mean free path?”

A

The mean distance that a molecule travels between collisions with neighboring molecules.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

41
Q

Describe continuum flow in terms of molecular collisions on an arbitrary body.

A

Continuum flow can be applied when the molecules of the fluid impact the body’s surface with such frequency that the body cannot distinguish individual molecular collisions, and the surface experiences the fluid as a continuous medium.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

42
Q

For continuum flow, how does the mean free path compare to the geometry of the body immersed in the flow?

A

The mean free path is orders of magnitude smaller than the physical dimensions of the body.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

43
Q

Describe “free molecular flow” in terms of molecular collisions on an arbitrary body.

A

If the mean free path is on the same order as the geometric dimensions of the body then molecular collisions are infrequent and random. As such, the body is able to distinguish individual molecular impacts.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

44
Q

Define “viscous flow.”

A

A flow that experiences one of the three main transport phenomena.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

45
Q

What are the three main transport phenomena?

A
  1. Mass diffusion
  2. Viscosity (friction)
  3. Thermal conduction

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

46
Q

Define an “inviscid flow” in terms of transport phenomena.

A

An inviscid flow is a fluid flow in which there is no mass diffusion, viscosity, or thermal conduction.

47
Q

Do true inviscid flows actually exist in nature?

A

No, however, there are many practical applications where transport phenomena in the flow can be safely ignored.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

48
Q

How does the magnitude Reynolds number relate to inviscid flow.

A

For inviscid flows, the Reynolds number is high but finite.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

49
Q

What is a boundary layer?

A

A small, vicious, region of air near a body’s surface in which the effects of transport phenomena are significant.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

50
Q

Define “incompressible flow” in terms of density.

A

An incompressible flow is one in which density is constant.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

51
Q

Define “compressible flow” in terms of density.

A

A compressible flow is one in which density must be treated as a variable.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.10

52
Q

Where would shear stress be found to exist in any arbitrary flow?

A

At any point in the flow where there is a velocity gradient across streamlines.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

53
Q

What is the source of friction drag on an aerodynamic body?

A

The viscous effects found in the boundary layer.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

54
Q

When flow separates from a surface, it dramatically changes the pressure distribution over the surface resulting in a large increase in drag called ____.

A

Pressure drag.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

55
Q

What is the cause of the large velocity gradients within the boundary layer?

A

Due to friction, the infinitesimally thin layer of air molecules immediately adjacent to the body surface sticks to the surface (zero velocity). This effect, called the “no-slip condition,” is responsible for the large velocity gradients.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

56
Q

The temperature inside the boundary layer is governed by what mechanisms?

A
  1. Thermal conduction.
  2. Frictional dissipation.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

57
Q

For liquids only, how does the viscosity coefficient change with temperature?

A

As the temperature goes up the viscosity coefficient decreases and vv.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

58
Q

For gasses only, how does the viscosity coefficient change with temperature?

A

As the temperature goes up the viscosity coefficient also goes up and vv.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

59
Q

What are the two types of viscous flow?

A
  1. Laminar
  2. Turbulent

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

60
Q

Define “Laminar flow.”

A

Streamlines are smooth and regular, and a fluid element moves smoothly along the streamline.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

61
Q

Define “turbulent flow.”

A

Streamlines break up and a fluid element moves in a random, irregular way.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

62
Q

How does the shear stress at the surface of an airfoil in turbulent flow compare to laminar?

A

The shear stress caused by turbulent flow will always be greater than laminar flow.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

63
Q

True or False

Turbulent aerodynamic heating is larger than laminar.

A

True

Ref: Anderson, Introduction to Aerodynamics, Ch 1.11

64
Q

For a low-speed flow,
the aerodynamic coefficients for a fixed shape at a fixed orientation to the flow
are functions of ____.

A

The Reynolds number.

Ref: Anderson, Introduction to Aerodynamics, Ch 1.12

Incompressible Flow (6 decks)

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