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Fluid Mechanics > Chapter 3 > Flashcards

Chapter 3 Flashcards

1
Q

True or False

In a flowing fluid, friction is associated with shear stress, and shear stress is associated with viscosity.

A

True

Ref: Pg 73 (Print Version)

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2
Q

Define “inviscid fluid.”

A

A fluid that has no viscosity (friction between layers of fluid).

Ref: Pg 74 (Print Version)

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3
Q

True or False
In practice, there is no such thing as an inviscid fluid, since every fluid supports shear stresses when it is subjected to a rate of strain displacement.

A

True

Ref: Pg 74 (Print Version)

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4
Q

Inviscid flow is governed by what two forces?

A
  1. Pressure forces
  2. Gravity

Ref: Pg 74 (Print Version)

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5
Q

Give the exact definition of “steady flow.”

A

None of the parameters of the fluid change with time at a given location in the flow field.

Ref: Pg 74 (Print Version)

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6
Q

Give the exact definition of “streamlines.”

A

Lines that are tangent to the velocity vectors throughout the flow field.

Ref: Pg 74 (Print Version)

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7
Q

For a 2D flow in the xz plane, the acceleration has how many components? Describe them.

A

One along the streamline = a_s
One normal to the streamline = a_n

Ref: pg 75 (Print Version)

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8
Q

True or False
In steady flow, we must take into account how the velocity changes with time as well as how it changes with the spatial coordinates.

A

False, velocity changes over time are ignored for steady flow analysis.

Ref: Pg 75 (Print Version)

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9
Q

The normal component of acceleration is commonly referred to as _______.

A

Centripetal acceleration.

Ref: Pg 75 (Print Version)

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10
Q

True or False

In a flowing fluid, the pressure varies from one location to another.

A

True

Ref: Pg 76 (Print Version)

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11
Q

True or False
In a flowing fluid, the pressure and weight forces do not balance–the force unbalance provides the appropriate acceleration and hence, particle motion.

A

True

Ref: Pg 77 (Print Version)

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12
Q

True or False
For steady, inviscid flow, the sum of a certain pressure, velocity, and elevation effects are constant along a streamline.

A

True

Ref: Pg 78 (Print Version)

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13
Q

Name all the assumptions that are used to derive the Bernoulli Equation.

A
  1. Inviscid flow
  2. Steady flow (no rotation or turbulence)
  3. Incompressible fluids
  4. Only applicable along a streamline
  5. No pumps or turbines

Ref: Lecture Notes Chapter 3

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14
Q

In steady flow, acceleration can be interpreted as arising from what two distinct occurrences?

A
  1. A change in speed along the streamline.
  2. A change in direction if the streamline is not straight.

Ref: Pg 84 (Print Version)

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15
Q

True or False

The pressure across streamlines is hydrostatic.

A

True

Ref: Pg 84 (Print Version)

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16
Q

What is a “stagnation point?”

A

A point where all the fluid’s kinetic energy is transformed into a pressure rise.

Ref: Pg 85 (Print Version)

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17
Q

The pressure at a stagnation point is greater than static pressure by what amount?

A

Dynamic pressure = (1/2)(density)(velocity)^2

Ref: Pg 86 (Print Version)

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18
Q

True or False

There is a stagnation point on any moving body placed into a flowing fluid.

A

False, the body must be stationary.

Ref: Pg 86 (Print Version)

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19
Q

Where does the stagnation streamline terminate?

A

The stagnation point on the body of interest.

Ref: Pg 86 (Print Version)

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20
Q

If elevation effects are neglected, the stagnation pressure is _______.

A

The largest pressure obtainable along a given streamline.

Ref: Pg 86 (Print Version)

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21
Q

The exit pressure for an incompressible fluid jet is equal to _______.

A

The surrounding pressure.

Ref: Pg 90 (Print Version)

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22
Q

Describe the “vena contracta” effect.

A

This phenomenon occurs in free jets when the exit is not a smooth, well-contoured nozzle. The diameter of the jet will be less than the diameter of the hole. The effect is the result of the inability of the fluid to turn a sharp corner.

Ref: Pg 91 (Print Version)

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23
Q

True or False

The diameter of a fluid jet is often larger than that of the outlet from which it flows.

A

False. The jet diameter is smaller than the outlet diameter due to the vena contracta effect.

Ref: Pg 91 (Print Version)

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24
Q

What is the physical interpretation of the continuity equation?

A

Mass cannot be created or destroyed.

Ref: Pg 92 (Print Version)

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25
Q

What is the equation for the mass flow rate?

A

m_dot = (rho)(Q)

Ref: Pg 92 (Print Version)

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26
Q

When does “cavitation” occur?

A

When the pressure in the fluid is reduced to vapor pressure.

Ref: Pg 96 (Print Version)

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27
Q

What conditions must be present for the energy line to remain constant along a streamline?

A

Steady, inviscid, incompressible flow.

Ref: Pg 103 (Print Version)

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28
Q

Define “energy line.”

A

The total energy head available to the fluid.

Ref: Pg 104 (Print Version)

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29
Q

True or False

Under the assumptions of the Bernoulli equation, the energy line is horizontal?

A

True

Ref: Pg 104 (Print Version)

30
Q

The hydraulic grade line is the sum of what two heads?

A

The elevation head (z) and the pressure head (p/gamma).

Ref: Pg 104 (Print Version)

31
Q

True or False

If the fluid velocity changes along a streamline, the HGL will be horizontal.

A

False. Every time the velocity changes the HGL will shift.

Ref: Pg 104 (Print Version)

32
Q

True or False
With respect to a filled tank; if the Bernoulli assumptions are satisfied, the energy line corresponds to the elevation of the free surface.

A

True

Ref: Pg 104 (Print Version)

33
Q

How does the hydraulic grade line (HGL) vary with respect to the energy line?

A

By a distance of V^2/(2g) which is the velocity head of the modified Bernoulli equation.

Ref: Pg 105 (Print Version)

34
Q

If a pipe with flowing fluid were to lie beneath the HGL, would the pressure be positive or negative with respect to atmospheric pressure?

A

Positive

Ref: Pg 105 (Print Version)

35
Q

If a pipe with flowing fluid were to lie above the HGL, would the pressure be positive or negative with respect to atmospheric pressure?

A

Negative

Ref: Pg 105 (Print Version)

36
Q

True or False

In some situations, it is possible to model flow as steady with the appropriate choice of coordinate system.

A

True

Ref: Pg 108 (Print Version)

37
Q

With respect to pressure gradient problems, the s_hat and x_hat directions are the same if ____.

A

The cross product of their unit vectors is zero (i.e., the x_hat and s_hat directions are exactly the same for the problem at hand).

Example 3.2

38
Q

Consider a scenario where a person is riding a bike. If the coordinate system is attached to the ground, the flow around the person’s helmet will be ___.

A

Unsteady.

Example 3.2

39
Q

Consider a scenario where a person is riding a bike. If the bike is accelerating or decelerating the flow will be ____ regardless of coordinate system choice.

A

Unsteady

Example 3.2

40
Q

The Bernoulli equation is a general integration of ___.

A

Newton’s second law.

Ref: Example 3.2

41
Q

With respect to the Bernoulli equation, if the boundary conditions along a streamline are known, the velocity distribution is _____ to find the difference in pressure.

A

Not needed

Ref: Example 3.2

42
Q

With respect to the streamline coordinate system, if rotation is perfectly circular, where does the normal vector point?

A

Away from the center of rotation.

Ref: Example 3.3

43
Q

With respect to the streamline coordinate system, if rotation is perfectly circular, where does the radial vector point?

A

Towards the center of rotation.

Ref: Example 3.3

44
Q

Is pressure the same tings as energy?

A

No, pressure is a stress.

Ref: Example 3.4

45
Q

With respect to the Bernoulli equation written in terms of heads, what are the first three terms?

A 
p/gamma = pressure head
v^2/rho = velocity head
z = elevation head

Ref: Pg 111 (Digital Version)

46
Q

In the alternate form of the Bernoulli equation

p/rho + v^2/2 +gz = constant

What are the physical interpretations of all three terms?
What are the units for this equation?

A 
p/rho = work done by pressure forces
v^2/2 = the kinetic energy of the particle
gz = work done by the weight (gravity) forces.

Energy per unit mass

Ref: Pg 111 (Digital Version)

47
Q

The pressure variation across straight streamlines is completely _____.

A

Hydrostatic (you can use the manometer equation on it).

Ref: Pg 112 (Digital Version)

48
Q

For steady flow, acceleration can be interpreted as arising from?

A

A change in speed along the streamline or a change in direction if the streamline is not straight.

Ref: Pg 112 (Digital Version)

49
Q

When a fluid particle travels along a curved path, a net force is directed ____.

A

Toward the center of curvature.

REf; Pg 112 (Digital Version)

50
Q

True or False
If the radius of curvature of the streamline is infinite, the pressure variation in the vertical direction (orthogonal) to the streamlines is as if the fluid were stationary.

A

True

Ref: Example 3.5

51
Q

Consider a scenario where two fluid layers are fluid. flowing over each other. In the standard coordinate system (x^,z^), the normal vector between the top and bottom layer will be___.

A

The negative of the main coordinate axis direction (e.g. dn= -dz)

Ref: Example 3.5

52
Q

In order to measure thermodynamic pressure, one would need to.

A

Move with the fluid particles thus being static relative to the moving fluid.

Extra Notes: This is why it is often called the “static pressure.”

Ref: Pg 113 (Digital Version)

53
Q

Consider the following form of the Bernoulli equation;

p +1/2rhoV^2 +gamma*z = constant.

Each term in this equation has units of ___.

A

Force per unit area (pressure)

Ref: Pg 85 (Print Version)

54
Q

The pressure at a stagnation point is greater than the static pressure by an amount equal to ___.

A

The dynamic pressure.

Ref: Pg 86 (Print Version)

55
Q

The pitot-static principle is based on what two pressures?

A

Static and stagnation pressure.

Ref: Pg 86 (Print Version)

56
Q

Pitot static tubes measure velocity by ____.

A

Converting velocity into pressure.

Ref: Pg 87 (Print Version)

57
Q

With respect to a pitot-static system, at the point where static pressure is measured an inaccurate value will result if there are _____.

A

Burrs or imperfections.

Ref: Pg 88 (Print Version)

58
Q

If streamlines are converging, the streamline acceleration is ___.

A

Greater than zero.

Ref: Pg 75 (Print Version).

59
Q

If streamlines are diverging, the streamline acceleration is ___.

A

Less than zero.

Ref: Pg 75 (Print Version).

60
Q

If streamlines are curved but not diverging or converging then the normal acceleration is ____.

A

Greater than zero.

Ref: Pg 75 (Print Version).

61
Q

If streamlines are curved and converging then the streamline acceleration is ____, and the normal acceleration is ____.

A

Greater than zero
Greater than zero

Ref: Pg 75 (Print Version).

62
Q

True or False

It is generally more difficult to measure static pressure as opposed to stagnation pressure.

A

True

Ref: Pg 89 (Print Version)

63
Q

How does a direction-finding pitot tube work?

A

The pressure taps are drilled into a circular cylinder and fitted to three pressure transducers. The cylinder is rotated until the pressure in the two side taps are equal, indicating that the center tap is directly into the streamline.

Ref: Pg 89 (Print Version)

64
Q

What is the equation for a free jet’s speed?

A

V = [2gh]^0.5
where h is the height of the fluid in the tank.
Ref: Pg 90 (Print Version)

65
Q

Consider a large tank with a hole on the side near the bottom. Assume the fluid in the tank is draining out. At a point at the bottom of the tank opposite (far away) from the outlet, can the pressure be considered hydrostatic?

A

Yes because at this point we assume that it is sufficiently far away from the jet such that the velocity is zero, therefore the static pressure is the same and the hydrostatic pressure.

Ref: Example 3.7

66
Q

In order for siphoning to occur what two conditions must be met?

A

The end of the tube is below the free surface
The maximum elevation of the tube is not too great

Ref: Example 3.10

67
Q

What three factors influence the flow rate through a sluice gate?

A
  1. Hight of the water behind the gate.
  2. Width of the gate
  3. Height of the opening

Ref: Pg 100 (Print Version)

68
Q

True or False

The flowrate under a sluice gate depends on the water depths on either side of the gate.

A

True

Ref: Pg 101 (Print Version)

69
Q

Any time an object is moving into/through a stationary fluid, where must the coordinate system be placed so ensure steady flow?

A

The coordinate system must be placed on the object and move with it. If it is placed at an intertial reference point the speed of the fluid will change with time as the object moves through it; rendering the flow unsteady. Examples 3.2 and 3.16 illustrate this fact.

Ref: Example 3.2 and 3.16

70
Q

If the flow is irrotational, then the particles do not ____.

A

Spin

Ref: Pg 109 (Print Version)

Fluid Mechanics (7 decks)

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