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Hydraulics and Hydrology 4 > Hydraulics > Flashcards

Hydraulics Flashcards

1
Q

How are hydro-statics and hydro-dynamics different in terms of the variables involved?

A

Statics uses pressure only
Dynamics uses pressure and shear

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2
Q

What does steady flow mean?

A

dV/dt = 0

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3
Q

What does uniform flow mean?

A

dV/dx = 0

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4
Q

What is the condition for laminar flow?

A

Re < 2000

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5
Q

What is ideal flow?

A

The assumption of frictionless flow

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6
Q

What is the equation for hydraulic radius R?

A

Cross-sectional area/wetted perimeter

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7
Q

What is the shear equation in uniform flow?

A

τ = ρgRS
R is hydraulic radius
S is slope angle

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8
Q

What is the Manning’s discharge equation?

A

Q = a/n * A * R^2/3 * sqrt(S0)

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9
Q

How do you calculate discharge Q for a compound channel?

A

Split the channel into simple sections and calculate Q for each, then sum

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10
Q

How do equations for total energy and specific energy differ?

A

Total energy includes height, z, above reference height

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11
Q

What is the specific energy equation?

A

E = y + αv^2/2g
α is the energy coefficient, usually close to 1

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12
Q

What’s an important relationship to remember with flow variables?

A

V = Q/A

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13
Q

What is assumed to calculate the flow depth on the other side of a sluice gate to the depth given?

A

Energy is conserved (E1=E2)

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14
Q

What does the conservation of mass assumption mean?

A

Q1 = Q2

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15
Q

What is true of energy in critical flow?

A

dE/dy = 0

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16
Q

What is the equation for Froude Number?

A

Fr = U/sqrt(gy)

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17
Q

What are two naturally occurring instances of critical flow?

A

Change of bed surface slope from /mild to steep/
Free outfall past a /mild/ slope

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18
Q

For a constant total head H, how do changes in bed profile affect flow depth?

A

The specific energy decreases when the
channel bottom rises

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19
Q

How does increasing energy affect flow depth?

A

Increases depth in subcritical flow
Decreases depth in supercritical flow

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20
Q

In supercritical flow, how do changes to flow affect the rest of the waterway?

A

Changes only affect the downstream system as waves travel slower than flow and so cannot travel upstream

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21
Q

What 3 key changes occur at a hydraulic jump?

A

Flow goes from super to subcritical
Depth increases at the expense of velocity
Specific energy is lost irreversibly

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22
Q

What does the significant loss of energy at a hydraulic jump mean for continuity analysis?

A

Conservation of energy cannot be used, momentum must be used instead

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23
Q

What is the equation for energy loss at a hydraulic jump?

A

ΔE = (y2-y1)^3/(4y1y2)

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24
Q

What is the equation for power dissipated at a hydraulic jump?

A

P = ρgqΔE

25
Q

What counts as rapidly varied flow (RVF)?

A

Sudden change in flow regime
Hydraulic jump
Flow around an obstruction

26
Q

What is the equation for the slope of the free surface profile?

A

dy/dx = (S0-Sf)/(1-Fr^2)

27
Q

How is the friction slope (Sf) defined?

A

The slope of the total head line
dH/dx = -Sf

28
Q

What are the 5 bed slopes and their conditions?

A

Mild: yn > yc
Steep: yn < yc
Critical: yn = yc
Horizontal: S0 = 0
Adverse: S0 < 0

29
Q

How does flow depth define the 3 zones in profile classification?

A

y > yn: Zone 1
yn > y > yc: Zone 2
yc > y: Zone 3

30
Q

How does flow depth approach normal depth?

A

Asymptotically

31
Q

How does flow depth approach critical depth?

A

Sharply, often almost vertically

32
Q

How does flow depth trend in Zone 1 conditions?

A

It trends towards a horizontal flow surface

33
Q

How do Zone 1 and Zone 3 profiles approach or diverge from the normal/critical depth on a critical slope?

A

Zone 1: Diverges sharply
Zone 3: Approaches asymptotically

34
Q

What is different about a sluice gate on a steep slope compared to a mild one?

A

On a steep slope, the hydraulic jump occurs before the sluice, whereas for mild it occurs after

35
Q

How does flow criticality affect the response of flow to a depression in the bed?

A

Subcritical flow will increase in depth
Supercritical flow will decrease in depth

36
Q

How are the energy equations for subcritical normal flow and flow over a weir related?

A

En = Ew
yn + Vn^2/2g = yw + hw + Vw^2/2g

37
Q

Why are hydraulic jumps often engineered to occur?

A

To reduce energy as a means of reducing speed, erosion etc.

38
Q

What is a stilling basin and why is it used?

A

A series of blocks in the water flow that induce a hydraulic jump
The aim is to shorten the jump so as to reduce scour

39
Q

From where do you start sketching flow profiles?

A

Locations of known flow depth

40
Q

How do you check where a hydraulic jump occurs when comparing two possibilities?

A

Compare the height of the jump to the normal depth after the jump and check if this difference makes sense given the slope profile

41
Q

How are the heights of two potential hydraulic jumps related?

A

yj1 = 1/2 yj2(sqrt(1+8Fr2^2)-1)

42
Q

What does hL signify? What is it equivalent to?

A

Head loss due to friction
hL = Sf * L

43
Q

How do you perform the direct step method?

A

Start from a known (y,V)
Proceed by solving for x assuming Δy is the same for each step

44
Q

How do you perform the standard step method?

A

Start from a known (y,V)
Proceed by iteratively solving for y by changing Δx until the target y is met

45
Q

What is an important part in the 2 step methods?

A

Finding the energy grade line using EGL = y + V^2/2g

46
Q

What is the version of Manning’s formula used to find energy loss due to friction?

A

Sf = (n * V)^2/(R^4/3)

47
Q

What is the equation of Δx in the direct step equation?

A

Δx = ΔE/(S0-Sfavg)

48
Q

What values need to be calculated (and can be tabulated) for each depth in the direct step method?

A

A
P
R
V
EGL
Sf
Δx

49
Q

What must be true of step size in the standard step method?

A

It must be small enough that changes in depth are small

50
Q

What changes about equations in natural channels?

A

Z is used instead of y, where Z is the distance from a datum to the water surface

51
Q

How do you know if your guess of y2 in the standard step method is correct?

A

E2 = E1 + (S0-Sfavg)Δx

52
Q

What must you do if discharge is one of the required outputs but slope profile is unknown?

A

Assume profile to calculate discharge

53
Q

What can R be assumed to be equivalent to in a very wide channel?

A

depth, y

54
Q

What is the area of a trapezium?

A

1/2 (a+b) * h

55
Q

What is the mnemonic for direct step method

A

YAPRR has Very HeaVy HeEps rolling down the Mean Slope

56
Q

What is the mnemonic for standard step method?

A

Start Deep, ZAPR’s Very HeaVy HEAD Sloped over and the Sloping Distance changed his HEAD too much

57
Q

What is the equation for normal depth of a wide channel?

A

yn = (Qn/(B * sqrt(S0)))^3/5

58
Q

How does the uniform flow assumption help with finding normal flow depth?

A

At normal depth, all variables are assumed to be constant rather than changing

Hydraulics and Hydrology 4 (3 decks)

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