Open Channel Flows Introduction Part 2

Question 1

How is regime defined?

Answer 1

Regime defined by Froude number Fr → most important in channel flow

Question 2

What is Froude number?

Answer 2

Fr = ratio of flow speed and the shallow-water wave velocity

Question 3

How can Froud number be calculated?

Answer 3

F_r = U/√gh

Question 4

What does a super-critical flow mean?

Answer 4

Fr> 1, does not affect flow upstream

Question 5

What does a critical flow mean?

Answer 5

Fr=1

Question 6

What does sub-critical flow mean?

Answer 6

Fr<1, may affect flow upstream

Question 7

When does a hydrualic jump form?

Answer 7

Forms between super and sub-critical flow

Question 8

When is a flow laminar?

Answer 8

When Re«Re critical

Question 9

How can the Reynolds number be calculated?

Answer 9

Re = UR/v

Question 10

What are all open channel flows classified as?

Answer 10

Turbulent

Question 11

What does steady flow mean?

Answer 11

No variation in time, no change in source discharge

Question 12

What does unsteady flow mean?

Answer 12

• Variations over time
• A flow in which quantity of liquid flowing per second is not constant

Question 13

What is a uniform flow?

Answer 13

• No spatial variations
• No along-channel variations in water depth, slope, discharge, etc.

Question 14

What is a non-uniform flow?

Answer 14

Variations in one or more of the variables e.g., depth, slope, discharge

Question 15

What are slow varying flows?

Answer 15

Gradual variations; pressure is usually hydrostatic and streamlines are straight

Question 16

What are rapidly varying flows?

Answer 16

Rapid variations; pressure is generally not hydrostatic and streamlines are curved

Question 17

What is the force balance like in uniform flow?

Answer 17

Equilibrated

Question 18

How can width be determined?

Answer 18

dA/dh

Question 19

When is pressure P hydrostatic?

Answer 19

Pressure p is hydrostatic (linear increase with depth) if streamlines are approximately straight and parallel (uniform/slowly varying flows).

Question 20

What is the pressure head at the channel bed?

Answer 20

P = ⍴gh

Question 21

What is the pressure head at the channel bed?

Answer 21

h = P/⍴g

Question 22

What are the underlying assumptions?

Answer 22

i) uniform/slowly varying flow,
ii) incompressible fluid
iii) steady flow
iv) Friction negligible

Question 23

What new behaviors can the flow encounter?

Answer 23

• Waves (due to wind, ships, etc.).
• Travelling shocks (e.g. surge waves due to sudden closing of sluice gates).
• Hydraulic jump.

Question 24

What is the piezometric head equal to?

Answer 24

Piezometric head is equal to the free surface under hydrostatic conditions

Question 25

What’s the equation for steady uniform flow?

Answer 25

F_1-F_2+F_g-F_f=0

Question 26

How is the gravity body force calculated?

Answer 26

ρgALs

Question 27

How is the wall friction force calculated?

Answer 27

𝜏_wPL

Question 28

Gravity balances…?

Answer 28

Fricition

Question 29

How can wall shear stress be calculated?

Answer 29

ρgRs

Question 30

How is the friction related to the velocity in turbulent flows?

Answer 30

For any turbulent flow, the friction (wall shear stress) varies approximately as the square of the mean velocity

Question 31

How can Chezy coefficient be derived?

Answer 31

• Assume flow is steady and uniform
• Momentum equation
• Friction forces balance with g forces

Question 32

What is the radius of a a channel if it is really wide?

Answer 32

h