6.17. (9/30) Bedload; suspended load; dissolved loads; shear stresses; frictional forces

Question 1

What is the Reynold’s Number?

Answer 1

• Re
• relates density (mass per unit volume), velocity of water flow (m/s), and size of particle/roughness/diameter (m) divided by viscosity ()
• tells you if a flow is turbulent or laminar
• no units/ dimensionless

Question 2

What is the equation for Reynold’s Number?

Answer 2

Re= -ρwUd50/μ
ρw - density of water
U - velocity
d50 - roughness/ diameter
μ - viscosity

Question 3

what happens if Re>10?

Answer 3

There is a turbulent flow (shaky movement, with air)

Question 4

what happens if 1>Re?

Answer 4

There is a laminar flow (steady, slow stream, pillar of water)

Question 5

What has to be big in the Reynold’s equation for a flow to be laminar?

Answer 5

• the viscosity
• for water, the diameter or roughness must be small

Question 6

What is flow separation?

Answer 6

as velocity gets bigger, flow lines go around a particle and separate creating an unstable vacuum
*Where trout hang out
*The truck pushes air out of the way and prevents the flow from hitting you

Question 7

what is form drag?

Answer 7

• holds particles back
• causes the biggest amount of friction
• unstable vacuum

Question 8

What is surface drag?

Answer 8

how smooth a particle is

Question 9

What are the surface and form drag like on a smooth particle?

Answer 9

surface drag is small, but form drag is large

Question 10

What are the surface and form drag like on a rough particle?

Answer 10

surface drag is large, but form drag goes to 0

Question 11

which is more powerful? surface or form drag?

Answer 11

form drag

Question 12

What is the equation for drag force?

Answer 12

Fd = 1/2ρwAU^2Cd

Question 13

What is the coefficient of drag?

Answer 13

depends on particle shape/ shape factor

Question 14

How do we make the coefficient of drag dimensionless?

Answer 14

Cd = Fd/ρwU^2D^2

Question 15

What determines the conditions in which a stream needs to pick up and move sediment?

Answer 15

coefficient of drag and Reynold’s Number

Question 16

What does the plot for Cd (y) vs. Re (x) look like?

Answer 16

negative line to parabola to constant

Question 17

What does the constant mean?

Answer 17

turbulent

Question 18

What does the parabola mean?

Answer 18

transitioning

Question 19

What does the negative line mean?

Answer 19

laminar

Question 20

Why does the Cd increase as a particle becomes more laminar?

Answer 20

form drag is forming

Question 21

What kind of grain size do you need for a Re of 1 in the world of water?

Answer 21

• small grain size/ roughness
• d50= silt

Question 22

What kind of grain size do you need for a Re of 1 in the world of water?

Answer 22

• large grain size/ roughness
  d50= gravel

Question 23

What kind of grain size do you need for a Re between 1 and 10 in the world of water?

Answer 23

sand

Question 24

What is the equation for the slope of the negative line?

Answer 24

Cd = 24/Re

Question 25

What does Stoke’s Law do?

Answer 25

it predicts how fast a fluid settles when you put a silt particle in a bucket of water
*settling velocity of fluid

Question 26

What is the equation for Stoke’s Law?

Answer 26

U = D^2(ρs-ρw)g/18μ
D- diameter

Question 27

What is Duboy’s equation?

Answer 27

• bed shear stress: τ0= ρghs
• what the water gives you (HAVE)
• s: slope

Question 28

what is a newton?

Answer 28

a force

Question 29

what units does shields criterion have?

Answer 29

θ is dimensionless

Question 30

What is the equation for the shields criterion?

Answer 30

critical shear stress (NEED) : τc= .045 (θ) * (ps-pw) * g * d50

Question 31

What does critical shear stress do?

Answer 31

it determines how much shear stress we need to move a grain of a certain grain size

Question 32

What is the density of water?

Answer 32

1000 kg/m^3

Question 33

What is the density of quartz sand?

Answer 33

2650 kg/m^3

Question 34

When θ vs Re is plotted what is the value of the constant? What does the graph look like?

Answer 34

0.045