6.17. (9/30) Bedload; suspended load; dissolved loads; shear stresses; frictional forces Flashcards
lab too
What is the Reynold’s Number?
- 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
What is the equation for Reynold’s Number?
Re= -ρwUd50/μ
ρw - density of water
U - velocity
d50 - roughness/ diameter
μ - viscosity
what happens if Re>10?
There is a turbulent flow (shaky movement, with air)
what happens if 1>Re?
There is a laminar flow (steady, slow stream, pillar of water)
What has to be big in the Reynold’s equation for a flow to be laminar?
- the viscosity
- for water, the diameter or roughness must be small
What is flow separation?
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
what is form drag?
- holds particles back
- causes the biggest amount of friction
- unstable vacuum
What is surface drag?
how smooth a particle is
What are the surface and form drag like on a smooth particle?
surface drag is small, but form drag is large
What are the surface and form drag like on a rough particle?
surface drag is large, but form drag goes to 0
which is more powerful? surface or form drag?
form drag
What is the equation for drag force?
Fd = 1/2ρwAU^2Cd
What is the coefficient of drag?
depends on particle shape/ shape factor
How do we make the coefficient of drag dimensionless?
Cd = Fd/ρwU^2D^2
What determines the conditions in which a stream needs to pick up and move sediment?
coefficient of drag and Reynold’s Number
What does the plot for Cd (y) vs. Re (x) look like?
negative line to parabola to constant
What does the constant mean?
turbulent
What does the parabola mean?
transitioning
What does the negative line mean?
laminar
Why does the Cd increase as a particle becomes more laminar?
form drag is forming
What kind of grain size do you need for a Re of 1 in the world of water?
- small grain size/ roughness
- d50= silt
What kind of grain size do you need for a Re of 1 in the world of water?
- large grain size/ roughness
d50= gravel
What kind of grain size do you need for a Re between 1 and 10 in the world of water?
sand
What is the equation for the slope of the negative line?
Cd = 24/Re
What does Stoke’s Law do?
it predicts how fast a fluid settles when you put a silt particle in a bucket of water
*settling velocity of fluid
What is the equation for Stoke’s Law?
U = D^2(ρs-ρw)g/18μ
D- diameter
What is Duboy’s equation?
- bed shear stress: τ0= ρghs
- what the water gives you (HAVE)
- s: slope
what is a newton?
a force
what units does shields criterion have?
θ is dimensionless
What is the equation for the shields criterion?
critical shear stress (NEED) : τc= .045 (θ) * (ps-pw) * g * d50
What does critical shear stress do?
it determines how much shear stress we need to move a grain of a certain grain size
What is the density of water?
1000 kg/m^3
What is the density of quartz sand?
2650 kg/m^3
When θ vs Re is plotted what is the value of the constant? What does the graph look like?
0.045
