lecture 3 - Fluid Flow Flashcards
Sheet flow
Flow that runs on the surface, eroding the top soil
What is Matrix flow? Is The ground permeable?
water flowing through porous material such as soil, which is is permeable
Pipe Flow
Flow in enclosed conduits (water pipes)
What is the continuity equation?
Q = W x D x V
Discharge = width x depth x velocity
or Q = VA
Discharge = velocity x cross-sectional Area (of width and depth)
How does Newton’s 2nd law affect water mechanics?
It is the law of conservation of Mass
P = m x v
Momentum = mass x velocity
Where is the least Flow resistance in natural rivers?
Water will be the fastest in the centre of the channel, just below the surface. This is due to natural friction from the channel bed and banks.
What is Darcy’s law?
It is a calculation for flow resistance
ff = 8gRs/v squared
ff = resistance coefficients
g = gravitational acceleration
s = water surface slope
R = hydraulic radius
v = flow velocity
What is the Manning’s equation?
Its based off the continuity equation.
v = R2/3x S1/2 / n
symbols are the same as darcy’s law
What are the 4 ways of describing flow?
a) Uniform / Non-uniform
b) Steady / Unsteady
c) Laminar / Turbulent
d) Sub-critical / Super-critical
Describe the flow in uniform and non-uniform flow
Uniform flow has a velocity constant with position.
Non-uniform flow has a velocity variable with position. Rivers are naturally non-uniform due to the channel beds and banks.
If a channel narrows, what happens to discharge, width, depth and velocity of the flow?
- The discharge will stay the same
- The depth will increase due to bed incision
- The width will decrease
- The velocity will increase
Describe the differences between steady and unsteady flow
Velocity is constant over time in steady flow.
Velocity is variable over time in unsteady flow.
Natural river channels have unsteady flow.
Where can laminar flow be found within a channel of water?
At the bottom of the channel, closest to the riverbed in the no-slip zone
What is sub-critical flow, and where can it be found?
In a hydraulic drop, it is found at the deeper, slower water before the drop in elevation.
In a hydraulic jump, it can be found downstream of the jump, where the water is deeper and has a slower velocity.
What is the froude number?
It is a number that defines the transition from sub-critical to super-critical flow.
The equation is F = V/ square root of g x d
F = froude number
(<1 =sub-crit; >1 =super-crit)
v = flow velocity
g = gravitational acceleration
d = flow depth
