Channel flow Flashcards
What is steady uniform flow
Occurs when gravity balances friction force
What are the conditions of steady uniform flow
- Flow is steady
- Bed slope is positive
- Artificial channel with constant roughness
- Channel is long, without any obstacle disturbances
What are two types of non-uniform open channel flow?
Rapid varied flow
Gradually varied flow
What is rapid varied flow?
Flow changes ins hort lengths of channel where friction can be ignored
What is gradually varied flow?
Flow changes in long lengths of channel where friction dominates
What is a flow control?
a device which creates a unique relationship between depth and discharge
What are the energy losses in open channel flow?
- Friction losses, accumulate with distance
- local energy losses, due to sudden changes in channel cross-sections
over short reaches of a channel what is the total head?
Constant
What is plotted on a specific energy curve?
Specific energy (E) vs. depth (y) for a fixed discharge (Q)
What is critical flow?
The point of minimum specific energy and maximum discharge
What is critical velocity?
The velocity corresponding to the critical flow
On a specific energy diagram where does super-critical flow occur?
below the critical flow line
On a specific energy diagram where does sub-critical flow occur?
Above the critical flow line
What does the froude number define?
The regime of flow (FR=1; critical flow)
Is uniform flow condition a flow control
Yes it is
Will both total head and specific energy reduce along the channel, and why?
Specific energy is part of total head, which reflects flow characteristics.
The difference between them is , the level of channel bed
-The total head would reduce, due to friction, but specific energy is related to the channel bed so could go up down or stay the same….
Equation for critical water depth (yc) when we have a constant q?
yc=(q^2/g)^(1/3)
Equation for critical water flow (yc) when we have a constant E (total head)
yc=2/3 E
Equation for critical velocity
Vc=(gyc)^0.5
What is E
Specific energy
What’s the equation to get the depth corresponding to critical flow for non-rectangular channels?
Q^2/g=A^3/B where B is the top width of the channel and this is because dA/dy=approx. B
What’s the equation for the Froude Number?
Fr=V/SQRT(gy)
What does a flow control do
Defines the depth at which water will flow in an open channel; often transitions a flow from subcritical to supercritical
What are some examples of engineered flow controls:
- Sluice Gates
- Weirs
- Flumes….
What occurs during flow under a sluice gate
No energy loss, depths remain same
What occurs during flow over a weir?
There is a change in height (Z)
What occurs in a Venturi Flume constriction
It is designed to produce critical; depth, so flow measurements can be taken
What’s the area of a trapezoidal cross section?
(b+my)y
What is the most economic cross-section
The cross section that will give maximum discharge for a given area.
How can the most economic cross section be calcualted?
For a given A and S, Q will be maximum for a maximum Rh
Rh=A/P so max Rh means having a minimum P
Use dp/dy=0 to find these values
What are non-erodible channels?
A technique of minimising lining material costs by creating teh most economic cross-section
What are erodible channels?
based on the principles of maximum permissible velocity, creating a stable hydraulic section.
What is Manning’s equation (velocity)
V=1/n Rh ^2/3 S^1/2
