Open-Channel Flow
•
any flow that occupies a defined
channel and has a free surface (free
surface flow)
•
the free surface is subject to
atmospheric pressure (if pipe flow, pipe
is not flowing full) - not completely
enclosed by solid boundaries
•
the flow is caused by gravity
component along the slope of the
channel (gravity flow)
•
the pressure distribution within the fluid
is hydrostatic
CIVL3011: HYDRAULICS
1
Classification of flow
• Open-channel flow
– There is a free surface
open to the
atmosphere.
– Driven by gravity.
– Examples:
• Streams, rivers
CIVL3011: Hydraulics
• Pressurized pipe
flow:
– There is not a free
surface.
– Mainly driven by
pressure.
2
Basic Terminologies
Hydraulic Grade Line (HGL)
line drawn through the tops of piezometer
columns
Energy Grade Line (EGL)
line indicating total head
Kinetic Energy
velocity Head
vertical distance between HGL & EGL
CIVL3011: HYDRAULICS
3
Definitions
1
1
2
1
ℎ𝑙 : headloss between
𝑣12
2𝑔
section 1 and section 2
ℎ𝑙
𝑆𝑓
y1
𝑆𝑓 : 𝐸𝑛𝑒𝑟𝑔𝑦 𝑠𝑙𝑜𝑝𝑒
𝑆𝑤 : 𝑊𝑎𝑡𝑒𝑟 𝑠𝑙𝑜𝑝𝑒
𝑆𝑜 : 𝐶ℎ𝑎𝑛𝑛𝑒𝑙 𝑠𝑙𝑜𝑝𝑒
𝑣1
𝑆𝑤
𝑣22
2𝑔
z1
L
𝑆0
𝑣2
Dx
Datum
y2
z2
Figure: Definition Sketch
y
Dx
L
=
=
=
CIVL3011: HYDRAULICS
flow depth (measured vertically)
Horizontal distance between sections
Length between sections (along the channel)
4
Channel Properties/Definitions
• flow depth - vertical distance from the lowest point in a
channel to the water surface
• flow stage - elevation of water surface relative to a
datum
• top width - with of the channel section at the water
surface
• flow area - cross-sectional area of flow normal to the
direction of flow
• wetted perimeter - length of the line which is the
interface between the fluid & the channel boundary
• hydraulic radius - ratio of flow area to wetted perimeter
• hydraulic mean depth - ratio of flow area to top width
CIVL3011: HYDRAULICS
5
Geometric Functions
Source: Mays (2011)
CIVL3011: HYDRAULICS
6
Channel Properties
Source: Chanson (2002)
CIVL3011: HYDRAULICS
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