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Open
channel flow occurs when liquid flows in a conduit or channel with a free
surface -- like rivers, streams,
canals, and irrigation ditches -- as opposed to the closed pipes common in
industrial and process control environments. Other examples of open channel flow include flow in water treatment
plants, storm and sanitary sewer systems, industrial waste applications,
sewage treatment plants, and irrigation systems.
Many towns and municipalities that monitor drinking water and
wastewater treatment flows use open channel measurement.
Some industrial applications also measure open
channel flow.
Open
channel vs. closed-pipe flow
What is slightly confusing about
the term open channel is that that the
flow of liquids in partially filled pipes -- when not under pressure -- is
also considered open channel flow. Water flowing through a culvert running underneath a street is
considered open channel flow, for example even though the channel is not
actually "open." Likewise,
flow in sewers and tunnels are classified as open channel flows, along
with other closed channels that flow partly filled.
One
way to understand the difference between open channel and closed-pipe flow
is to think of it as the difference between gravity-induced and
pressurized flow:
-
Flow in
uncovered channels like irrigation ditches depends on gravity.
Likewise, flow in partially filled closed conduits, such as
culverts and drain pipes, also is gravity-dependent.
-
Flow in closed pipes for industrial applications,
however, occurs under
pressure.
So open channel
flow might be called gravitational flow, while closed-pipe flow could be
called pressurized flow. This
explains why flows in both uncovered conduits and partially filled pipes
are considered to be open channel flow: They are both examples of
gravitational flow.
How to measure open
channel flow There
are two main ways to measure open-channel flow.
Hydraulic
structure
A very common method of open-channel flow involves the
use of a hydraulic structure such as a weir or flume as a primary device
-- a restriction placed in an open channel that has a known depth-to-flow relationship.
A flume is a specially
shaped portion of the open channel with an area or slope that is different from the channel’s slope or area. The
velocity of the liquid increases and its level rises as it passes through the flume. To determine
flowrate, liquid depth is measured at specified points in the flume.
A weir resembles a dam placed across an open channel. It
is positioned in such a way that the liquid can flow over it. Weirs are classified according to the shapes of their openings. Types
of weirs include: V-Notch; rectangular; and trapezoidal.
Water depth is measured at a specific place upstream from the weir.
Charts correlate various water
depths with flowrates, taking into account different types and sizes of weirs and flumes.
They associate an equation for determining flowrate with each type of
weir:
Velocity
method
Flow can
also be measured without a hydraulic structure. In the area velocity method, the mean
velocity of the flow is calculated at a cross-section, and this value is multiplied by the flow area. Normally, this method
requires that two measurements be made—one to determine mean velocity and another measurement to determine
depth of flow. The area velocity method is used when it is not practical to use a weir or
flume and for temporary flow measurements -- influx and infiltration studies and
sewer measurements, for example.
Click to learn
more about other technologies in:
Opportunities
in open channel flow While pressurized,
closed-pipe flow has grabbed much of the attention in the flowmeter
market, open-channel flow measurement is an exciting area that is worth a closer
look. Custody transfer of water, for example, is a critical application. Environmental regulations and the increasing importance of water as a
natural resource are generating growing interest in open channel flow.
In
addition, the variety of open flow technology types provides room for new
methods. Recent developments, for example, revolve around electronic
enhancements and improvements in communication. There is also room for truly novel developments involving new
or improved sensor technologies o, including Doppler technology.
For more
information about open channel flow, check out our comprehensive
studies on the world flowmeter market.
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