Venturi flume
Encyclopedia
A venturi flume is a critical-flow open flume
with a constricted flow which causes a drop in the hydraulic grade line, creating a critical depth.
It is used in flow measurement
of very large flow rates, usually given in millions of cubic units. A venturi meter would normally measure in millimetres, whereas a venturi flume measures in metres.
For measurement of discharge
with venturi flumes two measurements, one upstream and one at the throat (narrowest cross-section), are required, if the flow passes in a subcritical state through the flume. If the flumes are designed so as to pass the flow from sub critical to supercritical state while passing through the flume, a single measurement at the throat (which in this case becomes a critical section) is sufficient for computation of discharge. To ensure the occurrence of critical depth at the throat, the flumes are usually designed in such way as to form a hydraulic jump
on the downstream side of the structure. These flumes are called 'standing wave flumes'
s where the critical depth is created by a vertical constriction. First, the hydraulic head
loss is smaller in flumes than in weirs. Second, there is no dead zone in flumes where sediment
and debris
can accumulate; such a dead zone exists upstream of the weirs.
is given by
where K and n are functions of the throat width W.
A venturi flume is similar to the Parshall flume, without the contoured base, but the cross section is usually rectangular, the inlet shorter, and there is a general taper on the outlet similar to the venturi meter
.. Because of their size, it is usual for these meters to be open to their surroundings just like a river or stream and therefore this type of measurement is referred to as open-channel flow measurement. Parshall flumes are much more efficient than standard flumes and generate a standard wave to effect a measurement.
A good example can be found via google earth: 50°58'41.34"N, 5°51'36.81"E, eye altitude 200 m. This is in the 'Geleenbeek', near Geleen in Holland
Flume
A flume is an open artificial water channel, in the form of a gravity chute, that leads water from a diversion dam or weir completely aside a natural flow. Often, the flume is an elevated box structure that follows the natural contours of the land. These have been extensively used in hydraulic...
with a constricted flow which causes a drop in the hydraulic grade line, creating a critical depth.
It is used in flow measurement
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
of very large flow rates, usually given in millions of cubic units. A venturi meter would normally measure in millimetres, whereas a venturi flume measures in metres.
For measurement of discharge
Discharge (hydrology)
In hydrology, discharge is the volume rate of water flow, including any suspended solids , dissolved chemical species and/or biologic material , which is transported through a given cross-sectional area...
with venturi flumes two measurements, one upstream and one at the throat (narrowest cross-section), are required, if the flow passes in a subcritical state through the flume. If the flumes are designed so as to pass the flow from sub critical to supercritical state while passing through the flume, a single measurement at the throat (which in this case becomes a critical section) is sufficient for computation of discharge. To ensure the occurrence of critical depth at the throat, the flumes are usually designed in such way as to form a hydraulic jump
Hydraulic jump
A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface...
on the downstream side of the structure. These flumes are called 'standing wave flumes'
Comparison with weirs
Venturi flumes have two advantages over weirWeir
A weir is a small overflow dam used to alter the flow characteristics of a river or stream. In most cases weirs take the form of a barrier across the river that causes water to pool behind the structure , but allows water to flow over the top...
s where the critical depth is created by a vertical constriction. First, the hydraulic head
Hydraulic head
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
loss is smaller in flumes than in weirs. Second, there is no dead zone in flumes where sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
and debris
Debris
Debris is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, in geology, large rock fragments left by a melting glacier etc. The singular form of debris is debris...
can accumulate; such a dead zone exists upstream of the weirs.
Parshall flume
One of the most common weirs is the Parshall flume which was developed by Dr. Ralph L. Parshall of the U.S. Bureau of Reclamation in 1922. It consists of three sections, a converging section, a throat section and a diverging section. The critical depth develops in the throat section. The flow upstream and downstream of the throat is subcritical and supercritical respectively. The hydraullic jump forms in the diverging section. The flow in the flume is a 'free flow' if the ratio of the downstream head Hs, measured in the downstream stilling well to the upstream head Ha measured in the upstream stilling well, Hs/Ha is less than about 0.70. The discharge for the free- flow conditionFlow conditioning
Flow conditioning ensures that the “real world” environment closely resembles the “laboratory” environment for proper performance of inferential flowmeters like orifice, turbine, coriolis, ultrasonic etc.- Types of Flow :...
is given by
- Q=KHna
where K and n are functions of the throat width W.
A venturi flume is similar to the Parshall flume, without the contoured base, but the cross section is usually rectangular, the inlet shorter, and there is a general taper on the outlet similar to the venturi meter
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...
.. Because of their size, it is usual for these meters to be open to their surroundings just like a river or stream and therefore this type of measurement is referred to as open-channel flow measurement. Parshall flumes are much more efficient than standard flumes and generate a standard wave to effect a measurement.
A good example can be found via google earth: 50°58'41.34"N, 5°51'36.81"E, eye altitude 200 m. This is in the 'Geleenbeek', near Geleen in Holland