Flow Measurement

What is an open channel flow measurement?

What is an open channel flow?

The open channel flow is the flow of the liquid on a free surface, some of the open channel flows are dams and irrigation channels. The surface pressure will be equal to the atmospheric pressure in an open channel. There is some human-made open-channel flow and they are drainage ditches, sewers, and freshwater aqua-ducts. An open channel flow would have free airspace above, so by this, we can say that an open channel could be a visible channel or it could be a closed pipe that doesn’t have a full flow.

What is the difference between the open channel flow and the pipe flow?

  • The open channel flow would have a free surface while the pipe flow doesn’t have a free surface.
  • The open channel flow is subjected to atmospheric pressure while the pipe flow doesn’t have any direct atmospheric pressure instead of that it has hydraulic pressure.
  • The driving force in an open channel flow is mainly gravity and also the flow direction, in the case of the pipe flow, the driving force is mostly the pressure force along the flowing direction.
  • The flow area in the pipe flow is according to the pipe dimensions and the pipe is in a circular shape and thus the flow will be according to it. In the case of the open channel, the flow area is determined by the geometry of the channel and also the free surface level and it would change over time and also because of the flow direction
  • The pipe cross-section is circular but in the case of the open channel flow it could be irregular forms, the OCF is mostly natural streams.
  • The relative roughness is constant in the case of pipe but in the case of the OCF it is not, the relative roughness would vary according to the free surface level.

What are the different types of open channel flow and how gravity would affect the flow?

The depth of the flow would vary according to the obstruction, there are two types of open channel flow and they are rapidly varied flow and gradually varied flow. The rapidly varied flow would take place over a short distance near the obstacle, while the gradually varied flow would occur over a long distance. The gravity is the major part of an open channel flow, the driving force for the flow is due to the gravity and thus the flow would take place in a good manner.

How to do the open channel flow measurement?

This is a type of flow meter that would measure the flow through an open channel there would be an obstruction so when the liquid passes over this obstruction the liquid height will be measured. So this type of meter can be used to measure the liquid flow in an open channel, these meters can be used for continuous or precise flow monitoring in partially filled tunnels, pipes, canals, rivers, and streams. The open channel flow meters are easy to install, with the help of these flow meters we could determine the instantaneous flow rate by measuring the water depth or head. The open channel flow measurement is done in two ways they are weirs and flumes flow measurement.

How does an open channel flow meter work?

The water level is measured in an open channel by creating a hydraulic structure such as a weir or flume. The water depth will be converted to flow rate and this flow rate can be calculated over time and thus volumetric flow rate can be determined.

By doing the depth measurement the cross-sectional area of the flow can be calculated and also the flow velocity can be determined too. So by using this velocity and the cross-sectional area we could get the volumetric flow.

The open channel flow meter has two components and they are the primary hydraulic control element and the measuring device.

The primary hydraulic control element

The primary control element would control the hydraulic condition in a pipe or in a flume, there are many types of primary hydraulic control elements and we could choose them according to our application. The PHCE should be used if the suspended solids are a major concern. The PHCE should be selected according to the flow range. The major types of primary control elements are flumes, weirs, and nozzle.

Measuring device

These devices are used to determine the depth or velocity, the flow measurement is also depended on how the measuring device would influence the flow. The measuring devices are of contact and noncontact type certain censors are placed in contact with the flow stream while there are some other which is noncontact type.    

Types of open channel flow measurement

Weirs

The operation of this type of flow is based on the principle that if there is an obstruction in the water channel then it would make the water back up. So because of this, there will be a high level behind the barrier. The high level is due to the flow velocity and thus we could determine the flow rate through the device. The weirs are divided according to their shape there are three major types of weirs such as V-notch, rectangular, and trapezoidal. The weir structure would act as an obstruction to the water flow and thus it would increase the water level and this can be measured.

Rectangular weir

The rectangular weir can be used to measure higher flows, these weirs are mostly used in water supplies, wastewater, and sewage systems. This type of meter would have a sharp-edged plate. A rectangular weir has sharp edges and because of this the water would flow freely and thus accurate measurements can be done. In a rectangular weir, the measuring point is not at the weir crust. The disadvantage of the rectangular sharp-crested weir is that since some of them don’t have end contractions and the channel walls affect the flow over the weir, thus the wall condition could affect the flow.

V notch weir

The V-notch weir has a V-shaped opening and this type of weir is used to measure low flows within a narrow operating range. This type of weir would accurately measure low discharges. In this weir, as shown in the below image there will be a ninety-degree opening and its sides will be inclined forty-five degrees with vertical. This type of weir has no crest length and due to this, it is used for very smaller flow measurements. We can use V-notch weirs in case if there is frequent low flow is present in the overall flow.  The v notch weir as discussed above is designed in a way that it would allow the fluid to travel through a small opening and because of this the depth change in the fluid can be easily determined. So a V-notch weir should be selected according to the amount of flow through the V-notch and also according to the amount of change in the flow depth.

What are the major applications notch and weirs?

  • It can regulate the flow in channels
  • It could act as an emergency spillway in order to regulate the high flows
  • It would control the flow through an unsubmerged culvert that operates under the inlet control
  • Irrigation water measurement
  • Industrial process discharge
  • It can control the pond overflow
  • Wastewater treatment plant discharge

Flumes

This type is used when the head needs to be minimum or in case if there is a presence of suspended solids in the flowing liquid. The major types of flumes are Parshall and palmer bowlus designs. A flume is a type of flow channel which could be either on or above the ground so that the water can be transferred to the desired place. The flume is designed in a way that would restrict the channel area and it would also change the channel slope. Thus the liquid velocity would increase and also the level of the liquid would change too.

Parshall flume

The Parshall flume has a converging upstream section and a diverging downstream section. These are mostly used for smaller installations. The head loss of this type of open channel meter is really low and these devices are capable to measure the flow over a wide range of conditions. The location of the measuring point must be selected properly to get proper hydraulic conditions through the control element. The flow rate in this device is determined by measuring the level of the liquid one-third of the way into the converging section. While choosing a Parshall flume we must consider the throat width and other dimensions.

Palmer bowlus flume

This device can be used to measure the flow in a sewer system, and these are also easily adaptable to a variety of sewer diameters. The hydraulic principle is the same as the Parshall flume, the flow rate through this device can be determined by measuring the depth of the liquid at a point one-half pipe diameter upstream from the flume throat. These devices must be selected according to the size of the pipe into which they are going to be installed.

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