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What Is a Venturi and the Vena Contracta Zone?


This essay discusses the venturi and vena contracta zone. The vena contracta is explained as the narrowest central flow region in a jet found in the orifice of a regurgitant valve. It determines the fluid flow velocity. The vena contracta occupies less area as compared to the orifice owing to its periphery characteristics.

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The essay provides an insight into the control of velocity. The bore area of the vena contracta refers to the size of the opening that determines the amount of fluid flowing out. This part also controls the rate of flow of the fluid due to pressure difference.

In clinical situations, the vena contracta changes during a cardiac cycle. Various mechanical aspects of the venturi influence the pulsation rate of the fluid. Various applications of the venture and vena contracta zone are also presented in the essay.


A venturi is a small tube that consists of both convergent and divergent conical parts that are connected with a straight portion. The main features of this tube are long and short conical parts. The device is used for determining the flow rate of a fluid down a pipe. One of the areas measures the pressure difference between a venture inlet and neck while determining the flow rate.

The apparatuses are important in this experiment. They consist of several straight tube manometers organized into equally spaced intervals. The importance of this experiment is to determine the hydraulic grade line of the fluid when it passes through toe manometer.

The instrument also consists of a converging section that amplifies the velocity of the flowing fluid. The converging section is known as the vena contracta zone. It has the least diameter along the length of the venturi pipe. The constriction then widens again to match the actual dimensions of the pipe. At the vena contracta zone, the velocity of the fluid reaches the maximum point.

However, the speed is determined by the size of the constriction. There are two sides of the throat, the small and higher parts. They represent the cross-sectional areas of the throat. Since the amount of energy in the flowing fluid should be conserved, the drop in pressure is realized; hence, it helps in measuring the velocity of the fluid at the throat.

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The reading is converted into a volumetric flow rate that is calculated by multiplying the cross-sectional area. This essay explores the venturi with a view of discussing the significance of the vena contracta zone.

Vena Contracta Zone

The vena contracta zone is an important part of the venturi meter. It refers to the area of the tube where the diameter of the stream is smallest. This arrangement helps in increasing the fluid pressure thereby amplifying its speed to a maximum rate. This situation is observed in fuel nozzles in an orifice. In the vena contactor zone, there is minimum cross-section and maximum contraction (Fehske et al., 2003)

Practical Applications

At the outset, the venturimeter principle is used in carburetors to determine the amount of air flowing into the engine of a car. It helps in ensuring that the right ratio of air to fuel is fed to the engine cylinders for efficient combustion. Ensuring proper air and fuel mixture is paramount to the proper operation of the engine.

The engine temperature fluctuates depending on various factors such as idling and acceleration. As this the heat changes occur, a venturimeter installed in the system allows the carburetor to regulate the fuel-air mixture to ensure the generation of maximum power.

A car engine carburetor showing the application of the venturi
Figure 1: A car engine carburetor showing the application of the venturi (Fehske et al., 2003).

The vena contracta concept is also useful in administering treatment to patients suffering from valvular regurgitation. In this situation, the expulsion rate at the muzzle is dictated by the Doppler color streamline imaging (Fehske et al., 2003).

The definition of the vena contracta zone implies that it is the thinnest area of the jet that lies close to the orifice. It is necessary for reflecting the severity of regurgitation. The determination of the diameter of the orifice is relatively easy, especially in clinical settings. It is correlated with the effectively regurgitated orifice area as well as its volume (Fehske et al., 2003).

The vena contracta measurements are realized by determining the cutoff volumes. They are significant in the realization of critical regurgitation in aortic, mitral, and/or tricuspid regulators. In clinical practice, the vena contracta measurements are determined by the diameter, which is one of the methods of assessing the valvular regurgitation (Som, 2014; Fehske et al., 2003).

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The measurements are also useful in the future assessment of complex jet regurgitations. They are also applied to the three-dimensional Doppler flow imaging to improve the performance of the vena contracta quantities (Som, 2014).

The principle underlying the vena contracta is also applied to water flowing from a tank into a pipe. As it flows from the tank to the pipe, the pressure increases significantly due to the smaller opening. As a result, the flow speed amplifies.

The phenomenon is also seen when a pipe meter is pressed. Water jets out when the pipe is pressed at a high speed. The same effect is observed down the stream when its width changes in size. The streamline reduces in width meaning that there is an area of contraction.

In this case, the orifice meter is placed by the flinch between the narrow sections to the line connection. This effect is also found in pipeline connections that provide a coaxial contraction in relation to the path of the fluid as it streamlines to form a vena contracta zone (Som, 2014).

The vena contactor represents the minimum contractual flow in the cross-sectional area that is close to the orifice plate that leads to the downstream flow of the fluid (Som, 2014).

The science is useful in measuring the speed and force of fluids in various departments. In addition, the speed before the contraction is different to the velocity after contraction; hence, the force in the vena contracta zone increases significantly.

Although various explanations for the change of speed exist, the primary reason is that any matter in constant speed cannot abruptly change direction. For example, an unrestricted jet that has an abrupt diameter difference along its length can lead to the interruption of the streamlines due to the immediate angular change (Baum, 2015). Streamlines that tend towards a point adhere to an even path. This situation results in either a thin or a widespread jet from the opening of the pipe.


Vena contracta measurements are important in several areas that include calculating the speed of an engine horsepower, echocardiography, and dam construction. The measurements help in describing the smallest area of the fluid jet in a heart valve or other types of entry and exit points.

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Therefore, the venturimeter principle cannot be undermined in the modern world where airflow technology is still applied to many innovations. These devices are also used in some applications to reduce pressure losses while realizing diverse metering precisions.

Reference List

Baum, D. (2015). Selection and the Origin of Cells. Bioscience, 1(1), 678 – 684.

Fehske, W., Omran, H., Manz, M., Köhler, J., Hagendorff, A., & Lüderitz, B. (1994). Color-coded Doppler imaging of the vena contracta as a basis for quantification of pure mitral regurgitation. The American Journal of Cardiology, 73(4), 268-274.

Som, P. (2014). Fluid Mechanism. Kharagpur: Indian Institute of Technology.

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