A hydraulic jump 🌊 is a phenomenon that is frequently observed in open channels such as rivers, canals, spillways and weirs and is engineered for purposes of dissipating excess amounts of energy.

A hydraulic jump is formed when high-velocity liquid flow is discharged into a zone of lower velocity, creating an abrupt increase in depth hence, dissipating huge amounts of energy ⚡️.

This energy is the alteration of the flow’s initial kinetic energy to potential energy with some energy lost unavoidably in the form of friction, turbulence, eddying, heat and noise in the process.

The distinctive behaviour of supercritical flow and sub-critical flow is analysed to the extent of understanding the reason for the formation of the hydraulic jump.

Moreover, the parameter of the Froude number (Fr) is very useful towards the understanding of the occurrence of energy dissipation and the creation of the hydraulic jump. Conservation equations such as mass, linear momentum, Froude number and energy are used to investigate and detail analyse the flow of open-channel.

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### PRINCIPLES 📚

In 1914, Raleigh calculated the change in fluid depth associated with the shock wave from a hydraulic jump and introduced the principles of continuity and conservation of momentum.

To better understand the hydraulic jump an understanding of the Froude Number should be made. In simplification, Fr number is a dimensionless quantity that is an indicator of resistance of objects moving in the water and hence the type of the water flow can be defined.

In order to have a hydraulic jump, the Froude number needs to be greater than or equal to 1 which can be defined as Super-critical flow (Fr>1) or critical flow (Fr=1) respectively.

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As shown in figure1 the hydraulic jump is formed when liquid at high velocity (V1) discharges into a zone of lower velocity (V2).

This sudden change in velocity creates an abrupt increase in depth which is typically accompanied by violent turbulence, eddying, air entrainment and surface undulations which contribute to the inevitable loss of energy E of the flow.

V1 is the supercritical flow and it occurs at depths below the critical depth which is the depth at the point of minimum energy. Whereas, V2 is known as sub-critical flow and occurs above the critical depth.

Weak (Undular) jump (1 < Fr1 < 2.5) 💪

Oscillating jump (2.5 < Fr1 < 4.5) 💪💪

Steady Jump (4.5 < Fr1 < 9) 💪💪💪

Strong jump (Fr1 > 9) 💪💪💪💪💪

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### ENGINEERING USES 🛠

The most common use of hydraulic jumps is the dam's spillways. The hydraulic jump is used to dissipate large quantities of energy. The reason for this energy dissipation is due to some factors:

To decelerate the excess kinetic energy of the liquid so to not damage the structure of the dam and cause failure.

It prevents the erosion of the downstream dam surface since the velocity is lowered.

Hydraulic jumps are ideal for the mixing of chemicals for water purification and treatment plans purposes.

Additionally, because of the high turbulent flow, the phenomenon of air entrainment is observed which is useful for removing waste and pollution from the flowing river.

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REFERENCES

· Douglas, J. (2011). Fluid mechanics. Harlow: Prentice Hall, pp.547-550.

· Aboutcivil.org. (2019). Hydraulic Jump - Calculation, Effects & Applications of Hydraulic Jump. [online] Available at: https://www.aboutcivil.org/hydraulic-jump-calculations-effects-applications.html [Accessed 4 Mar. 2019].

· Aboutcivil.org. (2019). Hydraulic Jump - Calculation, Effects & Applications of Hydraulic Jump. [online] Available at: https://www.aboutcivil.org/hydraulic-jump-calculations-effects-applications.html [Accessed 4 Mar. 2019].

· The Constructor. (2019). Hydraulic Jump -Types and Characteristics of Hydraulic Jump. [online] Available at: https://theconstructor.org/water-resources/hydraulic-jump-types-characteristics/12091/ [Accessed 4 Mar. 2019].

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