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. The rapidly flowing liquid is abruptly slowed and increases in height, converting some of the flow's initial kinetic energy into an increase in potential energy, with some energy irreversibly lost through turbulence to heat. In an open channel flow, this manifests as the fast flow rapidly slowing and piling up on top of itself similar to how a shockwave forms.

1 Expression for Hydraulic Jump

 Y₂ =y₁  (1+ 8(F₁)²)^1/2   - 1

Where         Y₁      =       depth of flow at section 1-1

          Y₂      =       depth of flow at section 2-2

          F₁      =       Froude number at section 1-1

2 Loss of Energy due to HydraulicJump

h₁ = y₂-y₁³ /  4y₁y₂

Where         Y₁      =       depth of flow at section 1-1

          Y₂      =       depth of flow at section 2-2

3 Uses of Hydraulic Jump

The kinetic energy of flow after the hydraulic jump is greatly reduced, which may prevent erosion of the channel boundaries of downstream side.

4 Classification of Hydraulic Jumps

Based on Froude number (F), hydraulic jump can be classified into 5 types.

a.   Undulation jump: The Froude number F ranges from 1 to1.7 and the liquid surface does not rise shortly but having undulations of radically decreasing size.

b.     Weak jump: The Froude number F ranges from1.7 to 2.5 and the liquid surface remains smooth.

c.     Oscillating jump: The Froude number F ranges from 2.5 to 4.5 and there is an oscillating jet which enters the jump bottom and oscillating to the surface.

d.      Steady jump: The Froude number F ranges from 4.5 to9 and energy loss due to steady jump in between 45 and 70%.

Strong jump: The Froude number greater than 9 and the downstream water surface is rough. Energy loss due to strong jump may be up to 85%.