Jump

Pouring and shooting at an attack weir: just below the weir crest, the drain goes from pouring to shooting, dissolves into individual falling plumes of water and turns "white". Down in the water roller in front of the falling water, it jumps from shooting to pouring.

Hydraulic jump in the sink. In the center there is a shooting runoff, the roughly circular wave edge marks the jump to the flowing runoff.

The alternating jump (also water jump or hydraulic jump ) is a term from the hydromechanics of open channels . It is an abrupt change in flow from very fast to very slowly flowing water. A hydraulic jump is z. B. the water roller .

The reverse case (see section on delimitation below ) is also a flow change, but takes place continuously.

Discharge velocities related to the water depth

If the runoff in a channel is faster than the speed of propagation of disturbances ( waves ), then this is called torrential or supercritical runoff ( ). ${\displaystyle Fr>1}$

If the runoff is slower than the speed of propagation of disturbances, then this is called flowing or subcritical runoff ( ). ${\displaystyle Fr<1}$

The Froude number is used to differentiate :

${\displaystyle Fr={\frac {v}{\sqrt {gh}}}}$

With

• v = mean speed,
• g = acceleration due to gravity ,
• h = water depth.

Jump

An alternating jump (hydraulic jump) occurs at the flow transition from the shooting to the flowing discharge:

• Transition occurs suddenly (discontinuously)
• significant energy losses when shooting from the upper water into the lower water
• The water depth is greater than the water limit
• Speed ​​is less than the speed limit
• if the inflow is increasing with Fr <1.7, an undulating or undulating alternating jump occurs

An analogous phenomenon are shock waves in gas flows. The Mach number in gases corresponds to the Froude number in channel flows.

The mechanism of energy conversion in the hydraulic jump is to that in the breaking of breakers used.

Demarcation

A second type of flow transition (no jump) occurs from flowing to flowing drainage:

• The prerequisite is the channel discontinuity
• Change in water depth is continuous
• Speed ​​is greater than the speed limit.

See also

• Compression shock - analogous phenomenon in gases that behave as a compressible fluid
• hydraulic jump in the foehn weather phenomenon

literature

• Gerhard H. Jirka, Cornelia Lang: Introduction to Channel Hydraulics , Universitätsverlag, Karlsruhe 2009, ISBN 978-3-86644-363-1

Web links

• M. Koch: Technical hydraulics (PDF; 205 kB) , script, University of Kassel
• Gerhard H. Jirka, Cornelia Lang: Introduction to channel hydraulics. (PDF) Archived from the original on March 1, 2014 ; accessed on March 17, 2015 (8400 kB). 

Individual evidence

1. ↑ Wolfgang Schröder : Fluid Mechanics. Wissenschaftsverlag Mainz in Aachen 2004 , ISBN 3-86130-371-X , p. 125.