A hydraulic jump


When water hits water, a fluid-mechanical mystery arises.


A hydraulic jump created by water flowing into a sink.
A hydraulic jump created by water flowing into a sink.
James Niland

A hydraulic jump is an abrupt change in the level of a liquid that occurs when a fast-moving stream encounters a slower body of liquid.

In the image above, high-velocity water is flowing from a tap into a sink already filled with water. At first, the tap water spreads out quickly and smoothly, but as it loses energy it slows down and ‘piles up’ in a ring around the point of inflow. This ring is the hydraulic jump.

Leonardo da Vinci documented the phenomenon as long ago as the 1500s, but its precise mechanics have until recently been a mystery. Hydraulic jumps are harmless in our household sinks but they can cause violent waves, turbulence and whirlpools in deeper water.

Since the 1820s scientists have believed that hydraulic jumps occur partly as a result of the gravitational pull forcing the liquid downward.

A recent paper published in the Journal of Fluid Mechanics has disproved this longstanding theory.

Rajesh Bhagat, a PhD student at the University of Cambridge, fired jets of water upwards and sideways onto flat surfaces, and witnessed exactly the same hydraulic jumps as those when the water flowed downwards.

This result helped him to prove that hydraulic jumps are purely a result of the surface tension and viscosity of the fluid.

  1. https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/on-the-origin-of-the-circular-hydraulic-jump-in-a-thin-liquid-film/328F7B8C8A9419180DE89D9DE320BDE9
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