in

Hydraulic jump: phenomenon of rising water finally solved

Credits : Zeimusu / Common Files Wikipedia

After waiting five centuries to be solved, the hydraulic jump phenomenon, famously observed by Da Vinci, has finally found a satisfying explanation. Without even noticing it,  when you turn on the tap in your kitchen and look into your sink you will see a hydraulic jump.  Since the famous Italian painter and inventor coined the term in the 16th century, this phenomenon has remained a mystery until now.   

 What is a hydraulic jump?

When water hits the sinks surface, you will see a disk appear. The layer of water is very thin and flows very quickly. This very thin disk is surrounded by a thicker ring of turbulent water. The hydraulic jump refers to the moment when the water rises to form a thicker circle.

Not linked with gravity as previously thought

For centuries, mathematicians racked their brains to explain this phenomenon only to hit a brick wall.  The major role of gravity form the basis of almost all of their equations and theories.

The lead author of the new study concerning the hydraulic jump is a PhD doctorate from the Chemical Engineering and Biotechnology department at the University of Cambridge, England. Rajesh K. Bhagat and his team believe that if nobody has found gravity as the major force of the phenomenon already, then perhaps that suggests that gravity does not play an important role.

Rajesh K. Bhagat and his team created a simple experiment to definitively rule out gravity as a source.   The classic hydraulic jump is created by a jet of water hitting a flat, horizontal surface.  However the group tilted this flat surface in different ways so that in the end the water hit the ceiling. So as to not miss anything, they filmed the hydraulic jump with the help of a camera that could take an enormous amount of images per second.

In all cases, the hydraulic jump took place in the same way.  The thin and fast layer was the same size regardless of the surface orientation. If it is was gravity that brought about these jumps, the water would have been “deformed” when it wasn’t a flat surface. To conclude, Rajesh K. Bhagat explained that “this simple experiment proves that gravity is not the cause of the phenomenon.”

So what does create the thick circle during a hydraulic jump?

Viscosity

Here are different definitions of the word viscosity according to Collins dictionary

  • The state or property of being viscous
  • The extent to which a fluid resists a tendency to flow
  • A measure of the resistance, equal to the tangential stress on a liquid undergoing streamline flow divided by its velocity gradient.  It is measured in newton seconds per metre squared.
  • The viscosity of a particular fluid is how easily it flows.

For example, if you poured water into a glass and you poured mayonnaise into another, you will see a difference in the flow as the liquid lands at the bottom of the glass. If you wait five minutes the water will become calm and still whereas the mayonnaise will remain in the same initial state. These differences are explained due to the different viscosity levels between the two substances.

During the experiment carried our by Rajesh K. Bhagat the liquid was 25% more viscous than pure water.

viscosite GIF
Crédits : Common Files Wikipédia

Surface tension

Have you ever seen a full glass that is almost overflowing?  On top of the glass a sort of domed summit begins to form.  The peak is higher than the rim of the glass but the water does not spill.  Surface tension is the force that makes a surface area change state to “save” energy. This is why a dome forms at the top of the glass, or a drop of water can form.

In his experiment, Rajesh K. Bhagat always used a more viscous liquid but with a surface tension three times weaker.

goutte d'eau
Credits : Pixabay

Conclusions

These techniques allowed researchers to isolate the influence of each force,” explained Ian Wilson in Live Science, lead author of the study and professor at University of Cambridge. The authors were able to determine that the jump occurs when the surface tension forces and viscosity add up and balance the momentum of the liquid jet.

What does this discovery mean?

According to the authors of the study, their discovery will be useful in creating industrial cleaning products and for increasing the performance of cooling systems.

Sources : Journal of Fluid Mechanics – LiveScience

Related articles: