A self-portrait of Leonardo da Vinci done in red chalk.
(Leonardo da Vinci, ca. 1510-1515)
In the 16th century, Leonardo da Vinci described for the first time in a fascinating phenomenon in which water that later became known as the hydraulic jump. And just five centuries later, scientists have finally explained why it happens.
This leap is not some obscure feature that’s only visible to scientists. You really need to walk in your kitchen or jump in the shower to see it.
If you are on a crane, note what happens when water hits the surface of the sink. It creates a very thin, fast flowing, circular layer of water, surrounded by a thick, concentric ring of turbulent water. A hydraulic jump refers to the point where the water rises and forms the thicker layer. [Images: The world’s most Beautiful Equations]
From 1819 with Italian mathematician Giorgio Bidone, many researchers have tried to explain what causes the water to jump in this way. But all the explanations and comparisons to date relied on gravity as the main force, said lead author Rajesh K. Bhagat, a phd student in the department of chemical engineering and biotechnology at the University of Cambridge in England.
However, Bhagat and his team recently found that the force of gravity hardly anything to do with this hydraulic jumps. On the contrary, the large forces behind them are the surface tension and viscosity, they reported in their study, which was published July 31 in the Journal of Fluid Mechanics.
In order to exclude gravity, Bhagat and his team are performed with a simple experiment. They hit a flat, horizontal surface with a jet of water to create a simple hydraulic jump — the same kind that you would see if you turned on the water in the sink. But then, they tilted this surface in different ways: vertically, at an angle of 45 degrees and horizontal, so that in the latter, the flow of the water would be hitting a surface that was a ceiling. For the capture of the initial jump, she recorded what happened with high-speed cameras.
In any case, the hydraulic jump occurred at the same point. In other words, the thin, quick inner layer is of the same size no matter what orientation the plane was in. If gravity was the cause of the jumps, the water would be “twisted,” in one of the aircraft in addition to the horizontal, Bhagat said. “This simple experiment proves it is something, but the force of gravity.”
The new theory is not down with gravity
For the study of the other forces that may be at play, the researchers varied the water flow — viscosity- a measure of how much it can resist flow by mixing it with glycerol, a type of alcohol with a surface tension similar to water, but that is 1000 times more viscous than water.
They kept the viscosity constant, and reduced the surface tension — the attraction that holds liquid molecules together at the surface by mixing in a common ingredient in laundry detergent called sodium dodecyl benzene sulfonate (SDBS). Finally, they varied both the viscosity and the surface tension by mixing water and isopropyl alcohol, another form of alcohol, so that the solution was 25 percent more viscous than pure water, but had a surface tension three times weaker.
This allowed the researchers to isolate the influence of each force, senior author Ian Wilson, a professor of soft fabrics and surfaces, also at the University of Cambridge, told Live Science.
The point is to “be able to predict where this transition between a thin film and a thick film starts,” Wilson said. Many of the previous theories would not do that because the location of the hydraulic jump changes as soon as the thick layer hits a kind of edge, like the edge of the sink.
The jump happens at the place where the forces of surface tension and viscosity add up and balance out the momentum of the liquid jet, the authors found.
They know where the leap first occurs could have applications in the industry, Wilson said. The thin layer that forms for the jump contributes a lot more strength than the thicker layer, making it thinner areas more efficient in transferring the heat.
High-speed water jets are used in industrial applications, such as cleaning and in the processing of milk and the cooling of an aircraft turbine blades or silicon semiconductors , Bhagat said. Often in these applications, intermittent water jets are more efficient, Wilson said. To improve the efficiency of this intermittent jets, you should be able to predict where the first hydraulic jumps happen, ” he said.
Originally published on Live Science.