Thursday, August 09, 2018

Hydraulic jump: New insights into a very old phenomenon

Ever since I learned about them, I thought that hydraulic jumps were cool.  As I wrote here, a hydraulic jump is an analog of a standing shockwave.  The key dimensionless parameter in a shockwave in a gas is the Mach number, the ratio between the fluid speed \(v\) and the local speed of sound, \(c_{\mathrm{s}}\).   The gas that goes from supersonic (\(\mathrm{Ma} > 1\)) on one side of the shock to subsonic (\(\mathrm{Ma} < 1\)) on the other side.

For a looong time, the standard analysis of hydraulic jumps assumed that the relevant dimensionless number here was the Froude number, the ratio of fluid speed to the speed of (gravitationally driven) shallow water waves, \(\sqrt{g h}\), where \(g\) is the gravitational acceleration and \(h\) is the thickness of the liquid (say on the thin side of the jump).  That's basically correct for macroscopic jumps that you might see in a canal or in my previous example.

However, a group from Cambridge University has shown that this is not the right way to think about the kind of hydraulic jump you see in your sink when the stream of water from the faucet hits the basin.  (Sorry that I can't find a non-pay link to the paper.)  They show this conclusively by the very simple, direct method of producing hydraulic jumps by shooting water streams horizontally onto a wall, and vertically onto a "ceiling".  The fact that hydraulic jumps look the same in all these cases clearly shows that gravity can't be playing the dominant role in this case.  Instead, the correct analysis is to worry about not just gravity but also surface tension.  They do a general treatment (which is quite elegant and understandable to fluid mechanics-literate undergrads) and find that the condition for a hydraulic jump to form is now \(\mathrm{We}^{-1} + \mathrm{Fr}^{-2} = 1\), where \(\mathrm{Fr} \sim v/\sqrt{g h}\) as usual, and the Weber number \(\mathrm{We} \sim \rho v^{2} h/\gamma\), where \(\rho\) is the fluid density and \(\gamma\) is the surface tension.   The authors do a convincing analysis of experimental data with this model, and it works well.  I think it's very cool that we can still get new insights into phenomena, and this is an example understandable at the undergrad level where some textbook treatments will literally have to be rewritten.

1 comment:

Anonymous said...

This is a nice talk by Sir Roger Penrose on what the universe was before big bang;