How to Surf a Supersonic Shock Wave

Shock waves aren’t your ordinary kind of sound wave. They occur in air when the speed of the gas changes by more than the speed of sound, usually when being pushed by a solid body. When that happens, sound waves cannot escape and so form a region of high pressure called the shock front.

It’s hard to imagine why shock waves might be useful but the task is made a little easier today by Nick Parziale at the California Institute of Technology in Pasadena and a couple of pals who explain how to surf on them.

To study the phenomena they placed two nylon balls, one smaller than the other, in a supersonic wind tunnel and blasted them with a supersonic shot of air travelling at Mach 4. They then filmed the resulting shock wave with a technique known as Schlieren photography

The balls start off touching but immediately begin to separate. What’s interesting is that the trajectory of the smaller ball follows the bow wave created by the larger ball. In other words, the smaller ball surfs on the shock wave created by its larger partner.

Parziale and co have also created a 3D computer model of the motion in which they calculate the trajectory of the balls by working out the aerodynamic forces acting on them at any instant. They say the effect is highly sensitive to the shape and geometry of the system, in case you were thinking of reaching for your surf board.

They publish the results in the form of a video for the 2010 APS Gallery of Fluid Dynamics.

If you’re wondering why anybody would be interested in this kind of phenomenon (other than because of naked curiosity), shockwave surfing occurs regularly in nature when an object moving at supersonic speed breaks up. One well known example is the break up of meteorites, when the smaller fiery fragments surf on the shock wave of the primary body creating a kind of 3D image of the bow wave.

Ref: arxiv.org/abs/1010.3248: Experimental Investigation of Shock Wave Surfing

Videos: arxiv.org/src/1010.3248v2/anc