Supersonic phenomenon in the kitchen

According to a recent study (Gekle, et al., 2010), one can make a supersonic air jet flow by just dropping a solid object into water in the kitchen. The "jet" to scientists usually refers to a high-speed flowing column of material (e.g. air or water). The jet is quite common in daily life e.g. water out of a hose for cleaning. The aircraft is usually pushed forward by a supersonic air flow out from the back of engine, termed turbo-jet.

When we drop a heavy solid object into a liquid (e.g. a marble into a bowl of water), three jets will be generated during the process (Lathrop, 2010): one of upward supersonic air jet, an upward water jet and a  downward water jet toward the object (e.g. marble in this case). When the air escaping through the "neck" formed during this process, it can be greatly accelerated even to supersonic flow by the pressure difference between the collapsing cavity and outer space (Gekle, et al., 2010).

The impact of a heavy object on a fluid surface causes a sequence of events leading to the creation of three jets. From Lathrop (2010). Link

In Gekle, et al. (2010), a carefully controlled disk is used. The disk  is pushed into the water with certain speed and the whole process was recorded using high-speed camera. The physical process is similar like those described above. Fine smoke particles were used to measure the speed of the air flow. Disk with different shapes were also used to study the collapse of non-symmetric cavities (Enriquez et al., 2010).

Supersonic air flow. From Gekle et al. (2010, Fig.1). 

Collapse of non-symmetric cavities. From Enriquez et al. (2010). Link

Recorded sequences from the top of disk during collapse of non-symmetric cavities. From Enriquez et al. (2010). More information is available on the website of Physics of Fluids Group, University of Twente, Netherlands. Link

You can try it in your own kitchen!

References
J. Eggers and E. Villermaux, Rep. Prog. Phys. 71, 036601 (2008).
Enriquez, O.R. et al. (2010). Collapse of nonaxisymmetric cavities, Phys. Fluids 22, 091104. Link

D. P. Lathrop, Making a supersonic jet in your kitchen, Physics 3, 4 (2010). Link

S. Gekle, I. R. Peters, J. M. Gordillo, D. van der Meer, and D. Lohse, Phys. Rev. Lett. 104, 024501 (2010).

S. Gekle, J. M. Gordillo, D. van der Meer, and D. Lohse, Phys. Rev. Lett. 102, 034502 (2009).