The Weird Science of Tossing Stones Into a Lake

Interoperable writes "Researchers in Spain and the Netherlands add another piece to a centuries-old puzzle in physics: the dynamics of an object falling into water. This common occurrence has a complex anatomy that includes a thin 'crown splash' around the perimeter of the impact, a deep cavity of air following the impactor, and a high, narrow jet of water that results from the collapse of the cavity. The new research, recently published in Physical Review Letters, demonstrates that airflow through the neck of the collapsing cavity reaches supersonic speeds despite low relative pressures between the air in the cavity and ambient pressure. Such an effect has no analogue in aerospace engineering or other sciences because of the highly dynamic nature of the collapsing nozzle structure." It's funny that the APS wants to charge non-subscribers $25 to download what is available for free on the arXiv.

Comments

Re:Viscosity Changes the Sound Too

[mikael]

A similar event happened during the construction of the London flood barrier. For some reason, a large amount of material had to be dumped into the river Thames. This might have been cement or just rubble for part of the underground foundations. It was anticipated that this process might generate some sort of pressure wave that would travel at high speed through the water. Thus a warning was given not to be in the river or close any storm drains at this time. Unfortunately, this warning was not received by a particular tenant of an old apartment block which had one end of their combined storm drain/sewer overflow submerged in the river. Minutes after the construction process had begun there was a complaint through the telephone lines that the tenant in question had just stood up in their bathroom, when the toilet had erupted in a geyser that went everywhere.

This was from the same construction company that attempted to fill in a deep hole in the River Thames with liquid cement, only to find that they had filled in someone's basement / underground car park.