Saturday, July 14, 2012

Helpful Cavities

Exploring the problems in sailboats approaching 60 knots (I can dream) we saw how cavitation is a real problem.

But apparently this phenomenon, when pushed to the extreme can be a real boon - at least for some objects like torpedoes and maybe for boats.  I refer to supercavitation.

As you will remember, cavitation is liquid turning into vapor (bubbles) due to a pressure differential - for instance bubbles forming on the low pressure side of a foil when that reduced pressure goes below water's vapor pressure. Huge drag ensues.

In supercavitation you turn a real drag into a huge boost by "simply" making the bubble large enough to encapsulate the entire object, thus drastically the amount of wetted surface and drag.  The trick to doing this is to inject additional gas into the bubble until it gets large enough.

The photo above is from the University of Minnesota "cavitation and bubbly flows research group" (as Dave Barry would say, a great name for a rock band).  It shows axisymmetric ventilated supercavitation with ventilation increasing and cavitation index decreasing from top to bottom, but I am sure I didnt have to tell you that.

At any rate, the military superpowers have long been interested in supercavitation as a way of making torpedos and other underwater projectiles go very fast. The Russians made the Shkval VA-111 torpedo which allegedly goes 500 km/h.  DARPA has the Underwater Express program which envisions a submarine at 100 knots and according to some sources a one-quarter size model has already been tested off Rhode Island.   Tillerman, have you noticed any unusual currents lately?

And now a private company claims to have a developed a supercavitating stealth watercraft.

And finally if you have a hankering to build one yourself, Scientific American can give you some hints, Although they do mention that it will require "a deep understanding of the fluid mechanics and hydrodynamics of this novel flow regime" this shouldn't be a problem for my readers (note I did not include myself).

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