turning in on itself

Irelia20150604

Senior Member
Chinese
Hi everyone! I don't understand the phase "turning in on itself" here. What does it mean?

the context is as follows:
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This April, as undergraduates strolled along the street outside his modest office on the campus of the University of California, Los Angeles, the mathematician Terence Tao mused about the possibility that water could spontaneously explode. A widely used set of equations describes the behavior of fluids like water, but there seems to be nothing in those equations, he told me, that prevents a wayward eddy from suddenly turning in on itself, tightening into an angry gyre, until the density of the energy at its core becomes infinite: a catastrophic ‘‘singularity.’’

source: http://www.nytimes.com/2015/07/26/magazine/the-singular-mind-of-terry-tao.html
 
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