David Acheson



Research






Instability of Vortex Leapfrogging

This research gives a new twist to a 100-year old problem in the theory of vortex motion.



A E H Love (1863-1940)


In 1894, Love showed that two identical vortex pairs can engage in a perpetual leapfrogging motion, but it now turns out that this motion can be unstable, in a very curious way.


"Walkabout" instability of vortex leapfrogging


The research is described in European Journal of Physics Vol. 21, pp 269-273 (2000), and the animation software leapfrog.zip has been specially written to accompany that paper.

Download and unzip, or, alternatively click on the individual, ready-to-run programs

LEAP1.EXE contains pre-set animations of vortex leapfrogging
LEAP2.EXE shows the corresponding vortex trajectories
LEAP3.EXE allows the user to try different parameter values





Classical Dynamics and Control



An upside-down pendulum theorem

Not quite the Indian Rope Trick

Nonlinear inverted pendulum dynamics






Fluid Dynamics


Over-reflection

'Valve'-effect at a critical layer

Enhanced spin-up

'Stability' as a catalyst for instability






Geophysical Fluid Dynamics


Magnetic instabilities of rotating fluids

Westward drift of the Earth's magnetic field





Astrophysical Fluid Dynamics


Magnetically-aided instability of differential rotation

Magnetic buoyancy in the Sun

'Local' analysis of astrophysical instabilities





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