physics4me

Alejandro Jenkins

Illustration of the turbulent vortices generated by a flow of velocity v as it hits a circular obstacle of diameter d, on the left

Physicists are very familiar with forced and parametric resonance, but usually not with self-oscillation, a property of certain linear systems that gives rise to a great variety of vibrations, both useful and destructive. In a self-oscillator, the driving force is controlled by the oscillation itself so that it acts in phase with the velocity, causing a negative damping that feeds energy from the environment into the vibration: no external rate needs to be tuned to the resonant frequency. A paper from 1830 by G. B. Airy gives us the opening to introduce self-oscillation as a sort of “perpetual motion” responsible for the human voice. The famous collapse of the Tacoma Narrows bridge in 1940, often attributed by introductory physics texts to forced resonance, was actually a self-oscillation, as was the more recent swaying of the London Millenium Footbridge. Clocks are self-oscillators, as are bowed and wind musical instruments, and the heartbeat. We review the criterion that determines whether an arbitrary linear system can self-oscillate and describe the operation of two thermodynamic self-oscillators, the putt-putt toy boat and the Rijke tube, before concluding with a brief discussion of the relevance of the concept of self-oscillation to the semi-classical theory of lasers….
Read more: http://arxiv.org/PS_cache/arxiv/pdf/1109/1109.6640v1.pdf