Light Beating Study of the Transition to Turbulence
A light beating technique has been used to study the dynamical behavior of a fluid contained between concentric cylinders with the inner cylinder rotating (circular Couette flow). A laser was focused midway between the two cylinders, and backscattered Doppler-shifted light was mixed at the photocathode with unshifted laser light. The Doppler shift frequency, proportional to the radial component of the velocity for our geometry, is several orders of magnitude higher than the frequencies characteristic of the fluid flow; hence measurements of the frequency of the oscillating photocurrent for successive time intervals (long enough to contain many Doppler cycles) accurately reflect the time dependence of the velocity. The Doppler shift frequency was recorded for 8192 successive time intervals in a computer and then Fourier-transformed to obtain velocity power spectra and autocorrelation functions.