We have used the technique of laser-Doppler velocimetry to study the transition to

turbulence in a fluid contained between concentric cylinders with the inner cylinder

rotating. The experiment was designed to test recent proposals for the number and

types of dynamical regimes exhibited by a flow before it becomes turbulent. For

different Reynolds numbers the radial component of the local velocity was recorded as

a function of time in a computer, and the records were then Fourier-transformed to

obtain velocity power spectra. The first two instabilities in the flow, to time-independent

Taylor vortex flow and then to time-dependent wavy vortex flow, are well

known, but the present experiment provides the first quantitative information on the

subsequent regimes that precede turbulent flow. Beyond the onset of wavy vortex

flow the velocity spectra contain a single sharp frequency component and its harmonics;

the flow is strictly periodic. As the Reynolds number is increased, a previously

unobserved second sharp frequency component appears at RIR, = 10.1, where R, is

the critical Reynolds number for the Taylor instability. The two frequencies appear to

be irrationally related; hence this is a quasi-periodic flow. A chaotic element appears

in the flow a t RIR, 2: 12, where a weak broadband component is observed in addition

to the sharp components; this flow can be described as weakly turbulent. As R

is increased further, the component that appeared a t RIR, = 10.1 disappears at

RIR, = 19.3, and the remaining sharp component disappears at RIR, = 21.9, leaving

a spectrum with only the broad component and a background continuum. The observance

of only two discrete frequencies and then chaotic flow is contrary to Landau’s

picture of an infinite sequence of instabilities, each adding a new frequency to the

motion. However, recent studies of nonlinear models with a few degrees of freedom

show a behaviour similar in most respects to that observed.