Flow disturbance measurements through a constricted tube at moderate Reynolds numbers.

Instantaneous velocities in the field distal to contoured axisymmetric stenoses were measured with a laser Doppler anemometer. Upstream flow conditions were steady and spanned a range of Reynolds numbers from 500 to 2000. Autocorrelation functions and spectra of the velocity were employed to describe the nature of fluid dynamic disturbances. Depending upon the degree of stenosis and the Reynolds number, the flow field contained disturbances of a discrete oscillation frequency, of a turbulent nature, or both. If turbulence was detected in a given experiment, it was always preceded upstream by velocity oscillations at discrete frequency arising from vortex shedding. For mild degrees of stenosis (50% area reduction or less) the intensity of flow disturbances was relatively low until the Reynolds number exceeded 1000, thus highlighting difficulties to be expected in employing flow disturbance detection as a diagnostic tool in the recognition of early atherosclerosis in major arteries. In view of the relatively high noise levels inherent in noninvasive Doppler ultrasound systems employed clinically, it seems unlikely that detection of stenosis of less than 50% area reduction is feasible unless the Reynolds numbers exceed 1000 or unless pulsatility introduces new unsteady flow features beyond those studied here.