DOPPLER ULTRASOUND SPECTRAL SHAPE IN THE POSTSTENOTIC VELOCITY-FIELD

The shape of averaged and smoothed pulsed Doppler spectra from the she ar layer between jet and recirculation zones, one diameter downstream of three acrylic stenosis (27, 47 and 65% diameter reduction) in a ste ady flow dialysis tubing model, were found to be complex. Their shape contrasted with the shape of spectra from regions of jet breakup. The latter had a shape suggesting a Gaussian distribution of apparent velo cities consistent with varying magnitude and direction in turbulent no w. In contrast, the least complex spectra from the shear layer had a d ouble-peaked shape suggestive of two distinct major velocity component s or streamline groups. A subtraction method was used to resolve the l east complex spectra into two relatively narrow Gaussian components. M ore complex spectra could be resolved into multiple components by seri ally subtracting individual Gaussian components. Comparison with the p attern of particles revealed by flow visualization studies suggests th at most of the components are due to axially directed streamline group s of different velocities but that some may be due to radially directe d streamline groups. Broadened spectral components, which are apparent at higher flow rates, are consistent with development of vortices in the shear layer.