Plug Flow
In plug flow, all the red blood cells move at the same velocity, producing a flat wave front. The Doppler spectrum from this flow pattern is characterized by a narrow spectral line and a clear spectral window, which represent the absence of lower velocities (2). That waveform typically is seen in large arteries, such as the aorta.

Laminar Flow
In laminar flow, peripheral red blood cells move at a slower rate than central red blood cells do, because of friction offered by the vessel wall. This difference in flow velocities produces a parabolic wave front. In Doppler US spectra, laminar flow appears as broadening of the spectral line and filling of the spectral window . That waveform usually is seen in vessels with a diameter of less than 5 mm.

Turbulent Flow
Turbulent flow consists of a wide range of velocities, includes reversed flow components, and is readily appreciated as multiple colors on color Doppler images. In spectra, a turbulent flow pattern is visible as spectral broadening with components below the baseline. Turbulent flow is considered normal near vessel bifurcations (eg, the carotid bulb) but elsewhere is suggestive of abnormality .


http://radiographics.rsna.org/content/28/3/691.full