Case Studies of the Human Cardiovascular System - Computational Hemodynamics: Theory, Modelling and Applications - page 192

Fig. 7.11 Streamline plots pertaining to patient-specific carotid bifurcations. Streamline plots
based on anatomical realistic geometries can demonstrate the regions of high velocity and also
indicate the onset of turbulence from its laminar regrime. a shows a healthy normal carotid bifurca-
tion, while b and c show stenosed carotid bifurcations at the ECA and ICA respectively
which redistributes the flow velocity and further intensifies the skewness of the
flow structure (Vetel et al. 2009).
The velocity contours with streamlines passing through show that at the neck
of the carotid stenosis, there is increased velocity and many streamlines converge
through (Fig. 7.11 ). A higher flow velocity also corresponds to a higher Reynolds
number, which initiates turbulence. Streamlines are useful for identifying the tran-
sition from laminar to turbulent flow. For example, laminar flow usually produc-
es streamlines with tangential velocities aligned in their directions as shown by
Fig. 7.11a , whereas turbulent flows are characterized by more random motion and
indistinguishable paths. The latter usually occurs when there is a sufficiently high
Reynolds number flow. For example, flow pertaining to the peak volume flow rate
of the cardiac waveform through the stenosis of the carotid artery can result in tur-
bulence downstream as shown in Fig. 7.11b and c .
In summary, path streamlines can be used to show that recirculation was preva-
lent downstream of the stenotic region where the flow experiences sharp changes
in the flow conditions.
The complex asymmetric flow pattern within the stenosed carotid artery clearly
depict the fluid flow transiting to a state of weak turbulence downstream while re-
maining laminar upstream (Lee et al. 2008). A closer examination of the iso-surface
plot of the vorticity field by computational and experimental results is shown in
Fig. 7.12 . Upstream, the flow remained laminar and uniform in the CCA. Here,
the Reynolds number ranges between 485 and 768. Through the sinus region, the
flow in both ICA and ECA changed significantly and becomes more chaotic with
pronounced vortical coherent structures and strong central vortex threading being
formed through the stenosis. At the stenosed area of the ICA, the Reynolds number
can reach as high as 2100, which indicates the probable transition to weak turbulent