  Flow fluctuations have recently emerged as a promising hemodynamic metric for
understanding the rupture risk of intracranial aneurysms. Several
investigations have reported in the literature corresponding flow instabilities
using established computational fluid dynamics tools. In this study, the
occurrence of flow fluctuations is investigated using either Newtonian or
non-Newtonian fluid models in patient-specific intracranial aneurysms using
high-resolution lattice Boltzmann method simulations. Flow instabilities are
quantified by computing power spectral density, proper orthogonal decomposition
and spectral entropy, and fluctuating kinetic energy of velocity fluctuations.
Furthermore, these hemodynamic parameters are compared between the ruptured and
unruptured aneurysms. Our simulations reveal that the pulsatile inflow through
the neck in a ruptured aneurysm is subject to a hydrodynamic instability
leading to high-frequency fluctuations around the rupture position throughout
the entire cardiac cycle. At other locations, the flow instability is only
observed during the deceleration phase; typically, the fluctuations begin there
just after peak systole, gradually decay, and the flow returns to its original,
laminar pulsatile state during diastole. In the unruptured aneurysm, there is
only minimal difference between Newtonian and non-Newtonian results. In the
ruptured case, using the non-Newtonian model leads to a considerable increase
of the fluctuations within the aneurysm sac.
