  Spatial patterns of elevated wall shear stress and pressure due to blood flow
past aortic stenosis (AS) are studied using GPU-accelerated patient-specific
computational fluid dynamics. Three cases of moderate AS, one with a dilated
ascending aorta and two within the normal range (root diameter less than 4cm)
are simulated for physiological beat cycle waveforms obtained from
echocardiography data. The computational framework is built based on
sharp-interface Immersed Boundary Method, where aortic geometries segmented
from CT angiograms are integrated into a high-order incompressible
Navier-Stokes solver. We show that even though the wall shear stress is
elevated and oscillatory due to turbulence in the ascending aorta for all the
cases, its spatial distribution is significantly more focused for the case with
dilation than those without dilation. This focal area is linked to aortic valve
jet impingement on the outer curvature of the ascending aorta, and has been
shown in vivo using 4D flow MRI of aortic stenosis patients with aortic
dilation (van Ooij et al., J. Am. Heart Assoc., 6(9), 2017). We show that this
focal area also accommodates a persistent pocket of high pressure, which is
likely to have contributed to the dilation process through an increased
wall-normal forcing. The cases without dilation, on the contrary, showed a
rather oscillatory pressure behaviour, with no persistent pressure buildup
effect. We further argue that a more proximal branching of the aortic arch
could explain the lack of a focal area of elevated wall shear stress and
pressure, because it interferes with the impingement process due to fluid
suction effects. These phenomena are further illustrated using an idealized
aortic geometry. We finally show that a restored inflow condition eliminates
the focal area of elevated wall shear stress and pressure zone from the
ascending aorta.
