UI Experimental Fluids Research Laboratory
Model Studies of Flow
in Abdominal Aortic Aneurysms
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Dr.
Carla Egelhoff
Dr.
Ralph Budwig
Dr.
Don Elger
Dr.
Kaj Johansen (UW)
Mr.
Tariq Khraishi
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Abstract
Pulsatile flow in abdominal aortic aneurysm (AAA) models
has been examined in order to understand the hemodynamics that may
contribute to growth of an AAA. The model studies were
conducted by experiments (flow visualization and laser Doppler
velocimetry) and by numerical simulation using physiologically
realistic resting and exercise flow conditions. We
characterize the flow for two AAA model shapes and sizes emulating
early AAA development through moderate AAA growth (mean and peak
Reynolds numbers of 362 < Remean <1053
and 3308 < Repeak < 5696 with Womersley
parameter 16.4 <
a < 21.2). The results of our investigation indicate that AAA
flow can be divided into three flow regimes: (i)
Attached flow over the entire cycle in small AAAs at resting
conditions, (ii) vortex formation and translation in moderate size
AAAs at resting conditions, and (iii) vortex formation, translation,
and turbulence in moderate size AAAs under exercise conditions.
The second two regimes are classified in the medical literature as
disturbed flow conditions that have been correlated with
atherogenesis as well as thrombogenesis. Thus, AAA disturbed
hemodynamics may be a contributing factor to AAA growth by
accelerating the degeneration of the arterial wall. Our
investigation also concluded that vortex development is considerably
weaker in an asymmetric AAA. Furthermore, turbulence was not
observed in the asymmetric model. Finally, our investigation
suggests a new mode of transition to turbulence: vortex ring
instability and bursting to turbulence. The transition process
depends on a combination of the pulsatile flow conditions and the
tube cross-sectional area change.
Keywords:
Abdominal aortic aneurysm; Hemodynamics; Pulsatile flow;
Transition to turbulence
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Actual Photos: Dr. Carla
Egelhoff
Math Models: Tariq Khraishi
Resting waveform comparison of
computational fluid dynamics results to experiment for a moderately large
AAA model. Flow is from left to right as pictured. The top left
graph shows a bar indicating the "shutter open" time of the
photograph and the top right graph indicates the instantaneous time of each
streamline plot. (a) t/T= 0.5 early systole, attached flow throughout
the AAA, (b) t/T= 0.6 late systole, vortex formation, (c) t/T= 0, mid
diastole, vortex development and translation, (d) t/T= 0.3, late
diastole, vortex dominated flow with vortices traveling into tube proximal
to the AAA. |
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