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By Helene Bothorel1, Emiliano Votta1, Monica Soncini1,
Umberto Morbiducci2, Costantino DelGaudio2, Antonio Balducci2, Mauro Grigioni2,
and Alberto Redaelli1
1 Dept of Bioengineering, Politecnico di Milano, Italy
2 Laboratory of Biomedical Engineering, Istituto Superiore di Sanità,
Rome, Italy
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Pathlines through the aortic valve during the opening
phase at opening angles of 37.5°, 63° and 80°, from top to
bottom; pathlines are colored with respect to pressure values
The “Opening Valve Project” is a collaborative effort being
carried out by three organizations to study the behavior of a mechanical
aortic valve. The Department of Bioengineering at Politecnico di Milano
in Italy is performing CFD simulations, the Italian National Institute
of Health (Istituto Superiore di Sanità, or ISS) is carrying out
experimental tests, and St. Jude Medical Center in Fullerton, CA is providing
technical support.
Angular displacement (above) and angular velocity (below) during the opening
phase showing a comparison between experimental (red) and numerical (blue)
results
To date, the investigation has
focused on a St. Jude Medical HP
mechanical aortic valve, 27 mm in size,
mounted in a mock physiological flow
loop designed by Vivitro Systems of
Canada. Two rigid leaflets are mounted
on hinges at opposite sides of the
aortic cavity. Depending on the
varying pressures upstream and
downstream of the valve, the leaflets
change from completely closed to fully
open positions.
The dynamic mesh model in
FLUENT has been used for the CFD
simulations. To account for the fluid
structure interaction (FSI) that is an
important component of the valve
motion, a user-defined function
(UDF) was developed. The UDF computes
the forces acting on the valve
leaflets at the end of each time step,
then calculates the difference
between the prescribed leaflet
motion and that corresponding to
the computed forces. This information
is used to forecast the correct
motion of the leaflets in the subsequent
time step on the basis of a relaxation
scheme. To avoid flow field
instabilities, a small relaxation constant
is used. The method was first
tested and refined using a 2D
model, and has subsequently been
applied to a 3D case.
Preliminary experimental tests were
performed at ISS using ultrafast cinematography.
These tests provided
the transient boundary conditions (inlet
and outlet pressure) for the numerical
simulations. A mesh of about
320,000 cells was used along with
a time step of 0.02 ms to allow for
optimal remeshing and accuracy of
the results. Simulations were performed
on a single processor Intel
Pentium PC (1.7 GHz with 2GB RAM)
and took about 150 h of CPU time.
The CFD results show a reasonably
good correlation with experimental
measurements of leaflet
motion (angular displacement) and
angular velocity during the opening
and closing phases. The simulated
angular displacement is delayed with
respect to the experimental data by
about 7 ms. This value is considered
acceptable with respect to the
opening phase time lapse (45 ms).
A better agreement with experimental
data is observed for the predicted
angular velocities, which are just slightly
underestimated (7%). These
results suggest that the dynamic
mesh model, in conjunction with the
FSI customization in FLUENT, is reliable
for valve opening simulations
when an accurate description of the
valve kinematics is not required.
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