  Computational fluid dynamics (CFD) is increasingly used to study blood flows
in patient-specific arteries for understanding certain cardiovascular diseases.
The techniques work quite well for relatively simple problems, but need
improvements when the problems become harder in the case when (1) the geometry
becomes complex (from a few branches to a full pulmonary artery), (2) the model
becomes more complex (from fluid-only calculation to coupled fluid-structure
interaction calculation), (3) both the fluid and wall models become highly
nonlinear, and (4) the computer on which we run the simulation is a
supercomputer with tens of thousands of processor cores. To push the limit of
CFD in all four fronts, in this paper, we develop and study a highly parallel
algorithm for solving a monolithically coupled fluid-structure system for the
modeling of the interaction of the blood flow and the arterial wall. As a case
study, we consider a patient-specific, full size pulmonary artery obtained from
CT (Computed Tomography) images, with an artificially added layer of wall with
a fixed thickness. The fluid is modeled with a system of incompressible
Navier-Stokes equations and the wall is modeled by a geometrically nonlinear
elasticity equation. As far as we know this is the first time the unsteady
blood flow in a full pulmonary artery is simulated without assuming a rigid
wall. The proposed numerical algorithm and software scale well beyond 10,000
processor cores on a supercomputer for solving the fluid-structure interaction
problem discretized with a stabilized finite element method in space and an
implicit scheme in time involving hundreds of millions of unknowns.
