# Team : reo

## Section: Scientific Foundations

### Multiscale modeling

Multiscale modeling is a necessary step for blood and respiratory flows. In this section, we focus on blood flows. Nevertheless, preliminary investigations are currently carried out in our team on respiratory flows.

#### Arterial tree modelling

Problems arising in the numerical modeling of the human cardiovascular
system often require an accurate description of the flow in a specific
sensible subregion (carotid bifurcation, stented artery, *etc.*).
The description of such local phenomena is better addressed by means
of three-dimensional (3D) simulations, based on the numerical
approximation of the incompressible Navier-Stokes equations, possibly
accounting for compliant (moving) boundaries. These simulations
require the specification of boundary data on artificial boundaries
that have to be introduced to delimit the vascular district under
study. The definition of such boundary conditions is critical and, in
fact, influenced by the global systemic dynamics. Whenever the
boundary data is not available from accurate measurements, a proper
boundary condition requires a mathematical description of the action
of the reminder of the circulatory system on the local district. From
the computational point of view, it is not affordable to describe the
whole circulatory system keeping the same level of detail. Therefore,
this mathematical description relies on simpler models, leading to the
concept of *geometrical multiscale* modeling of the circulation [39]. The underlying idea consists in coupling
different models (3D, 1D or 0D) with a decreasing level of accuracy,
which is compensated by their decreasing level of computational
complexity.

The research on this topic aims at providing a correct methodology and a mathematical and numerical framework for the simulation of blood flow in the whole cardiovascular system by means of a geometric multiscale approach. In particular, one of the main issues will be the definition of stable coupling strategies between 3D and 1D models that generalizes the work reported in [37] to general geometries coming from medical imaging.

When modeling the arterial tree, a standard way consists in imposing a
pressure or a flow rate at the inlet of the aorta, *i.e.* at the
network entry. This strategy does not allow to describe important
features as the overload in the heart caused by backward travelling
waves. Indeed imposing a boundary condition at the begining of the
aorta artificially disturbs physiological pressure waves going from
the arterial tree to the heart. The only way to catch this
physiological behavior is to couple the arteries with a model of
heart, or at least a model of left ventricle.

A constitutive law for the myocardium, controlled by an electrical command, has been recently developped in the ICEMA project [40]. One of our objectives is to couple artery models with this heart model.

A long term goal is to achieve 3D simulations of a system including heart and arteries. One of the difficulties of this very challenging task is to simulate the aortic valve. To this purpose, we plan to mix arbitrary Lagrangian Eulerian and immersed boundary or fictitious domain approaches.

#### Respiratory tract modelling

Work is in progress to develop a multiscale modelling of the respiratory tract. Intraprenchymal airways distal from generation 7 of the tracheabronchial tree (TBT), which cannot be visualized by common medical imaging techniques, are modelled either by a single simple model or by a model set according to their order in TBT. The single model is based on straight pipe fully developed flow (Poiseuille flow in steady regimes) with given alveolar pressure at the end of each compertment. It will provide boundary conditions at the bronchial ends of 3D TBT reconstructed from imaging data. The model set includes three serial models. The generation down to the pulmonary lobule will be modelled by reduced basis elements. The lobular airways will be represented by a fractal homogenization approach. The alveoli, which are the gas exchange loci between blood and inhaled air, inflating during inspiration and deflating during expiration, will be described by multiphysics homogenization.