Pressure and flow

Because of the problem’s scale, the researchers chose a Lattice Boltzmann approach that approximates segments of fluid as particles and follows the momentum of their interactions with neighbors along a lattice structure within the geometry of the arteries. The approach was ideal because of the geometry’s complexity and the work across parallel processors.

Blood itself is not a simple fluid and contains a variety of suspended components such as red cells, white cells, lipids and platelets. In this simulation, the researchers focused on the movement of red blood cells, mimicking their bialy-like shapes as 300 million ellipsoids. Molecular dynamics methods helped calculate their movements and interaction in this vascular system.

With those nearly 300,000 processors involved, one important challenge was partitioning the data so it would use the whole Blue Gene system effectively. With the problems of both fluid flow and red blood cell movement layered on top of each other, Peters says, the team had to ensure that information about the momentum of red blood cells was coupled to momentum of fluid flow. “So you have two simulations going at two different length scales simultaneously, and [we’re] trying to keep those coupled and moving together.”

To link those effectively, the researchers chose the same partitioning method for the blood cells and fluid flow. Memory also presented a challenge, and the researchers tried several different file-processing schemes before finding one that worked.

Blood behaves as a non-Newtonian fluid, which accounts for its ability to flow through tiny capillaries without clogging them. Red blood cells are the component responsible for this type of fluid behavior, Kaxiras says. “You cannot capture that without accounting for red blood cells in the flow.”

Because red blood cells are just one of many components that comprise the complex mixture of blood, this simulation is just a first step in understanding the many factors that could contribute to endothelial shear stress and heart disease, Kaxiras says.

In further simulations, Peters and her colleagues will look at other factors that can contribute to a heart attack: white blood cells, motion within blood vessels and the complexities that arise when arterial geometry changes as blood pulses through.

Once researchers can identify the most important components, Peters adds, it might be possible to scale this type of simulation down to a tool that a physician could use to diagnose individual patients.