Computational Fluid Dynamics (CFD)

The use of CFD as alternative or supplement to experimental investigations is very important to understand the flow phenomena in blood vessels or in the vicinity of heart valve protheses. These flow phenomena are very complex (3-dimensional, transient/pulsatile, partly turbulent with non-Newtonian behaviour of blood and wall elasticity) and includes stagnation flow and recirculation zones, which leads to a complex distribution of the wall shear stress. The assessment of these three-dimensional effects with experimental methods, such as Laser Doppler Velocimetry (LDV) or Particle Image Velocimetry (PIV) is very difficult. The knowledge of these phenomena is important to understand processes like thrombus formations and the atherosklerotic disease. In the last years the CFD was accepted as a tool for design and evaluation of mechanical heart valve prostheses, grafts, blood pumps and blood chambers for Ventricular Assist Devices (VAD).

A comparison of experimentally obtained flow patterns with numerical simulated flow patterns showed good agreement in major flow regions. However, not all hemo-dynamically relevant details such as location and extension of stagnation areas as well as the location of separation points could exactly be modelled. This is result of some model simplifications, which must be improved in future.

In the Biofluid Mechanics Laboratory, a commercial software of FLUENT Inc. is used to solve different problems:

• The simulations of the blood flow through heart valves in order to optimise the duct geometry of the heart valves and therefore to reduce thromboembolic complications (example of a new designed S-shape heart valve) and (example of a heart valve with purge flow).
• Reconstruction of the blood flow in the carotid bifurcations to investigate the relations between flow parameters and atherosclerotic processes (example of a wall shear stress distribution in a carotid bifurcation).

FLUENT is a general purpose CFD program for modelling fluid flow, heat transfer, and chemical reaction even in complex geometries. This program uses structured and/or unstructured grids.

FLUENT includes the following products:

• FLUENT the solver,
• GeoMesh, and preBFC, the pre-processors for geometry modelling and unstructured triangular surface mesh generation,
• Gambit, the pre-processor for geometry and mesh generation,
• Tgrid, the 3D tetrahedral mesh generator and
• grid filters for CAD/CAE packages.

S-shaped heart valve with a vector plot of the velocity magnitude in the central cross-section.

S-shaped heart valve

Heart valve with a purge flow shows a vector plot of the velocity magnitude in the central cross-section. The wall is colored by the wall shear stress.

Heart valve with purge flow

Distribution of the wall shear stress in a human carotid bifurcation calculated with a CFD program FLUENT (Re = 200 — Diastole, Flow rate 75/25 — carotis interna/carotis externa).

Wall shear stress in bifurcation