Turbulent Blood Flow in a Ventricular Assist Device

Application Challenge AC7-03 © copyright ERCOFTAC 2021

Best Practice Advice

Key Fluid Physics

When calculating blood flow through complex medical devices, it should always be kept in mind that blood is a non-Newtonian, multiphase fluid. However, in simulations in ventricular assist devices, blood is approximated as a Newtonian, single-phase fluid. The former is justified because blood has asymptotic viscosity under high shear rates. In the latter, a blood-analogous fluid is assumed, which has comparable density and viscosity to blood. This assumption is necessary because it is impossible with current computational technology to account for the multiphase character of blood in a VAD simulation. This is partly due to the fact that the dimensions of the blood components are much smaller than the vortex structures calculated by the simulation. Therefore, much larger computational grids than in the current literature are needed to accommodate the blood components (size order of erythrocytes ≈ 10-6 m) to be integrated in the simulation.

Application Uncertainties

There are several uncertainties that can explain the differences between experiments and numerics:

It is important that the experimental validation uses a blood analog fluid that adequately represents the simulated fluid properties. As in this study, a mixture of glycerol-water is often used, which has a density 5% greater than the numerical fluid. Despite the same dynamic viscosity, this has an impact on the VAD flow field, since the density in the conservation equations is coupled to the pressure and velocity of the fluid. is coupled. The deviation is estimated to be ≈ 3 mmHg for the present case.

An additional deviation in the head is determined by the influence of the rotating shaft on the flow in the model. The shaft induces an additional swirl in the discharge flow and also "blocks" part of the discharge pipe. From URANS calculations of the VAD with rotating shaft, deviations in in the head of ≤ 1 mmHg were determined.

The inflow conditions between experiment and numerical simulation should be identical. Using additional CFD calculations, the influence on the head between a uniform and developed inflow profile can be estimated to be ≈ 1.5 mmHg.