The Effect of Turbulence Modelling on the Assessment of Platelet Activation

Abstract

Abstract Pathological platelet activation induced by abnormal shear stresses is regarded as a main clinical complication in recipients of cardiovascular biomedical implantable devices and prostheses. In order to improve their performance computational fluid dynamics (CFD) has been used to evaluate flow fields and related shear stresses. More recently CFD models have been equipped with mathematical models that describe the relation between fluid dynamics variables, and in particular shear stresses, and the platelet activation state (PAS). These mathematical models typically use a Lagrangian approach to extract the shear stresses along possible platelet trajectories. However, in the case of turbulent flow, the choice of the proper turbulence closure model is still debated for both concerning its effect on Lagrangian statistics and shear stress calculation. In our study five numerical simulations of the flow through a mechanical heart valve were performed and then compared in terms of Eulerian and Lagrangian quantities: a direct numerical simulation (DNS), a large eddy simulation (LES), two Reynolds-averaged Navier-Stokes (RANS) simulations (SST k-ω and RSM) and a “Laminar” (no turbulence modelling on a Taylor microscale-based grid) simulation. Results exhibit a large variability in the PAS assessment depending on the turbulence model adopted. “Laminar” and RSM estimates of platelet activation are about 60% below DNS, while LES is 16% less. Surprisingly, PAS estimated from the SST k-ω velocity field is only 8% less than from DNS data. This appears more artificial than physical as can be inferred after comparing frequency distributions of PAS and of the different Lagrangian variables of the mechano-biological model of platelet activation. Our study indicates that turbulence closures can lead to a severe underestimation of platelet activation and suggests that turbulence should be fully resolved by DNS when assessing blood damage in blood contacting devices.