Abstract
Pathological platelet activation by abnormal shear stresses is regarded as a main clinical complication in recipients of cardiovascular mechanical devices. In order to improve their performance computational fluid dynamics (CFD) are used to evaluate flow fields and related shear stresses. CFD models are coupled with mathematical models that describe the relation between fluid dynamics variables, and in particular shear stresses, and the platelet activation state (PAS). These models typically use a Lagrangian approach to compute the shear stresses along possible platelet trajectories. However, in the case of turbulent flow, the choice of the proper turbulence closure is still debated for both concerning its effect on shear stress calculation and Lagrangian statistics. In this study different 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) 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 how much turbulence closures may affect platelet activation estimates, in comparison to an accurate DNS, when assessing blood damage in blood contacting devices.
Highlights
It is well known that cardiovascular devices determine thromboembolic consequences for which anticoagulant therapy is mandatory [1 – 4]
An intricate pattern of interacting vortices extending downstream the valve is predicted by both direct numerical simulation (DNS) and large eddy simulation (LES), while the other turbulence models cannot capture the spatial structure of the flow field
Reliable predictions of blood damage can only be obtained when the complex hemodynamics of the devices are accurately solved. This is fairly straightforward when the flow is laminar, but it is challenging in the transitional flow regime, which typically occurs in blood contacting devices [18, 47, 48]
Summary
It is well known that cardiovascular devices determine thromboembolic consequences for which anticoagulant therapy is mandatory [1 – 4]. In 2008 the United States Food and Drug Administration (FDA) launched a Critical Path initiative to assess the predictive capability of computational fluid dynamics (CFD) simulations and blood damage models used to evaluate bloodcontacting medical devices [11] This initiative led to the development of a specific FDA Guidance Document aimed at regulating the use of CFD in the field of medical device assessment [12]. New numerical models [13, 14] have been developed for the computation of the platelet activation state (PAS), due to flow shear stresses These models are based on the Lagrangian analysis of possible platelet trajectories along which flow field metrics involved in the process of platelet activation are extracted. The nonlinear structure of the PAS model equations implies that the accuracy of the flow solution and the associated Lagrangian computations – especially close to geometric singularities – are of utmost importance when computing platelet activation along the trajectories
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