Abstract
Cardiovascular diseases are the leading cause of death globally and there is an unmet need for effective, safer blood-contacting devices, including valves, stents and artificial hearts. In these, recirculation regions promote thrombosis, triggering mechanical failure, neurological dysfunction and infarctions. Transitional flow over a backward facing step is an idealised model of these flow conditions; the aim was to understand the impact of non-Newtonian blood rheology on modelling this flow. Flow simulations of shear-thinning and Newtonian fluids were compared for Reynolds numbers ( R e ) covering the comprehensive range of laminar, transitional and turbulent flow for the first time. Both unsteady Reynolds Averaged Navier–Stokes ( k − ω SST) and Smagorinsky Large Eddy Simulations (LES) were assessed; only LES correctly predicted trends in the recirculation zone length for all R e . Turbulent-transition was assessed by several criteria, revealing a complex picture. Instantaneous turbulent parameters, such as velocity, indicated delayed transition: R e = 1600 versus R e = 2000, for Newtonian and shear-thinning transitions, respectively. Conversely, when using a Re defined on spatially averaged viscosity, the shear-thinning model transitioned below the Newtonian. However, recirculation zone length, a mean flow parameter, did not indicate any difference in the transitional Re between the two. This work shows a shear-thinning rheology can explain the delayed transition for whole blood seen in published experimental data, but this delay is not the full story. The results show that, to accurately model transitional blood flow, and so enable the design of advanced cardiovascular devices, it is essential to incorporate the shear-thinning rheology, and to explicitly model the turbulent eddies.
Highlights
While blood flow in the healthy cardiovascular system is predominantly laminar, insight into the transition from laminar to turbulent blood flow is vital for understanding the development of cardiovascular diseases and for the design of blood-contacting devices
We only examined the behaviour of the primary recirculation zone (x1 ), as the focus was on the transition to laminar-turbulent flow behaviour between the two rheologies
The primary recirculation zone normalised by the step height (x1 /S) was computed and used to compare the recirculation zone normalised by the step height (x1 /S) was computed and used to compare the resolution of the meshes for both the k − ω SST model and the Smagorinsky subgrid scale (SGS) model
Summary
While blood flow in the healthy cardiovascular system is predominantly laminar, insight into the transition from laminar to turbulent blood flow is vital for understanding the development of cardiovascular diseases and for the design of blood-contacting devices (for example: ventricular assist devices, mechanical heart valves and stents). Occluded vessels, such as those seen in arterial stenoses, can trigger laminar-turbulent transition, leading to the development of recirculating flow regions.
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