Influence of shear-thinning blood rheology on the laminar-turbulent transition over a backward facing step

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 (Re) covering the comprehensive range of laminar, transitional and turbulent flow for the first time. Both unsteady Reynolds Averaged Navier-Stokes (k 􀀀 w SST) and Smagorinsky Large Eddy Simulations (LES) were assessed; only LES correctly predicted trends in the recirculation zone length for all Re. Turbulent-transition was assessed by several criteria, revealing a complex picture. Instantaneous turbulent parameters, such as velocity, indicated delayed transition: Re = 1600 versus Re = 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

Managing the Risk from Children’s Travel Cups

Introduction UK national newspapers have reported cases of children (and adults) who have got their tongue trapped in a Disney travel mug lid, causing extreme distress to the patients, their parents and ED staff. Potential risks include oral endotracheal intubation necessitating emergency tracheostomy to secure the airway, tongue necrosis and dental trauma. Although Disney has withdrawn their original mug from the global market, the same dangers can occur with other internationally available brands. Our aim was to design, test and present an alternative lid.Methods and materials We designed an alternative lid to fit onto the original Disney mug; our addition of two parallel bars prevented tongue protrusion into the lid. Prototypes of the original and new lids were three-dimensional printed for testing. A tongue substitute was developed and a representative 0.2 bar suction force was generated. The bottle was mounted in a material test machine, attached to the load cell fixture. Four samples each for the existing and new design were tested. The data were analysed by a custom Matlab script to extract the maximum force required to remove the tongues from the cup.Results The new design resulted in a significant (p=0.0286, Mann-Whitney U) reduction in pullout force. For the existing design, the median pullout force was 4.64 N (minimum 3.86 N, maximum 4.91 N), while it was 2.37 N (minimum 2.20 N, maximum 2.53 N) for the new design. Trauma to the materials used with the original lid design was evident but not observed with our design.Conclusion Our lid appears to offer a safer design that can avoid injuries. However, absolute safety remains unproved, as testing did not account for other body parts which may get trapped in the lid, nor did we test a range of tongue substitute sizes, and laboratory testing only was completed