![]() ![]() ![]() With increasing flow velocity and shear strain rate, blood flows more smoothly ( Moon et al., 2014) and its viscosity decreases toward a constant, which has been commonly used as the viscosity of blood in a Newtonian model ( Jahangiri et al., 2017). In most of the previous CFD studies on ICAS, blood was simulated as a Newtonian fluid for simplicity ( Leng et al., 2014, 2019 Nam et al., 2016 Liu et al., 2018 Chen et al., 2020), despite the fact that blood is a non-Newtonian fluid with a shear-thinning nature ( Nader et al., 2019). Both indices have been associated with the risk of stroke relapse in patients with symptomatic ICAS: those with a lower PR (i.e., larger translesional pressure gradient) and excessively elevated focal WSS at the ICAS lesion had significantly higher risk of recurrent stroke despite optimal medical treatment ( Leng et al., 2019). On the other hand, the relative change of wall shear stress (WSS) at the stenotic throat as compared to WSS at proximal “normal” vessel segment, has also been proposed to reflect the hemodynamic impact of an ICAS lesion on plaque growth and rupture ( Lan et al., 2020). For instance, translesional pressure ratio (PR), calculated as the ratio of the pressures distal and proximal to an ICAS lesion obtained in a CFD model, has been put forward to reflect the hemodynamic significance of ICAS ( Liebeskind and Feldmann, 2013). In recent years, computational fluid dynamics (CFD) modeling based on conventional neurovascular imaging has been applied to simulate in vivo cerebral blood flow and quantify cerebral hemodynamic metrics in the presence of ICAS, which cannot be achieved with conventional neurovascular imaging alone ( Liebeskind et al., 2016 Linfang Lan, 2017 Liu et al., 2018 Chen et al., 2020).Ĭomputational fluid dynamics modeling studies have indicated that global and focal cerebral hemodynamics may play an important role in governing the risk of stroke recurrence in patients with symptomatic ICAS ( Leng et al., 2014, 2019). The table below summarises four types of non-Newtonian fluids.Intracranial atherosclerotic stenosis (ICAS) is a major cause for ischemic stroke and transient ischemic attack (TIA) in Asian populations ( Wong, 2006). Some non-Newtonian fluids react as a result of the amount of stress applied, while others react as a result of the length of time that stress is applied. Not all non-Newtonian Fluids behave in the same way when stress is applied – some become more solid, others more fluid. In this case, the oobleck’s viscosity or resistance to flow increases with applied stress. You can roll it into a solid ball in your hand, but if you stop moving it, it reverts to liquid and oozes out through your fingers. You can hit a bowlful with a hammer, and instead of splashing everywhere, the particles lock together. This liquid is a runny goo until you apply stress to it, and then it suddenly acts like a solid. Oobleck is a mixture of cornflour and water (similar to uncooked custard) named after a substance in a Dr Seuss book. In this case, the sauce’s viscosity decreases and it gets runnier with applied stress. This causes the tomato sauce to become more liquid and you can easily squirt some out. So what do you do? You shake or hit the bottle. You know there is some in there, but when you turn the bottle upside down, nothing comes out. Say you want to get some tomato sauce out of the bottle. Remove the stress (let them sit still or only move them slowly) and they will return to their earlier state. If you apply a force to such fluids (say you hit, shake or jump on them), the sudden application of stress can cause them to get thicker and act like a solid, or in some cases it results in the opposite behaviour and they may get runnier than they were before. Non-Newtonian fluids change their viscosity or flow behaviour under stress. ![]()
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