Asterios Pantokratoras
Comment on Conjugate heat transfer of mixed convection for viscoelastic fluid past a stretching sheet, by Kai-Long Hsiao and Guan-Bang Chen, Mathematical Problems in Engineering, Volume 2007, article 17058, 21 pages
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Viscoelastic flows are pervasive in a host of natural and industrial processes, where the emergence of nonlinear and time-dependent dynamics regulate flow resistance, energy consumption, and particulate dispersal. Polymeric stress induced by the advection and stretching of suspended polymers feeds back on the underlying fluid flow, which ultimately dictates the dynamics, instability, and transport properties of viscoelastic fluids. However, direct experimental quantification of the stress field is challenging, and a fundamental understanding of how Lagrangian flow structure regulates the distr…
In this work, we study the role of viscoelastic instability in the mechanical dispersion of fluid flow through porous media at high Peclet numbers. Using microfluidic experiments and numerical simulations, we show that viscoelastic instability in flow through a hexagonally ordered (staggered) medium strongly enhances dispersion transverse to the mean flow direction with increasing Weissenberg number (Wi). In contrast, preferential flow paths can quench the elastic instability in disordered media, which has two important consequences for transport: First, the lack of chaotic velocity fluctuatio…
Flow and Heat Transfer of a MHD Viscoelastic Fluid in a Channel with Stretching Walls: Some Applications to Haemodynamics
本文通过数值方法研究了在外部磁场作用下，具有拉伸壁的平行板通道中稳态不可压缩粘弹性导电流体的流动与传热，发现拉伸壁导致中心线附近出现回流，而强磁场可抑制回流，且磁场增强会降低流速但升高温度，对心血管系统血液动力学有潜在应用。
The dynamics of fibers dispersed in viscoelastic turbulent flows
本研究通过双向耦合直接数值模拟，探究粘弹性湍流中聚合物弹性、纤维刚度与密度差对纤维拍动、曲率及对齐特性的影响，发现聚合物对中性浮力纤维作用更显著，惯性纤维几乎不受弹性影响。
Liquid Transfer for Viscoelastic Solutions
本文实验研究了平行圆盘间毛细桥在拉伸流动中的粘弹性液体转移，发现提高聚合物质量分数、溶剂粘度及拉伸高度会减少转移，且改变液体体积时牛顿与粘弹性溶液行为相反。
Drop Dynamics of Viscoelastic Filament
利用直径-空间-时间图研究粘弹性细丝断裂前附着液滴的层级、不对称性及迁移振荡合并等动力学，并通过德博拉数预测液滴最大数量，以最小直径测量拉伸粘度揭示聚合物浓度与变细方向的影响。
The effects of fluid elasticity on the swimming behavior of the nematode \emph{Caenorhabditis elegans} are experimentally investigated by tracking the nematode's motion and measuring the corresponding velocity fields. We find that fluid elasticity hinders self-propulsion. Compared to Newtonian solutions, fluid elasticity leads to 35% slower propulsion speed. Furthermore, self-propulsion decreases as elastic stresses grow in magnitude in the fluid. This decrease in self-propulsion in viscoelastic fluids is related to the stretching of flexible molecules near hyperbolic points in the flow.
This study presents a numerical investigation of how fluid viscoelasticity influences the flow dynamics past a transversely forced oscillating cylinder in the laminar vortex shedding regime at a fixed Reynolds number of 100. In particular, we examine how fluid viscoelasticity influences the boundary between the lock-in and no lock-in zones and the associated wake topology compared to that seen in a simple Newtonian fluid. All in all, we find that the fluid viscoelasticity facilitates the synchronization of the vortex street with the cylinder motion at lower oscillation frequencies than that re…
Early turbulence in viscoelastic flow past a periodic cylinder array
本研究通过二维直接数值模拟发现，添加聚合物使流经周期性圆柱阵列的粘弹性流在极低雷诺数下发生早期转捩，揭示了由弹性力主导的混沌波动状态及阻力增强机制。
The flow of viscoelastic fluids in porous media is encountered in many practical applications, such as in the enhanced oil recovery process or in the groundwater remediation. Once the flow rate exceeds a critical value in such flows, an elastic instability with fluctuating flow field is observed, which ultimately transits to a more chaotic and turbulence-like flow structure as the flow rate further increases. In a recent study, it has been experimentally shown that this chaotic flow behaviour of viscoelastic fluids can be suppressed by increasing the geometric disorder in a model porous media …
Drawing of Weakly Viscoelastic Fluid Tubes
本研究针对轴对称粘弹性管拉伸过程，采用Giesekus模型推导长波方程，发现弹性效应通常促进孔洞闭合但弱拉伸时相反，且惯性可忽略时总是失稳，与无孔细丝情况相反。
Simulations of undulatory swimming in viscoelastic fluids with large amplitude gaits show concentration of polymer elastic stress at the tips of the swimmers.We use a series of related theoretical investigations to probe the origin of these concentrated stresses. First the polymer stress is computed analytically at a given oscillating extensional stagnation point in a viscoelastic fluid. The analysis identifies a Deborah number (De) dependent Weissenberg number (Wi) transition below which the stress is linear in Wi, and above which the stress grows exponentially in Wi. Next, stress and velocit…
The rimming flow of a viscoelastic thin film inside a rotating horizontal cylinder is studied theoretically. Attention is given to the onset of non-Newtonian free-surface instability in creeping flow. This non-inertial instability has been observed in experiments, but current theoretical models of Newtonian fluids can neither describe its origin nor explain its onset. This study examines two models of non Newtonian fluids to see if the experimentally observed instability can be predicted analytically. The non-Newtonian viscosity and elastic properties of the fluid are described by the Generali…
Expansion of a hole in a viscoelastic liquid sheet
本文针对自由悬挂粘弹性液体薄膜的轴对称孔洞扩张问题，采用Oldroyd-B模型推导薄膜方程，发现粘弹性应力会提高孔洞扩张的指数增长率，其结果与实验和纯粘性理论的差异相关。
Non-monotonic variations in pressure drop and chaos in viscoelastic fluid flows through an ordered microporous medium
通过数值模拟研究粘弹性流体在有序微孔介质中的流动，发现双折射应力丝主导了混沌和压力降的非单调变化，并受固体体积分数影响。
We give the first correction to the suspension viscosity due to fluid elasticity for a dilute suspension of spheres in a viscoelastic medium. Our perturbation theory is valid to $O(ϕ\mathrm{Wi}^2)$ in the Weissenberg number $\mathrm{Wi}=\dotγλ$, where $\dotγ$ is the typical magnitude of the suspension velocity gradient, and $λ$ is the relaxation time of the viscoelastic fluid. For shear flow we find that the suspension shear-thickens due to elastic stretching in strain hot spots near the particle, despite the fact that the stress inside the particles decreases relative to the Newtonian case. W…
We studied experimentally the breakup of liquid bridges made of aqueous solutions of Poly(acrylic acid) between two separating solid surfaces with freely moving contact lines. For polymer concentrations higher than a certain threshold ($\sim$ 30 ppm), the contact line on the surface with the highest receding contact angle fully retracts before the liquid bridge capillary breakup takes place at its neck. This means that all the liquid remains attached to the opposing surface when the surfaces are separated. This behavior occurs regardless of the range of liquid volume and stretching speed studi…
The research is aimed at creating a method for rheological testing of viscoelastic fluids, the droplets of which, when stretched, form thinning filaments, i.e. exhibit the property of spinning. The typical example of such fluid is oral fluid (or mixed saliva). Among other things, the rheological features of the oral fluid actively affect the mechanisms of transmission of infection by airborne droplets. In this work, the oral fluid was studied both from the point of view of a participant in the transmission of infection and as a model for studying the rheology of viscoelastic fluids. The method…
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