| Περίληψη: | In this work we study the extensive flow of materials exhibiting elastoviscoplastic behavior. The sample, which is bounded between two coaxial discs, initially forming a cylindrical filament, is stretched as the upper disk is lifted upwards. This type of flow has not received the necessary spotlight compared to the corresponding shear, although it provides a complete characterization of the material. The material complies with the constitutive model proposed by Saramito [1], in particular the Herschel-Bulkley version [2], which predicts shear-thinning, and yields in accordance with the Von Mises criterion [3]. The induced deformation and pinching-off of the filament are studied numerically using the Finite Element Method [FEM] while the tesselation of the deformed physical domain is based on the elliptic grid generation as developed by Dimakopoulos and Tsamopoulos [4]. In addition, axial symmetry is considered to simplify the underlying equations. We attempt to investigate how important material properties, such as yield stress τy, elastic modulus, G, and shear exponent n, affect the structure of the filament by monitoring the evolution of its shape, and in particular the minimum radius, velocity, and appearing stresses. We demonstrate that the elastic response of the fluid can be evident by increasing the speed at which the upper plate is lifted. Consequently, lower speeds (of a magnitude or smaller) may lead to misguiding interpretations of the fluid’s nature as having a perfect viscoplastic behavior. A major advantage of using computational simulations compared to experimental results is that the investigator is given the opportunity to have a visual estimate of the fluidized and non-fluidized regions of the filament and how they evolve over time due to fluctuations of stresses. The shear-exponent n constitutes an interesting parameter that needs to be investigated. By increasing its value, the shear-thinning of the material decreases and consequently we reach the state of infinitely extensional viscosity. At this point, the capillary forces responsible for the collapse of the filament are much smaller than the elastic forces. As a result, the radius of the neck is not reduced and the material is pulled from the lower part due to the elastic forces. Finally, the temporal evolution of the extensional viscosity is examined to determine under what conditions the coaxial extensional flow approach can be used to calculate the extensional viscosity.
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