This paper examines pressure-driven tube flows of inelastic yield-stress materials with thixotropic effects. In contrast to previous works based on structural kinetic models, we employ a fluidity-based constitutive model that uses the material fluidity as a measure of the material structuring level. The model relies on rheological material properties that can be determined from standard experimental tests, avoiding the introduction of phenomenological functions to describe the mechanisms of microstructure buildup and breakdown in the flow. We analyze the transient evolution of velocity and fluidity fields toward the steady-state condition as functions of the material initial structuring state and the plastic number, a dimensionless parameter that measures the intensity of the material plasticity. When the material is initially fully structured, the results show that the avalanche effect essentially depends on the applied pressure gradient. Likewise, the process of microstructure buildup when the material is initially fully unstructured is a strong function of the applied stress. The yield surface might split the flow into two regions where the microstructure builds up at different rates, leading to a discontinuity in the transient evolution of fluidity and shear rate fields similar to that associated with transient shear banding. Finally, we show that the steady-state flow is determined by the imposed pressure gradient only and does not depend on the material initial structuring condition. These predictions bring new insights to fundamentally understand the flow of thixotropic viscoplastic materials and then optimize the operating conditions of processing flows of structured materials in many applications.

中文翻译：

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触变粘塑性材料的压力驱动流动：基于流动性的新型本构模型的性能

本文研究了具有触变效应的非弹性屈服应力材料的压力驱动管流。与之前基于结构动力学模型的工作相比，我们采用基于流动性的本构模型，该模型使用材料流动性作为材料结构化水平的度量。该模型依赖于可通过标准实验测试确定的流变材料特性，避免引入现象学函数来描述流动中微观结构的建立和分解机制。我们分析了速度和流动性场向稳态条件的瞬态演变，作为材料初始结构状态和塑性数的函数，塑性数是测量材料塑性强度的无量纲参数。当材料最初完全结构化时，结果表明，雪崩效应主要取决于施加的压力梯度。同样，当材料最初完全非结构化时，微观结构的形成过程是所施加应力的强函数。屈服面可能将流动分成两个区域，其中微观结构以不同的速率建立，导致流动性和剪切速率场的瞬态演变不连续，类似于与瞬态剪切带相关的场。最后，我们表明稳态流动仅由施加的压力梯度决​​定，而不取决于材料的初始结构条件。