Pressure-driven flows of a thixotropic viscoplastic material: Performance of a novel fluidity-based constitutive model

Abstract

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.