The avalanche effect of an elasto-viscoplastic thixotropic material on an inclined plane
Oishi, Cassio M. [UNESP]
Martins, Fernando P. [UNESP]
Thompson, Roney L.
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The so-called avalanche effect is one of the fingerprints of thixotropic materials. This self-reinforcing process where the decrease in viscosity, due to a rejuvenation process triggered by a stress field, induces a motion which in turn contributes to decrease the viscosity again, is well exemplified by the inclined plane problem. In this situation, the material in its fully-structured state is placed on an inclined plane with respect to the gravity force which is responsible for the beginning of the breakdown process. These thixotropic systems generally have a yield stress, a strength that must be overcome in order to induce rejuvenation. In addition, they exhibit elastic features, especially in the pre-yield state. In the present work we numerically solve the transient evolution of an elasto-viscoplastic thixotropic material subjected to the action of gravity on an inclined plane. In order to handle with the moving free-surface boundary condition encountered in the avalanche effect, we have used a combination of the Marker-And-Cell (MAC) method with the front-tracking scheme. This formulation was successfully employed for this kind of material in the recent paper of Oishi et al. (2016) . In the present work, we have adapted our finite difference formulation to analyze the effects associated with an extended Herschel-Bulkley model in the simulation of a transient complex free surface flow. Concerning the parameters of the flow curve, it is shown that the dimensionless yield stress (plastic number) is the most significant one. However, for a fixed plastic number, different combinations of dimensionless consistency index and dimensionless Newtonian viscosity plateau can lead to a diversity of responses. The thixotropic equilibrium time had a significant impact on shifting the instant when the flow regime changes from an accelerating (when the front part of the material accelerates) to a retardation one (when this front part decelerates). Higher elasticity, as captured by the Weissenberg number, led to longer distances covered by the material. (C) 2017 Elsevier B.V. All rights reserved.
Elasto-viscoplastic thixotropic materials, Avalanche effect, Finite difference Marker and Cell method, Transient computations, Free-surface boundary conditions
Journal Of Non-newtonian Fluid Mechanics. Amsterdam: Elsevier Science Bv, v. 247, p. 165-177, 2017.