Interaction between a free-falling sphere and structure dynamics in a heterogeneous thixotropic fluid

Abstract

The impact of thixotropy on the settling behavior of a solid sphere is investigated utilizing a finite element-computational fluid dynamics simulation. Flow behavior is evaluated by coupling the Navier–Stokes equations with the dynamic evolution of an initially heterogeneous fluid's microstructure. Studying the structure dynamics around the settling sphere allows us to identify a variety of irregular and linear settling regimes. Settling regimes are varied by the degree of structuring, the degree of associated heterogeneity, the local morphology of the heterogeneous microstructure, and the stress induced by the sphere. In addition, the settling velocity profile of the relatively light spheres temporarily fluctuates in a case where the settling time of the sphere is long enough to capture the local heterogeneity. Ultimately, we compare the results of the simulation of dropping spheres with those of the numerical simulation of different rheological tests. This illustrates that the competition between kernels of orthokinetic and perikinetic build-up and shear-induced break-down of the microstructure indeed allows an understanding of the connection between the fluids' flow curve and the settling behaviors. Furthermore, settling regimes are characterized based on the rates of build-up and break-down of the microstructure. Moreover, the loss of fore-aft symmetry is observed in the flow field around the settling sphere as a result of a viscosity gradient behind and ahead of the sphere.