Influence of thixotropic parameters on a non-Newtonian fluid flow

Non-Newtonian fluids exhibiting complex rheological characteristics, such as yield stress and thixotropy, are frequently encountered in nature and industries. Thixotropy is a time-dependent shear thinning property, associated with the microstructural evolution of materials. During a flowing process, two microstructure transition mechanisms are considered to take place simultaneously: the recovery and the breakdown; the former makes the materials more solid, while the latter makes them more liquid. The microstructure is characterized by a dimensionless structural parameter, whose evolution is modeled by a rate equation consisting of two terms representing the rate of the two mechanisms. A brief review on thixotropic models for different materials is first carried out. It is then assumed that the recovery rate depends linearly on the structural parameter, and the breakdown one is a complex function of it and the shear rate. This work aims at investigating the influence of the parameters that control the recovery and breakdown rates on the flow of a thixotropic fluid past a circular cylinder. In addition, the Bingham and/or Herschel–Bulkley model with Papanastasiou’s regularization is utilized. Various flow characteristics, such as the microstructure evolution and the flow field including the yielded and unyielded zones, are analyzed and discussed in detail. The simulation results show that the size and shape of both static and moving unyielded zones are considerably affected by the thixotropic parameters.