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.

This paper presents a numerical study of a complex fluid flow past a circular cylinder. The fluid is a thixotropic viscoplastic material. Thixotropy is a time-dependent rheological property associated with the material’s microstructure, which can be broken and/or built up during a flowing process. It is modeled using a structural parameter representing the structural state, which can vary from being completely broken to fully structured. Effects of the thixotropic parameters controlling the rate of the structural breakdown and recovery processes on the flow characteristics are investigated. Either the Herschel–Bulkley model or the Bingham model is employed together with Papanastasiou’s regularization scheme. A review on the determination of the regularization parameter is carried out; a new approach to this is thus proposed. Various flow aspects such as the unyielded/rigid zones, the structural evolution, and the vortex structures are analyzed. Results show that at Re = 150 and Bn ≤ 5, the flow is nonstationary with periodic vortex shedding. The structural state is found to correlate well with the vortex structures. A full structural recovery can happen either within or beyond the yield boundary, or even beyond the computational domain. Moreover, the drag coefficient is reported and discussed.

No abstract

No abstract

In this work, a numerical study of polymer flow behaviors in a lid-driven cavity, which is inspired by the coating process, at a broad range of Oldroyd numbers (0≤Od≤50), is carried out. The Reynolds number is height-based and kept at Re=0.001. The fluid investigated is of Carbopol gel possessing yield stress and shear-thinning properties. To express rheological characteristics, the Herschel–Bulkley model cooperated with Papanastasiou’s regularization scheme is utilized. Results show that the polymer flow characteristics, i.e., velocity, viscosity, and vortex distributions, are considerably influenced by viscoplastic behaviors. Additionally, there exist solid-like regions which can be of either moving rigid or static dead types in the flow patterns; they become greater and tend to merge together to construct larger ones when Od increases. Furthermore, various polymer flow aspects in different cavity configurations are discussed and analyzed; the cavity width/aspect ratio and skewed angle are found to have significant impacts on the vortex structures and the formation of solid-like regions. Moreover, results for the critical aspect ratio at which the static dead zone is broken into two parts and the characteristic height of this zone are also reported in detail.