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Abstract: A standard method used to determine material properties of semi-solid slurries is the squeeze flow experiment; a fixed amount of material is squeezed under constant force or velocity and the relation between the force and the displacement of the sample provides information about the rheology of the slurry. The objective of this work is to contribute to the further development of the squeeze flow methodology in order to accurately determine material properties. This is achieved by a model that accounts for the finite yield stress and the thixotropy of the slurry. More specifically a structural viscoplastic model based on the Bingham plastic constitutive equation is proposed. The yield stress is assumed to vary linearly with the structural parameter which follows a first-order rate equation accounting for the material structure break-down and build-up. Numerical experiments of squeeze flow under either constant load or constant velocity are presented and discussed. Comparisons with their non-thixotropic counterparts are made in the case of compression under constant load. The development of the yielded/unyielded regions in relation to material structural changes is analyzed. The numerical results show that initially thixotropy does not affect the flow. However, once the structure is destroyed, the unyielded regions grow slower than the non-thixotropic case allowing for longer compression of the sample. Under constant force the structure may be destroyed at the early stages of the compression but at a later time it re-builds steadily till the cessation of the flow experiment. Under constant velocity, however, the structure is destroyed steadily. Depending then on the case, the final internal structure of the squeezed material can vary significantly. This is an important issue that needs to be taken into consideration in the evaluation of the material parameters. [Copyright &y& Elsevier]

The article discusses the highlights of the 3rd meeting on "Viscoplastic Fluids: From Theory to Application, organized by the Hellenic Society of Rheology, held in Limassol, Cyprus on November 1 to 5, 2009. The event tackled various topics including elastocapillarity and viscoelastoplasticity, yield-stress liquids and the concept of yield stress. It also featured a workshops on the elastic behavior of viscoplastic fluids, the discrete element method simulations of unsteady granular flows, and freezing fouling of waxes.

Abstract: Semisolid materials are suspensions of metal alloys which are processed while in a mushy state. In this state, the suspensions behave as viscoplastic materials with time-dependent material properties. During processing, the material is injected at high speed into a mold cavity with the process lasting only fractions of a second. The short-term transient material response is thus very important for the understanding of the rheology and the further development of the process. A theory based on the Herschel–Bulkley flow model appropriate for the short-term breakage of welded bonds in semisolid metal slurries is proposed. The “strength” of the slurry due to these bonds is assumed to be a function of a coherency parameter which is proportional to the number of welded bonds that break during the application of shear. The evolution of this parameter is described by a first-order kinetics. Using a novel and simple computational method, well suited for free and moving-boundary problems, we study the flow between two coaxial cylinders and obtain accurate results for the evolution of the structure, the extent of the yielded domain and the evolution of the stress. The results and conclusions apply directly to the design and analysis of rotational rheometer experiments for semisolid metal slurries and suspension systems exhibiting similar behavior. [Copyright &y& Elsevier]

We study the flow of a thixotropic fluid around a cylinder. The rheology of the fluid is described by means of a structural viscoplastic model based on the Bingham constitutive equation, regularised using the Papanastasiou regularisation. The yield stress is assumed to vary linearly with the structural parameter, which varies from zero (completely broken structure) to one (fully developed skeleton structure), following a first-order rate equation which accounts for material structure break-down and build-up. The results were obtained numerically using the Finite Element Method. Simulations were performed for a moderate Reynolds number of 45, so that flow recirculation is observed behind the cylinder, but vortex shedding does not occur. The effects of the Bingham number and of the thixotropy parameters are studied. The results show that the viscous character of the flow can be controlled within certain limits through these parameters, despite the fact that the Reynolds number is fixed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Results for a wide range of Reynolds and Bingham numbers are provided. [•] The performance of SIMPLE/multigrid for solving viscoplastic flows is assessed. [•] High truncation errors near the yield surfaces limit the finite volume accuracy. [•] Adaptive mesh refinement is efficient in reducing the discretisation errors. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the performance of the finite volume method in solving viscoplastic flows. The creeping square lid-driven cavity flow of a Bingham plastic is chosen as the test case and the constitutive equation is regularised as proposed by Papanastasiou [J. Rheol. 31 (1987) 385–404]. It is shown that the convergence rate of the standard SIMPLE pressure-correction algorithm, which is used to solve the algebraic equation system that is produced by the finite volume discretisation, severely deteriorates as the Bingham number increases, with a corresponding increase in the non-linearity of the equations. It is shown that using the SIMPLE algorithm in a multigrid context dramatically improves convergence, although the multigrid convergence rates are much worse than for Newtonian flows. The numerical results obtained for Bingham numbers as high as 1000 compare favourably with reported results of other methods. [Copyright &y& Elsevier]