Bingham-Papanastasiou model

An important class of non-Newtonian fluids presents a yield stress limit which must be exceeded before significant deformation can occur – the so-called viscoplastic fluids or Bingham plastics. In order to model the stress-strain relation in these fluids, some fitting have been proposed such as the linear Bingham equation and the non-linear Herschel-Bulkley and Casson models.^[1] Analytical solutions exist for such models in simple flows. For general flow fields, it is necessary to develop numerical techniques to track down yielded/unyielded regions. This can be avoided by introducing into the models a continuation parameter, which facilitates the solution process and produces virtually the same results as the ideal models by the right choice of its value.^[2]

Viscoplastic materials like slurries, pastes, and suspension materials have a yield stress, i.e. a critical value of stress below which they do not flow are also called Bingham plastics, after Bingham.^[3]

Viscoplastic materials can be well approximated uniformly at all levels of stress as liquids that exhibit infinitely high viscosity in the limit of low shear rates followed by a continuous transition to a viscous liquid. This approximation could be made more and more accurate at even vanishingly small shear rates by means of a material parameter that controls the exponential growth of stress. Thus, a new impetus was given in 1987 with the publication by Papanastasiou^[4] of such a modification of the Bingham model with an exponential stress-growth term. The new model basically rendered the original discontinuous Bingham viscoplastic model as a purely viscous one, which was easy to implement and solve and was valid for all rates of deformation. The early efforts by Papanastasiou and his co-workers were taken up by the author and his coworkers,^[5] who in a series of papers solved many benchmark problems and presented useful solutions always providing the yielded/unyielded regions in flow fields of interest. Since the early 1990s, other workers in the field also used the Papanastasiou model for many different problems.^{[citation needed]}

Papanastasiou

Papanastasiou in 1987, who took into account earlier works in the early 1960s^[6] as well as a well-accepted practice in the modelling of soft solids^[7] and the sigmoidal modelling behaviour of density changes across interfaces.^[8] He introduced a continuous regularization for the viscosity function which has been largely used in numerical simulations of viscoplastic fluid flows, thanks to its easy computational implementation. As a weakness, its dependence on a non-rheological (numerical) parameter, which controls the exponential growth of the yield-stress term of the classical Bingham model in regions subjected to very small strain-rates, may be pointed. Thus, he proposed an exponential regularization of eq., by introducing a parameter m, which controls the exponential growth of stress, and which has dimensions of time. The proposed model (usually called Bingham-Papanastasiou model) has the form:

[math]\displaystyle{ \vec{\vec{\tau}} = \left( \mu +{\tau_y \over \mid\dot{\gamma}\mid} [1-\exp(-m\mid\dot{\gamma}\mid)] \right) (\vec{\vec{\dot{\gamma}}}) }[/math]

and is valid for all regions, both yielded and unyielded. Thus it avoids solving explicitly for the location of the yield surface, as was done by Beris et al.^[9]

Papanastasiou's modification, when applied to the Bingham model, becomes in simple shear flow (1-D flow):

Bingham-Papanastasiou model:

• [math]\displaystyle{ \tau = \tau_y [1-\exp(-m\dot{\gamma})] + \mu \dot{\gamma} }[/math]
• [math]\displaystyle{ \eta = \mu + {\tau_y \over \mid\dot{\gamma}\mid} [1-\exp(-m \mid\dot{\gamma}\mid)] }[/math]

where η is the apparent viscosity.

References

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