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[FAQ]

Concrete is known to exhibit a yield stress. The viscosity should be suitably described by means of a Bingham model. What parameters can be selected? Following information found in the literature, a typical model may involve a yield stress (ystr) of some 500 Pa and a viscosity factor (fac) of 500 Pa.s. In POLYFLOW, an additional parameter “gcrit” is used which describes the transition between the yield behaviour to the flow regime. This can receive a relatively small value, e.g. of 0.01 s-1. A smaller value will generate a sharp transition and can make the calculation a bit more difficult (more iterations can be required), a larger value generates a smoother transition and can lead to a possible departure of the model properties vs. the fluid ones. Using the modified Bingham law in POLYDATA is probably the first choice as it involves an analytical and continuous expression for all shear rates.