Robertson-StifF model

C0 yield stress constant, Robertson-Stiff model (s 1)... 

Beirute, R.M. and Flumerfelt, R.W., An Evaluation of the Robertson - Stiff Model Describing Rheological Properties of Drilling Fluids and Cement Slurries,... 

Comparison with Eq. (6.24) shows that the Herschel-Bulkley model is the power-law model with the addition of a yield stress. Another such derivative model is the Robertson-Stiff [379] model ... 

Figure 2 7. Comparison of fit of power law, Bingham plastic and Robertson-Stiff rheological models to experimental data from bentonite drilling fluid. (Data from reference 106.)...

The model of Robertson (Robertson, 1966 Escaig and G Sell, 1982 Cook et al., 1998) is based on a statistical distribution of stiff and flexible bonds (gauche —> trans conformation). The applied stress improves the density of flexible bonds up to a critical value where the material becomes equivalent to a liquid. 

From eqs. (5) to (10) it is possible to obtain the pressure drop for the laminar regime of the suspensions that follows the Robertson and Stiff of this model flowing on a given annular geometry. The development parallels that one made by Hanks J for the Herschel and Bulkley rheological model. 

Robertson, R.E. and Stiff, H.A., An Improved Mathematical Model for Relating Shear Stress to Shear Rate in Drilling Fluids and Cement Slurries,... 

Mendelson, A., Plasticity Theory and Applications. MacMillan, New York, 1970. Robertson, R.E. and Stiff, H.A., An improved mathematical model for relating shear stress to shear rate in drilling fluids and cement slurries. Soc. Pet. Eng. J., (February), 31-36 (1976). 

---