The Herschel Bulkley model is applied on fluids with a non linear behaviour and yield stress. It is considered as a precise model since its equation has three adjustable parameters, providing data (Pevere & Guibaud, 2006). The Herschel Bulkley model is expressed in equation 5, where to represents the yield stress.

T = t0 + К * y" (5)

The consistency index parameter (К) gives an idea of the viscosity of the fluid. However, to be able to compare К-values for different fluids they should have similar flow behaviour index (n). When the flow behaviour index is close to 1 the fluid’s behaviour tends to pass from a shear thinning to a shear thickening fluid. When n is above 1, the fluid acts as a shear thickening fluid. According to Seyssiecq and Ferasse (2003) equation 5 gives fluid behaviour information as follows:

To = 0 & n = 1 ^ Newtonian behaviour To > 0 & n = 1 ^ Bingham plastic behaviour T0 = 0 & n < 1 ^ Pseudoplastic behaviour T0 = 0 & n > 1 ^ Dilatant behaviour

1.3.1 Ostwald model

The Ostwald model (Eq. 6), also known as the Power Law model, is applied to shear thinning fluids which do not present a yield stress (Pevere et al., 2006). The n-value in equation 6 gives fluid behaviour information according to:

T = К * y(n_1) (6)

n < 1 ^ Pseudoplastic behaviour n = 1 ^ Newtonian behaviour n > 1 ^ Dilatant behaviour

1.3.2 Bingham model

The Bingham model (Eq. 7) describes the flow curve of a material with a yield stress and a constant viscosity at stresses above the yield stress (i. e. a pseudo-Newtonian fluid behaviour; Seyssiecq & Ferasse, 2003). The yield stress (t0) is the shear stress (t) at shear rate (y) zero and the viscosity (л) is the slope of the curve at stresses above the yield stress.

t = T0 + л * y (7)

T0 = 0 ^ Newtonian behaviour T0 > 1 ^ Bingham plastic behaviour