For thick suspensions and paste it is found that no flow occurs until a certain critical stress, known as the yield stress, is reached. The simplest model of a fluid with a yield stress is the Bingham model, given by = 0 + for > 0 where is the shear stress, the shear rate, 0 > 0 the yield stress, and the plastic viscosity. In the case of 0 = 0, then the Bingham fluid model reduces to the Newtonian fluid model