Following [33], we solve for the 2D creeping flow between two coaxial cylinders. The inner cylinder rotates at a constant speed. The outer cylinder is fixed. The viscosity is a function of the second principal invariant of the shear strain rate tensor:
D= 


where D_{ij}=(∂_{i}u_{j}+∂_{j}u_{i})/2.
We use a general HerschelBulkley formulation of the form:
µ(D)= 
 +µD^{N−1}, 
where τ_{y} is the yield stress. The solutions obtained for the stationary tangential velocity profiles for Newtonian, Power law (N=0.5), HerschelBulkley (µ=0.0672, τ_{y}=0.12, N=0.5) and Bingham (µ=1, τ_{y}=10, N=1) fluids are illustrated on Figure 67, together with the analytical solutions given by [3].
The Bingham fluid case is a particularly severe test of the diffusion solver, as the outer part of the fluid ring (r>0.35) behaves likes a rigid body attached to the outer boundary.