In order to discretise the Stokes equations in the case of variable viscosity, conservative finite differences should be used. Such finite differences provide conservation of stresses between nodal points and thus allowa correct numerical solution.
Below,
examples of non-conservative and conservative finite differences for the 1D incompressible
Stokes equation (
∂σ
xx
∂x
− ∂P
∂x
= 0, where σ
xx
= 2η
∂vx
∂x
) are compared
for 1D staggered grid (Fig. 7.10).