The divergence of the deviatoric stress in case of uniform viscosity is given by: ∇ ∇ ⋅ ⋅ τ τ = 2 μ μ ∇ ∇ ⋅ ⋅ ε ε = μ μ ∇ ∇ ⋅ ⋅ ( ∇ ∇ u + ∇ ∇ u T ) = μ μ ∇ ∇ 2 u because ∇ ∇ ⋅ ⋅ u = 0 for an incompressible fluid.