The effect of stress in the continuum flow is represented by the ∇ p and ∇ ⋅ τ terms; these are gradients of surface forces, analogous to stresses in a solid. Here ∇ p is the pressure gradient and arises from the isotropic part of the Cauchy stress tensor. This part is given by the normal stresses that occur in almost all situations. The anisotropic part of the stress tensor gives rise to ∇ ⋅ τ, which usually describes viscous forces; for incompressible flow, this is only a shear effect. Thus, τ is the deviatoric stress tensor, and the stress tensor is equal to: