 Viscous dissipation potential

A convex scalar function $\Omega (D)$ on symmetric rate-of-strain tensors generates stress by $S=\partial \Omega /\partial D$. For incompressible flow, its argument is trace free and a pressure reaction must be added to the stress. At nondifferentiable points the law is $S\in \partial \Omega (D)$, where the subdifferential is defined by $\Omega (E)-\Omega (D)\ge S:(E-D)$ for every admissible $E$. This is a strain-rate potential, in contrast with potentials parameterized by thermodynamic forces.