= Viscous dissipation potential
{title2=$S\in\partial\Omega(D)$}
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 $\Omega(E)-\Omega(D)\geq S:(E-D)$ for every admissible $E$. This is a strain-rate potential, in contrast with potentials parameterized by thermodynamic forces.
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