Energy-variational structure in evolution equations

We consider different measure-valued solvability concepts from the literature and show that they could be simplified by using the energy-variational structure of the underlying system of partial differential equations. In the considered examples, we prove that a certain class of improved measure-valued solutions can be equivalently expressed as an energy-variational solution. The first concept represents the solution as a high-dimensional Young measure, whereas for the second concept, only a scalar auxiliary variable is introduced and the formulation is relaxed to an energy-variational inequality. We investigate four examples: the two-phase Navier–Stokes equations, a quasilinear wave equation, a system stemming from polyconvex elasticity, and the Ericksen–Leslie equations equipped with the Oseen–Frank energy. The wide range of examples suggests that this is a recurrent feature in evolution equations in general.