In this section, an abstract framework is presented to define a particular family of compressible fluids inwhich the stress is considered to be equal in all directions for any given strain rate. The present study is restricted to this class of (inviscid) superfluids, but the theory can be
extended to encompass other kinds of constitutive equations as proposed in [9] and [10]. By definition, for this class of fluids, the free energy ψ is a differentiable function of the absolute temperature θ and of the density