In this section we discuss models that result when additional constraints are made on the properties of the fluids. We focus on the modeling of superfluid systems, using as our example the case of superfluid [61, 94, 110]. The equations describing more complex systems are readily obtained by generalizing our discussion. We contrast three different descriptions: (i) the model that follows from the variational framework that has been our prime focus so far if we impose that one constituent is irrotational, (ii) the potential formulation due to Khalatnikov and collaborators, and (iii) a “hybrid” formulation which has been used in studies of heavy-ion collisions with broken symmetries.
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