fluid, i.e., the fluid with an equation of state $p=p(\epsilon)$, the solution of the relativistic Euler equation can be given by Boyer’s condition determining the surfaces of constant pressure through the “equipotential surfaces” of the potential $W(r,\theta)$ by the relations[@Abr-Jar-Sik:1978:ASTRA:] $$\begin{aligned}
  &&\int_0^p \frac{\d p}{p+ \epsilon} = W_\mathrm{in} - W =
    \ln (U_t)_\mathrm{in} - \ln (U_t) +
    \int_{\ell_\mathrm{in}}^{\ell}
    \frac{\Omega\,\d\ell}{1- \Omega \ell};                       \label{eqv9}\end{aligned}$$ the subscript “in” refers to the inner edge of the disc.

The equipotential surfaces are determined by the condition $$W(r,\theta) = \mathrm{const},
\label{eqv10}$$ and in a given spacetime can be found from Eq. (\[eqv9\]), if a rotation law $\Omega = \Omega(\ell)$ is given. Equilibrium configurations of test perfect fluid are determined by the equipotential surfaces which can be closed or open. Moreover, there is a special class of critical, self-crossing surfaces (with a cusp), which can be either closed or open. The closed equipotential surfaces determine stationary toroidal configurations. The fluid can fill any closed surface – at the surface of the equilibrium configuration pressure vanishes, but its gradient is non-zero.[@Koz-Jar-Abr:1978:ASTRA:] On the other hand, the open equipotential surfaces are important in dynamical situations, [*e.g.*]{}, in modeling of jets.[@LyB:1969:NATURE:; @Bla:1987:300YoG:] The critical, self-crossing closed equipotential surfaces $W_\mathrm{cusp}$ are important 