2. Kinematics

2.1 Phase distribution

With Lagrange variables, in relations such as r = G (a ₀ , t ), two-phase mechanics differs very little from single-phase mechanics, except that the interface separating the two phases can be crossed by material particles; the particle then undergoes discontinuities in physical properties at the instant of its passage across the interface.

With Euler variables, we must first define the domain occupied by each phase by their Eulerian distribution (figure 1 ): $\begin{array}{l}{\mathrm{\chi }}_1(\end{array}$