  We are interested in the evolution of a compressible fluid under its
self-generated gravitational field. Assuming here Gowdy symmetry, we
investigate the algebraic structure of the Euler equations satisfied by the
mass density and velocity field. We exhibit several interaction functionals
that provide us with a uniform control on weak solutions in suitable Sobolev
norms or in bounded variation. These functionals allow us to study the local
regularity and nonlinear stability properties of weakly regular fluid flows
governed by the Euler-Gowdy system. In particular for the Gowdy equations, we
prove that a spurious matter field arises under weak convergence, and we
establish the nonlinear stability of weak solutions.
