Variational Approach to Hydrodynamics - from QGP to General Relativity
  We derive the special and general relativistic hydrodynamic equations of motion for ideal fluids from a variational principle. Our approach allows to find approximate solutions, whenever physically motivated trial functions can be used. Illustrating this, a Rayleigh-Plesset type equation for the relativistic motion of a spherical (QGP) droplet is obtained. Also the corresponding general relativistic effective Lagrangian for spherically symmetric systems is presented. 
H.-Th. Elze, T. Kodama, Y. Hama, M. Makler and J. Rafelski