Newtonian hydrodynamic equations with relativistic pressure and velocity
  We present a new approximation to include fully general relativistic pressure and velocity in Newtonian hydrodynamics. The energy conservation, momentum conservation and two Poisson's equations are consistently derived from Einstein's gravity in the zero-shear gauge assuming weak gravity and action-at-a-distance limit. The equations show proper special relativity limit in the absence of gravity. Our approximation is complementary to the post-Newtonian approximation and the equations are valid in fully nonlinear situations. 
Jai-chan Hwang, Hyerim Noh, J'ulio Fabris, Oliver F. Piattella,   Winfried Zimdahl
gr-qc astro-ph.CO