On geometrical and physical consequencesof using certain special classes of comoving physical frames ofreference

This paper develops the study of the geometrical characteristics of the class of frames of reference associated with the two well known sets of coordinates: Levi-Civita's curvature coordinates and gaussian polar coordinates. In particular, through the separate invariant formulation of the initial data problem and of the restricted evolution problem, the analysis is applied to a spherically symmetric perfect uid, whose stream lines coincide with those of the above mentioned class of frames.