Gürses M.Plaue, M.Scherfner, M.2015-07-282015-07-2820110264-9381http://hdl.handle.net/11693/12247Shear-free flows or observer fields are important objects of study in general relativity; stationary or rigid observers are important examples of shear-free reference frames. In this paper, we introduce a geometric structure based on a local coordinate expression of metrics admitting a shear-free reference frame. Furthermore, we investigate a large sub-class of these models ('tilted' warped products) that includes the Robertson-Walker spacetime, the Gödel spacetime and other models of Gödel type. We present a novel example of a rotating and expanding cosmological model that is contained in this class. Finally, we describe the geodesic barotropic perfect fluid solutions. © 2011 IOP Publishing Ltd.English04.20.Cv04.20.Jb04.20.GzArticle10.1088/0264-9381/28/17/1750091361-6382