PURPOSE AND SCOPE OF THIS DATA ITEM This Data Item introduces the subject of flow in applications where rotating machinery components induce flow rotation. There are many examples of rotating and swirling flow in engineering such as the flow between a stationary disc and a rotating disc@ in a gas turbine engine or turbocharger@ and the flow in an annulus with a rotating inner cylinder@ between the armature and stator of an electric motor. The aim of this Data Item is to introduce the phenomena involved in rotating flows and to provide the reader with guidance and techniques for modelling specific rotating flow applications. The flow applications considered in this Data Item relate to disc and cylindrical geometries@ i.e. rotating discs@ cylinders and cavities@ predominantly in steady-state@ stable flow conditions. The correlations provided are based on analytical@ experimental and computational fluid dynamics (CFD) modelling of the flow and can be used in parametric design studies and optimisation. An Introduction to this Data Item is given in Section 3.