3. Three-dimensional approaches to flow

In what follows, the flow will be considered as stationary. This means that the aerodynamic or hydrodynamic study of machine components will be based on stationary considerations and calculations, even if it means subsequently checking whether unsteady effects are significant or not.

This means that if an isolated stationary wheel is fed by a stationary axisymmetric flow, the flow will remain stationary. This will also be the case for an isolated moving wheel, provided we consider the case of relative motion, for a constant rotational speed, i.e. excluding start-up transients.