The geometric characteristics of the unidirectional flows discussed in Chap. 6 are their infinite extension in the flow direction and the fact that the flow cross-section does not change in the flow direction. Because of these kinematic restrictions the nonlinear terms in the equations of motion vanish, simplifying the mathematical treatment considerably. Now unidirectional flows do not really occur in nature, but they are suitable models for the flows often met in applications whose extension in the flow direction is much larger than their lateral extension. Frequently the cross-section is not constant, but varies, even if only weakly, in the flow direction. As well as the channel and pipe flows with slowly varying cross-section, a typical example is the flow in a journal bearing (Fig. 6.3), where a flow channel with slightly varying cross-section is formed due to the displacement of the journal.