However, when frictional effects are small, a flow may be approximately reversible. For instance, in the flow through a large duct where the boundary layers are very thin, viscous effects are confined to a rather small region, and the fluid friction may sometimes be neglected. This is not the case for most practical flows. In most pipe and duct flows, for example, the velocity gradients extend over the entire cross-section and frictional stresses are all important. Even if the boundary layers are thin to begin with, they can thicken rapidly under some circumstances, and the flow can separate , as in the diffuser flow shown in figure 10.
Flow is from left to right.
From Japan Society of Mechanical Engineers. Large losses can occur, as the mechanical energy of the fluid is used to drive large unsteady eddying motions which eventually dissipate into heat. This type of flow can occur for quite small angles. Flow in a contraction. Flow is from left to right.