ABSTRACT
In the discussion of flow through ducts given up to this point, it was assumed, in almost all cases, that the effects of viscosity were negligible. This is often an adequate assumption when dealing with flow through nozzles or short ducts. For long ducts, however, the effects of viscosity, i.e., the effects of fluid friction at the walls, can in fact be dominant. This is illustrated in Figure 9.1, which shows typical Mach number and pressure variations in a constant area duct with and without friction. In incompressible flow through a duct of constant cross-sectional area, the friction only affects the pressure, which drops in the direction of flow. The velocity in such a situation remains constant along the duct. In compressible flow, however, friction effects all of the flow variables, i.e., the changes in pressure cause changes in density, which lead to changes in velocity.
