The total enthalpy rise imparted to the fluid by the impeller is termed the impeller work input. An accurate prediction of the impeller work input is fundamental to all aspects of centrifugal compressor aerodynamic design and analysis. As discussed in Section 1.3, the work input supplied by the impeller blades is the useful work, which supplies the pressure rise. There are also various sources of parasitic work, which can be considered wasted energy. The fluid in the clearance gaps between the impeller and the housing exert frictional forces on the impeller, consuming part of the power supplied to it. This is a parasitic work commonly called the windage and disk friction loss. For covered impellers, fluid leakage from the impeller discharge back to the inlet through the impeller eye seal (see Fig. 1-4) is another source of parasitic work, known as the leakage loss. Since the pressure rise imparted to this fluid is dissipated in the clearance gap and seal, the impeller works on this portion of the fluid twice, with no addition contribution to the pressure rise. These two sources of parasitic work are easily recognized physical phenomena. From experience, it is known that other parasitic work models are needed to explain the observed work input curves from compressor test results. These take the form of models postulated from fluid dynamics reasoning and are validated by comparing predictions with experimental data. Here, the models of Aungier (1993b, 1995) will be reviewed. It is reasonable to also expect a leakage loss in open impellers, where the fluid leaks through the blade clearance gaps, dissipating a portion of the pressure rise, to be re-energized by the impeller in the next blade passage. In highly diffusing impellers, another parasitic work, the recirculation loss, is observed. It is believed to result from a portion of the fluid at the impeller tip reversing to re-enter the impeller at the tip, again dissipating part of its pressure rise and requiring the impeller to re-energize it. It is convenient to express the impeller work in dimensionless form as the work input coefficient defined in Eq. (1-6).