The foundations for designing and dimensioning turbocharger components for supercharging combustion engines were already laid in previous chapters. With the aid of the methods discussed, we can determine not only such basic dimensions and geometrical parameters as diameters, cross-sectional profiles, radii, and forms, but also other values, such as nominal rotation speed. This ensures that the relevant turbo-machine fulfils the basic requirements placed on it with respect to these parameters. The methods discussed, however, are based on fluid-mechanical and thermodynamic simplifications, empirical approaches or, in part, on diagrams which offer visualizations of the basic relationships and allow graphic definitions. For example, many of the equations are used on the basis of the assumption that the stationary flow filament theory applies, i.e. that the flow can be regarded as purely one-dimensional. Also, it is usually assumed that the flow can be considered nonviscous. For initial approximations, these assumptions are certainly justified. However, flows in turbo-machines are generally much more complex than the above premises would suggest. In particular, in conjunction with the rotation of the impeller, the surfaces, which can be highly bent (see Fig. 15.1), cause three-dimensional and, in part, unsteady flow processes which, because of the typically high Reynolds numbers involved, are turbulent and highly vortex-dominated. For this reason, the geometry of a component should be verified and confirmed at an early stage preceding final manufacture with the aid of a three-dimensional flow simulation. Also, turbo-machines have since become so efficient that neither a further optimization nor the analysis of deteriorations in the development process is possible any longer without more penetrating insights into the detailed flow processes involved.