Introduction

In this chapter we combine the flow-field time dependency with a fully three dimensional solution domain. The result is a large-size computational model requiring a great deal of computer resources. In view of how involved the problem is, several CPU time-saving techniques are devised and implemented.

Example

The relative motion between the stator and rotor subdomains within an axial (Figure 15.1) or centrifugal turbomachinery stage creates an unsteady-flow field that is periodic in time. Limiting the discussion to the axial turbine stage case, the statorcascade wake pattern around the circumference (see Figure 15.1) not only will shape the rotor flow behavior but also will expose its blades to a pattern of oscillating pressure that may very well lead to premature fatigue failure. In fact, the close proximity of the two (stator and rotor) cascades (Figure 15.2) in a predominantly subsonic flow field places the stator vanes in the same fluctuating-stress environment, but with lesser amplitude by comparison.

The small stator/rotor axial-gap length within a typical turbomachinery stage is a double-edge sword. On the one hand, the smaller the gap the less is the total pressure loss within it. This loss is a natural outcome of the boundary layer growth over the endwalls, which, together with the profile losses, constitutes a significant part of the stage losses. However, a small gap length magnifies the cyclic fluctuations within the rotor subdomain as a result of wake cutting, upstream vortex shedding, and potential flow interaction between the stationary and rotating blade rows.