Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 22 Typical precessional modes: generalized Laval–Jeffcott rotor. (a) First
forward mode, (b) second forward mode.
Disk Flexibility Effects
Rotordynamics analyst should carefully evaluate whether disk flexibility
may significantly influence the lateral dynamics of a rotating assembly. A
flexible disk or bladed disk assembly is usually cyclosymmetric. These
flexible disks have a large number of natural frequencies consisting of
‘‘umbrella-type’’ modes that have only circular nodal lines, and ‘‘diametraltype’’
modes where diametral and circular nodal lines may exist. Examples
of these types of modes are illustrated in Fig. 24.
The umbrella-type modes are coupled only with the axial motion of
the rotating assembly. Since axial motion is usually of minor importance for
rotordynamics systems, these modes are generally not of concern. All of the
remaining modes, those with diametral nodal lines, are self-equilibrating
with the exception of modes with a single nodal diameter such as illustrated
in Fig. 24(b). The term ‘‘self-equilibrating’’ implies that the summations of
Copyright © 2003 Marcel Dekker, Inc.