Handbook of Turbomachinery Second Edition Revised - Ventech!

stator blades, the unsteady disturbances from the two rows can only ‘‘beat’’
each other after the whole annulus, so the circumferential wavelength is the
whole annulus, as indicated by the instantaneous static pressure pattern
shown in Fig. 10. On the other hand, if the stator blade number is changed
to 12, the two rows beat twice for the whole annulus, so the wavelength is
half the annulus (Fig. 11). The fundamental interference wavelength is thus
determined by the difference of the blade numbers of the two blade rows.
On the aeroelastic side, for both flutter and forced response problems
we have similar patterns. Vibratory patterns of blade and disk assemblies
are typically featured by a cyclic symmetry mode, rotating circumferentially
at a constant speed. For this kind of traveling wave modes, the radial lines
with zero displacements are called ‘‘nodal diameters’’. So the circumferential
wavelength is defined by the number of nodal diameters.
In both blade-row aerodynamic interaction and aeroelastic problems,
each blade will be subject to unsteadiness of a circumferential traveling wave
pattern. Since the circumferential wavelength is not the same as the blade
pitch (usually much longer for those problems of interest), we no longer
have the direct periodicity between adjacent blade passages. Instead, there is
Figure 10 Instant static pressure contours (NBr ¼ 10, NBs ¼ 9, circumferential
interference wavelength ¼ whole annulus).
Copyright © 2003 Marcel Dekker, Inc.