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Handbook of Turbomachinery Second Edition Revised - Ventech!

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phase lag relative to the input) <strong>of</strong> the ith displacement coordinate <strong>of</strong> the<br />

structure due to a unit amplitude harmonic force associated with the jth<br />

coordinate. The complete set <strong>of</strong> dynamic flexibility FRFs forms the dynamic<br />

flexibility matrix, ½FssðoÞŠ, for the structure, and the inverse <strong>of</strong> this matrix is<br />

the dynamic stiffness matrix, ½SssðoÞŠ, for the structure. A typical dynamic<br />

stiffness FRF, SijðoÞ, may be interpreted as the ith harmonic force<br />

associated with a unit amplitude harmonic displacement <strong>of</strong> the jth<br />

coordinate with all other coordinates constrained to zero.<br />

The dynamic flexibility matrix for a support structure may be<br />

established experimentally using modal testing techniques, e.g., Ewins [7],<br />

or analytically from a finite-element model <strong>of</strong> the support structure.<br />

Typically, only the support structure coordinates that dynamic flexibility<br />

modeling <strong>of</strong> the support structure. The dynamic stiffness matrices for the<br />

rotating assembly(s), ½SraðoÞŠ, and interconnecting components, ½SicðoÞŠ,<br />

may be determined analytically from their respective governing equations.<br />

The dynamic stiffness matrix for the system is then assembled and appears<br />

typically in the form shown in Eq. (25). It is mathematically convenient to<br />

express this type <strong>of</strong> equation in complex form. The coordinates linked with<br />

the interconnecting components constitute a subset <strong>of</strong> both the rotating<br />

assembly and support structure components, i.e., fqraðoÞg and fqssðoÞg,<br />

respectively. Thus, the interconnection components are not separately<br />

identified in Eq. (25). For additional information on rotor-foundation<br />

effects and analysis, see Gasch [9, 10].<br />

2<br />

3<br />

SraðoÞ<br />

4 SicðoÞ 5 fqraðoÞg fqraðoÞg SssðoÞ<br />

¼<br />

fFraðoÞg ð25Þ<br />

f0g The harmonic force vector, fFraðoÞg, is typically the steady rotating<br />

assembly unbalance force vector, which is synchronous with the rotor spinspeed.<br />

The solution <strong>of</strong> Eq. (25) for a range <strong>of</strong> spin-speeds provides<br />

coordinate response information as well as the location <strong>of</strong> the critical speeds<br />

for the rotor-bearing-support structure system. When the rotor spins near a<br />

support structure natural frequency, the dynamic stiffness <strong>of</strong> the support<br />

structure tends to diminish significantly and the rotating assembly behaves<br />

as if it were decoupled from the support structure. A model such as<br />

suggested in Eq. (25) provides an analyst with a valuable tool to investigate<br />

the effect <strong>of</strong> the support structure design on the overall system dynamic<br />

characteristics. Based on this information, it may be possible to make<br />

support structure design changes to improve the overall system performance.<br />

Copyright © 2003 Marcel Dekker, Inc.

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