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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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Figure 17 Laval–Jeffcott rotor: rotatory motion.<br />

effects <strong>of</strong> the disk) and are given by the relations<br />

obðOÞ ¼ OIp<br />

2Id<br />

<strong>of</strong> ðOÞ ¼ OIp<br />

þ<br />

2Id<br />

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

OIp<br />

2<br />

þ kr<br />

2Id Id<br />

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

OIp<br />

2Id<br />

2<br />

þ kr<br />

Id<br />

ð20aÞ<br />

ð20bÞ<br />

The natural whirl frequency, obðOÞ, is associated with a backward circular<br />

whirl <strong>of</strong> the rotor system and the frequency, <strong>of</strong> ðOÞ, is a forward-whirl speed.<br />

These undamped natural whirl frequencies are plotted in a whirl speed map,<br />

Fig. 18, for the following arbitrary choice <strong>of</strong> system parameters:<br />

Id ¼ 0:094 kg-m 2 (321:2lbm-in: 2 ), kr ¼ 95:43 kNm ð844:610 3 lbf-in:) and<br />

two different values <strong>of</strong> the polar mass moment <strong>of</strong> inertia (Ip ¼ 0:80Id and<br />

Ip ¼ 1:10Id).<br />

The forward synchronous critical speed is associated with an<br />

intersection <strong>of</strong> the forward natural frequency line in Fig. 18 with an<br />

excitation frequency equal to the rotor spin-speed, i.e., oexc ¼ O. The<br />

forward synchronous critical speed is<br />

nf ¼<br />

s<br />

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

kr<br />

Id Ip<br />

ð21aÞ<br />

Similarly, the backward synchronous critical speed is associated with the<br />

intersection <strong>of</strong> the excitation line with the backward natural frequency line.<br />

Copyright © 2003 Marcel Dekker, Inc.

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