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Handbook of Turbomachinery Second Edition Revised - Ventech!

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one period Tp (neglecting change <strong>of</strong> the vibration amplitude) will be<br />

Z Tp<br />

WA ¼<br />

FAVBdt<br />

¼ pAFAX sinðf FXÞ<br />

ð11Þ<br />

A flutter instability would occur when the net work input to blade vibration<br />

is positive (i.e., a negative aerodynamic damping when the induced unsteady<br />

force leads the blade vibration displacement, 0 < f FX < p).<br />

MODELING ISSUES<br />

As for any other engineering problems, there are two basic considerations in<br />

prediction <strong>of</strong> unsteady flow effects: accuracy versus efficiency. Very <strong>of</strong>ten, a<br />

blading designer or method developer may face a choice between a<br />

seemingly more complete model and a truncated/reduced one. Usefulness <strong>of</strong><br />

a complete model is likely to be dictated by numerical resolution limited by<br />

computing power, while the truncated model apparently would neglect some<br />

phenomena deemed to be unimportant for the problems <strong>of</strong> interest. In the<br />

unsteady aerodynamics and aeroelasticity area, a judgment can be helped by<br />

appreciation <strong>of</strong> a common modeling issue, flow nonlinearity.<br />

Unsteadiness/Time-Averaged Flow Interaction (Nonlinearity<br />

and Deterministic Stresses)<br />

In relation to blading designs, aerodynamic designers normally are only<br />

interested in time-mean performances, rather than details <strong>of</strong> unsteady<br />

disturbances. The basic question is, can we obtain the time-mean parameters<br />

by solving the steady flow equations under the time-mean flow conditions?<br />

Effectively, what we ask is the difference between the steady flow equations<br />

and the time-averaged unsteady flow equations. The fundamental principles<br />

can be illustrated in a very simple situation. Let us consider a 1D inviscid<br />

flow governed by the following equations:<br />

Continuity :<br />

Momentum :<br />

Energy :<br />

qr qru<br />

þ ¼ 0<br />

qt qx<br />

ð12aÞ<br />

qru qruu<br />

þ ¼<br />

qt qx<br />

qP<br />

qx<br />

ð12bÞ<br />

qre qruh0<br />

þ ¼ 0<br />

qt qx<br />

ð12cÞ<br />

This set <strong>of</strong> equations is applicable to one-dimensional steady and unsteady<br />

Copyright © 2003 Marcel Dekker, Inc.

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