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Handbook of Turbomachinery Second Edition Revised - Ventech!

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(random and periodic) flows. A purely steady flow will satisfy the equations<br />

with the time derivatives being zero. To answer the question raised earlier,<br />

we apply the standard time-averaging technique. An instantaneous flow<br />

variable can be expressed in terms <strong>of</strong> a time-averaged part and a fluctuation:<br />

where<br />

f ðx; tÞ ¼f ðxÞþf 0 ðx; tÞ ð13Þ<br />

f ðxÞ ¼ 1<br />

T<br />

Z<br />

f ðx; tÞdt and f 0 ðx; tÞ ¼0<br />

If the flow is randomly unsteady (turbulent), the time scale T over which the<br />

averaging is carried out should be much larger than the scales <strong>of</strong> turbulence<br />

fluctuations. For a periodic flow, T is the time length <strong>of</strong> the period. We<br />

substitute the time-averaged and fluctuation parts into the unsteady flow<br />

equations [Eq. (12)]. Then the time-averaged equations would be as follows:<br />

qru<br />

¼ 0<br />

qx<br />

ð14aÞ<br />

qruu<br />

qx þ qðruÞ0u0 ¼<br />

qx<br />

qP<br />

qx<br />

ð14bÞ<br />

qruh0<br />

qx þ qðruÞ0h0 0 ¼ 0 ð14cÞ<br />

qx<br />

The above time-averaged equations can be compared to their counterparts<br />

for purely steady flows:<br />

qru<br />

¼ 0<br />

qx<br />

ð15aÞ<br />

qruu<br />

¼<br />

qx<br />

qP<br />

qx<br />

ð15bÞ<br />

qruh0<br />

¼ 0<br />

qx<br />

ð15cÞ<br />

Comparing the time-averaged equations [Eq. (14)] with the steady equations<br />

[Eq. (15)], we have the following observations:<br />

1. The mass continuity equation remains unchanged, i.e., the timeaveraged<br />

mass flow at the inlet and outlet <strong>of</strong> a flow domain must<br />

be conserved.<br />

2. For the momentum and energy equations, the time-averaging<br />

generates extra terms, due to nonlinearity <strong>of</strong> the equations. These<br />

terms need extra relationships or equations to close.<br />

Copyright © 2003 Marcel Dekker, Inc.

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