Handbook of Turbomachinery Second Edition Revised - Ventech!

integrity (stress levels) will have to be more adequately examined if aero
loading is to be increased, which challenges widely used empirical design
rules on blade flutter and forced responses. All these requirements would
point to closer coupling and more simultaneous (concurrent) interaction
between aerodynamic design and structural mechanical design, since a
lengthy aeromechanical iterative path can be very costly.
The competitive situation demanding better understanding and
prediction of unsteady effects is further compounded by ‘‘nonstandard’’
problems arising from increasingly important environmental concerns and
legislation. For example, a situation may arise where a required low NOx
combustion has to generate a large-scale circumferential temperature
distortion which can pass through a 5-stage low-pressure turbine with little
decay (Manwaring et al. 1997) [1]. The analysis of the blade aeromechanical
responses in this kind of nonstandard environments is becoming a necessary
part of standard procedures. An aero design that has a high efficiency in a
normal undistorted flow condition but produces an excessive stress level
under the distortion condition is obviously not to be accepted. A similar
example is the increasing priority to reduce aerodynamics-related noise
generation and propagation in future aero-engine developments due to
increasingly important environmental concerns and legislation. Again, an
aerodynamically efficient blading design will not be acceptable if its
associated noise level (especially that of fans or LP turbines at off-design
conditions) is high.
Today computation of unsteady turbomachinery flows has become a
very active research and development area. Carefully executed and analyzed
unsteady CFD computations have provided useful information and insights
to help our understanding of complex unsteady flow problems. However, it
needs to be recognized that with computer resources currently available,
full-scale time-dependent CFD calculations for unsteady turbomachinery
flows are still very expensive and are not suitable for daily design purposes.
In order to make effective use of CFD methods, we need to have a clear
appreciation of relevant flow physics and length scales and be aware of the
capabilities as well as the limitations of different flow models and numerical
methods.
In this chapter, various basic sources of periodic unsteady disturbances
and their aerodynamic and aeroelastic impacts are described first.
Relevant parameters controlling the basic unsteady flow characteristics are
then described. Particular attention is paid to the role of circumferential
wavelength. Finally several modeling issues on computational efficiency and
accuracy are discussed.
Copyright © 2003 Marcel Dekker, Inc.