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Experimental investigations on the aeroelastic stability of compressor blades by changing bending and torsion modeshapes
- Figure 1: Decomposition of fundamental bending mode shape (a) into plunge (b) and twist (c) components
[1]- © Vahdati & Cumpsty 2016
Flutter is an aeroelastic instability, where the
vibration of a mode shape of an aerodynamic body such as a blade row
is excited by the unsteady aerodynamic forces induced by that
vibration. In the absence of sufficient mechanical damping, this
positive feedback mechanism causes an exponential growth in vibration
amplitude which can quickly lead to component failure due to material
fatigue.
The mode shapes of fans and compressors most
susceptible to flutter are those with the lowest natural frequencies,
typically the fundamental bending (or “flap”, 1F) and torsion mode
(1T). These mode shapes generally have different natural frequencies
and very little aeroelastic coupling, due to the large mass ratio, and
can therefore be considered separately for aeroelastic analysis
(unlike in wings, whose flutter stability is largely determined by
combined bending-torsion-flutter). However, because of the
three-dimensional blade design and the strong annulus contraction, the
fundamental bending mode shape of modern fan and compressor blades
usually has a more or less pronounced twist component, meaning that
the blade sections move not only perpendicular to their chord (plunge
motion) but also rotate about their mid-chord axis (twist motion), as
in figure 1.
- Figure 2: 1 Nodal diameter rotor assembly mode
-
[2]
- © TUB F1
It is known that this twist component adversely
affects the aeroelastic stability of the bending mode, which, for
attached subsonic flow, is otherwise inherently stable. In addition,
the blades in a blade row are coupled mechanically (through the disc)
and aerodynamically (through potential flow effects as well as the
change in passage area). This leads to vibration patterns with nodal
diameters, in which all blades vibrate in the same mode shape, but
with a phase-lag called inter-blade phase angle (IBPA), as in figure
2. At appropriate IBPAs, a mode shape which would be aerodynamically
damped for an isolated blade, may become unstable.
In this
research project, which is conducted in cooperation with the Imperial
College London (ICL), the effects of a variable twist component in a
bending mode shape on the flutter stability of a linear compressor
cascade at different reduced frequencies are to be investigated
experimentally (ILR/TU Berlin) and numerically (ICL). The goals of the
project are to develop aeroelastic design rules for avoiding flutter
and to provide a set of measurement data for reference cases, which
can be used to validate aeroelastic simulation codes. For this
purpose, a vibrating blade module for the aeroelastic wind tunnel at
ILR is being developed and manufactured, which allows implementation
of a prescribed blade motion with variable twist component (figure
3).
- Figure 3: Mode shape implementation using lever with adjustable torsion axis location
- [3]
- © TUB F1
The aeroelastic stability of the cascade over the
full IBPA range can then be assessed employing the aerodynamic
influence coefficient (AIC) method, based on the unsteady pressure
distribution measured on the surface of the vibrating blade and its
neighbours.
Person of contact: Dipl.-Ing. Julian Gambel
[4]
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Contact
Dipl.-Ing. Julian GambelTel. +49 (0)30 314-29481
Institute for Aeronautics & Astronautics
Room F 013
e-mail query [5]
Website [6]
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