THE MODE LOCALIZATION PHENOMENON IN TURBINES BLADES

Resumo

We present a numerical study of vibration modes localization phenomenon in blades of
aeronautical turbines. This kind of equipment is an example of a class of structures called periodic, that
are composed of a sequence of a large number of nominally identical substructures, the blades
themselves, lightly coupled by their hub. In the case they are really equal, the vibration modes will
extend to the whole set of substructures. If, as is to be expected of real machinery, there are very small
differences between the dynamic characteristics of the blades (stiffness, mass etc.), the mode localization
phenomenon may occur.
For localized modes, energy may get to be restricted to very few or even a single one of the substructures,
the turbine blades. This unexpected behavior may lead to catastrophic failure of the affected part due to
excessive vibration amplitude or accelerated fatigue considerations.
Our work is carried out using matrix structural analysis and we consider real geometric characteristics
of jet turbine motors for a more accurate modelling. Geometric stiffness due to traction centrifugal forces
are also taken into account, as such turbines rotate at very high frequencies. A case study of a N2 turbine
of the CFM56 engine is presented.