Georges Jacquet-Richardet

Rotating Internal Damping in the Case of Composite Shafts

There is an increasing range of applications for rotors made of composite materials and operating at supercritical speeds. Design of such structures involves specific features which have to be accounted for in order to allow safe operations. A proper modeling of the mechanical characteristics of the composite is first needed. But, as far as the structure is rotating, the effect of stress stiffening and spin softening may be considered and the effect of internal damping has to be studied in order to avoid possible instability. Internal or rotating damping modeling remains an active field of research where both theoretical developments and experimental results are needed.

Identification of Contact Area From Full Field Displacement Surface Measurements

One of the most common failure modes for turbomachinery wheels is associated to high-cycle fatigue of blades. A practical way to extend working life is obtained through the introduction of specific devices that allow a reduction in vibrational magnitudes during resonance. Different kinds of components are used such as shrouds and wires for power industries and under platform dampers for aeronautics. The dry friction phenomenon between those devices and the blades induces nonlinear behaviors and flatten associated frequency response functions. This phenomenon is now well known and different modeling techniques of contact are available within numerical simulation softwares. Nevertheless, it is always difficult to estimate or to measure with sufficient precision the actual contact characteristics needed to run those softwares. Due to practical experimental capabilities, measurements are only possible quite far from the contact zone and major quantities such as the transverse loading for example are often unreachable directly. n nIn this paper, a new inverse methodology is presented. This method uses surface displacement measurements (obtained usually experimentally from conventional accelerometer and fast camera) in order to identify the characteristics of contact zones within elastic body assemblies. The new methodology is validated and illustrated by a numerical approach based on an academic set up

USE AND LIMITATIONS OF THE HARMONIC BALANCE METHOD FOR RUB-IMPACT PHENOMENA IN ROTOR-STATOR DYNAMICS

In the present paper, a Harmonic Balance Method (HBM) coupled with a pseudo-arc length continuation algorithm is pre- sented for the prediction of the steady state behaviour of a rotor- stator contact problem. The ability of the HBM to reproduce the four most common phenomena encountered during rotor to sta- tor contact situations (i.e. no-rub, full annular rub, partial rub and backward whirl/whip) is investigated. A modified Jef- fcott rotor model is used and results of the proposed algorithm are compared with traditional time marching solutions and ana- lytical predictions. The advantages and limitations of the HBM for this kind of problem are analyzed. It is shown that the HBM is orders of magnitude faster than transient simulations, and pro- vides very accurate results. However, in its current form it is unable to predict quasi-periodic behaviour. Detailed analysis of the transient solutions yields valuable information for the future extension of the HBM to efficient quasi-periodic simulations.

Effect of contact parameter uncertainties of the dynamic behaviour of rubbing cyclic assemblies

Fatigue with high number of cycles (HCF) is a current and dangerous mode of failure for blades of turbo shaft engines. It is induced by the high dynamic stresses generated at resonance in the operating range. Rubbing devices based on the use of dry friction, such as shrouds or under-platform dampers, make it possible to reduce vibratory amplitudes and even to push back resonances out of the operation zone. The governing parameters of rubbing contacts are often not known with accuracy and then are not easily controlled. They may also be the source of potentially high level responses of blades due to mistuning effects. To illustrate the influence of uncertainties, a stochastic model has been developed allowing the study of the dynamic behaviour of bladed assemblies induced by random dry friction damper parameters. These parameters are expanded on the Karhunen-Loeve base with random variables following the Gaussian law. The random nonlinear problem is based on the Monte Carlo simulation and gives the statistical characteristics of the forced response in the case of a lumped cyclic system. Results obtained confirm and illustrate the effects of variable contact parameters on the dynamic behaviour of nonlinear rubbing systems.