The formulation of standard procedures to determine the proneness to flutter or the forced-response vibration amplitude due to an external stimulus is generally based on modelling that assumes that all disk blades are identical – the so-called ‘tuned’ rotor. But real rotors – even when they are brand new – already show deviations (mistuning) from this ideal shape due to manufacturing and material tolerances, etc. These deviations have an (usually positive) effect on the tendency to flutter and a (usually negative) impact on the blade forced-response amplitude. To assess this impact, the Institute is developing numerical and experimental methods that take this phenomenon into account. With the aid of substructure technology and the use of reduced-order modelling techniques, the forced response amplitude is statistically estimated. By breaking the rotor model down into disc submodels and single blades, it is possible to generate different mistuning models through the variation of the blade parameters. The following statistical process ends in a Weibull probability distribution for the maximum amplitude, for which the computation time required is only a fraction of that needed for the conventional determination of a statistically verified maximum amplitude.
The data for real rotors should be incorporated into the models in parallel with the numerical studies. To do so, an experimental set-up through which the mistuning pattern of Blisk rotors can be identified is being developed. First, a generic rotor, as well as the corresponding chuck, are being manufactured as the test structure in a simple plate construction. On this structure, different measurement and stimulation techniques will be trialled for identification. In the final stage, real Blisk rotors will be studied and the influence of geometry variations will be determined in order to reversibly be able to affect the aeroelastic behaviour by targeted mistuning (such as reducing the tendency to flutter at unchanged or only moderately elevated response amplitudes).