Effects of mistuning and snubbing on bladed disks of turbo-machinery

Mistuning of bladed disks in turbo-machinery is practically unavoidable. Small mistuning may result in larger vibration amplitudes of single blades than larger mistuning, but sometimes single blade vibration amplitudes may be much larger than the tuned blade row amplitude, despite the fact that mistuned blade rows average vibration levels are lower. More recently blades with shrouds mounted on the disks with some inter-shroud clearance have become popular, because this arrangement may restrict the maximum blade vibration levels in some cases. This effect, called snubbing mechanism, has been analyzed for perfectly tuned rows. In this paper a first analysis is made with a numerical model of a bladed disk investigating the possible effects of mistuning on snubbing behavior. The model of the bladed disk, in which each blade is represented by a single degree of freedom system with parameters derived from single blade modal analysis, is used for analyzing its dynamical behaviour. Different amounts of randomly distributed mistuning are introduced and their effect on natural frequencies and associated mode shapes are checked. Finally the contact between shrouds is introduced and its effect on the vibrations is calculated for the tuned blade row and for the different mistuned blade rows. The results show to which extent and in which condition snubbing is helpful in reducing stress levels in vibrating blade rows.