Mistuned Flutter Analysis of Strongly Coupled Bladed Disks in Conjunction with Reduced Order Modeling

Kurzfassung

Bladed disks are often analyzed under the assumption that the whole structure consists of perfectly identical substructures or sections. But real bladed disks exhibit small variations in geometry and material properties among the sectors. These differences, called mistuning, can have a large impact on the dynamic behavior and stability of the system. A thorough review of reduced order models is performed for the description of the dynamic behavior of mistuned rotors. A flutter analysis for a mistuned cantilevered and shrouded bladed disk is conducted using a higher order approach and compared against the reduced Fundamental Mistuning Model. For both cases, the first mode family is analyzed which is a bending mode for the cantilevered and a mixed bending-torsion mode for the shrouded bladed disk. An alternate mistuning pattern is implemented by proportionally scaling the Young’s modulus of the structural finite element model. This allows taking into account the complete mistuning effects since all the perturbations including the eigenmode, frequency and aerodynamics are present in the flutter computations. The cantilevered bladed disk model, representing a structurally weakly coupled system, is created by removing the cyclic symmetry boundary condition at the blade tip of the shrouded bladed disk. Using the higher order approach, it is shown that the effect of mistuning has less influence on the structurally strongly coupled system compared to the weakly coupled bladed disk. A small mistuning level positively affects the cantilevered bladed disk. Further increase in mistuning stabilizes the system and the aerodynamic work becomes independent of the inter-sector phase angle. On the other hand, alternate mistuning of rotors with a high number of blades does not alter the eigenmodes of the shrouded bladed disk greatly enough, especially the lower nodal diameters. On the contrary, an unfavorable effect of alternate mistuning is observed which is seldom reported in the literature. While the tuned results agree quite well between the higher order and reduced order approach, there are some noticeable differences for the mistuned bladed disk. Despite some quantitative differences, the Fundamental Mistuning Model correctly predicts the stabilizing trend with increasing mistuning amplitude of the cantilevered bladed disk. However, the reduced approach fails to qualitatively and quantitatively resemble the mistuned results of the higher order approach for the shrouded case.