Aerodynamically and Structurally Coupled Vibration of Multiple Co-Rotating Disks

This paper studies vibration of multiple, co-rotating, identical disks coupled by air flow and structural flexibility. In particular, the study focuses on coupled vibration of disk modes with two or more nodal diameters. First, frequency response functions of multiple co-rotating disks are measured in air and in vacuum to study the effects of aerodynamic coupling. In vacuum, vibration modes from each rotating disk are aerodynamically uncoupled; therefore, corresponding travelling waves from each disk have the same natural frequency. When the air is present, the air couples the corresponding travelling waves and rearranges them into a group of N traveling waves with distinct frequencies, where N is the number of the disks. A perturbation analysis is developed to prove the phenomenon of frequency splitting. Aside from the air coupling, finite element analyses and experimental measurements indicate that the flexibility of the clamp and spacers between the disks can also couple the disk vibration in the same manner. Moreover, the aerodynamic coupling is more significant for disk modes with high number of nodal diameters (e.g., 4-nodal-diameter modes). In contrast, structural coupling through spacer flexibility is more pronounced for disk modes with low number of nodal diameters (e.g., 2-nodal-diameter modes). Also, parametric studies using FEA indicate that frequency splitting from structural coupling will remain significant over a wide parameter range.