Some Insights into the Simultaneous Forward and Backward Whirling of Rotors

One of the greatest advantages of using complex coordinates in the study of rotating machinery is the gain of physical insight into the forward and backward precessional modes. Together with the complex modal analysis formulation, other interesting tools (like the Shape and Directivity Index Plot) were developed to characterize the shape and directivity of the whirl modes. These tools are helpful in understanding the bizarre phenomenon of mixed operational modes in rotors where some stations describe their precessional motion in the forward direction while others move in the backward direction. This study is carried out using a complex coordinate based finite element model of rigid as well flexible rotors supportad on anisotropic bearings.