Numerical investigation on the forced vibration induced by the low engine order under boundary layer ingestion condition

Boundary layer ingestion propulsion systems have attracted much attention due to the significant potential to reduce the fuel consumption of future commercial aircraft. However, the fan blade, as a key component, needs to be designed to overcome the challenge of distorted inflow due to the boundary layer ingestion. Besides the negative effects on aerodynamic performance, the aeroelastic issues such as the forced response can be caused by the low engine order due to inflow distortion. To investigate this problem, the transonic fan NASA Rotor 67 with a BLI-type inlet distortion is used as the test case. The vibration mode of a single rotor blade is analyzed and there is an intersection between the 2EO and first-order bending mode at the design rotor speed. As demonstrated by three-dimensional unsteady simulations and modal force analysis, the first-order bending vibration mode of the rotor occurs with relatively high intensity, compared to the modal force of the forced vibration induced by the effects of stator downstream. The results of the Zig-zag-shaped excitation lines in the nodal diameters versus frequency graph present the vibration mode of the full-annulus rotor blade is a 1B-2EO-2ND. Based on the unsteady flow field analysis, the periodic evolution of the flow through a passage due to the various inlet conditions could be the main reason for the propagation of 2EO.