Forced Response Induced by Low Engine Order Under Circumferential Inlet Distortions With Different Extents

Boundary-layer ingestion systems have the potential to improve propulsion efficiency using fluid with lower momentum near the airframe. However, they also bring variations in the velocity and total pressure, leading to a complex distortion of the flow at the inlet stage in the form of a circumferential step variation with significant strength. Previous simulation of NASA Rotor 67 under a 120-deg sector distortion revealed the occurrence of first-order bending mode excited by the second engine order (2EO) under circumferentially distorted inflow. To explore if the variation of distortion extent could affect the excited mode, five cases are simulated for different sector angles of distortion extent: 60, 90, 120, 150, and 180 deg. The result of modal-force analysis demonstrates that the modal force increases from the 60-deg case to the 90-deg case and then decreases from the 90-deg case to the 180-deg case, and the lowest value is reached in the 180-deg case. To further investigate the excitation sources leading to such variations of modal force, the harmonic force caused by periodic flow is analyzed based on the Fourier decomposition. The results indicate two main sources for the excitation of 2EO: (1) the harmonic force induced by the distorted inflow, and (2) the dynamic response of the fan blade caused by a sudden drop of inlet total pressure and controlled by the time constant.