Numerical Investigation of the Influence of Tyler-Sofrin Modes on the Increase in Blade Unsteady Pressure in a Subsonic Axial Compressor

The present paper describes an investigation on the increase in unsteady pressure on rotor blade due to Tyler-Sofrin modes generated by stator-rotor interaction. The investigation is conducted numerically using CFD, which is unsteady RANS calculation on a quasi-3D model of a single stage cascade extracted from a 3.5 stages subsonic axial compressor. Several parameters such as stator vane count, rotor speed and axial gap between stator and rotor are varied to change characteristics of Tyler-Sofrin modes. As a result, it is found that the pressure amplitude of the 1st-harmonic of vane passing frequency on the blade surface increases at some specific conditions of the vane count and rotor speed. The modal phase velocity of a Tyler-Sofrin mode is related to the pressure amplitude. A parameter called Phase Speed Ratio (PSR) is introduced to express the relationship between the modal phase velocity of Tyler-Sofrin modes and the velocity of acoustic propagation in the circumferential direction. In the region where the PSR is lower than 1.0, the pressure amplitude gradually increases ranging from around 0.5 and peaks around 0.9. Then, the pressure amplitude decreases in the range of PSR from 0.9 to 1.0. After the PSR exceeds 1.0, the pressure amplitude rapidly decreases. This result indicates that the pressure amplitude of the 1st-harmonic of vane passing frequency increases when the modal phase velocity approaches the velocity of acoustic propagation. It is also found that the pressure amplitude peaks close to PSR = 1.0 when the axial gap between stator and rotor is large. It suggests that the non-monotonically increase in the pressure amplitude on the rotor blade is influenced by upstream stator.