Investigation of non-synchronous vibration mechanism for a high speed axial compressor using delayed DES

This paper uses the delayed detached eddy simulation (DDES) of turbulence to investigate the mechanism of non-synchronous vibration (NSV) of a multistage high speed axial compressor. DDES is a hybrid model for turbulence simulation, which uses RANS model within the wall boundary layer and uses large eddy simulation outside of the wall boundary layer. Time accurate Navier-Stokes equations are solved with a 3rd order WENO reconstruction for the inviscid flux and a 2nd order central differencing for the viscous terms. A fully conservative rotor/stator sliding BC is used to resolve the unsteady interaction between the rotor and the stationary blades. A 1/7th annulus sector is employed with the time shifted phase lag BC at the circumferential boundaries. The DDES shows that the NSV of the compressor occurs due to the rotating flow instability in the vicinity of the rotor tip at a stable operation condition. The tornado-like tip vortex causes the NSV of the rotor blades as it propagates to the next blade passage in the counter rotating direction above 80% rotor span. The tornado vortex travels fast on the suction surface of the blade and stays relatively longer at the passage outlet crossing to the next blade leading edge. Such a tornado vortex motion trajectory generates two low pressure regions due to the vortex core positions, one at the leading edge and one at the trailing edge, both are oscillating due to the vortex coming and leaving. These two low pressure regions create a pair of coupling forces that generates a torsion moment causing NSV.