Influence of the Tip Clearance on a Compressor Blade Aerodynamic Damping

The flutter CFD calculations conducted in industry generally use a single-row, single-passage approach with a minimum of detailing to reduce the computational time as much as possible. The question arises of what level of detailing is necessary for accurate aerodamping predictions. In this study, the influence of tip clearance size on the steady flowfield and aerodynamic damping is assessed for a high-pressure compressor second-stage rotor. The steady analyses are conducted on an isolated blade passage by imposing the experimental boundary conditions at the inlet and outlet of the passage. Two operating points are considered: 74 and 100% corrected speed. It is observed that, at the same pressure ratio, the mass flow decreases with a larger tip gap due to the tip clearance flow rolling into a vortex on the blade suction side. Then, unsteady analyses are conducted by enforcing the first torsion mode shape at the corresponding natural frequency. For all interblade phase angles, the same trend is observed: the damping first increases (stabilizing effect) with the tip clearance before reaching a maximum and dropping for the larger gaps. Such changes in damping would have a significant impact on both flutter and forced response predictions.