ANALYSIS AND IDENTIFICATION OF THE APPEARANCE OF UNSTEADINESS EFFECTS IN AN AXIAL COMPRESSOR CASCADE

The flow in a rectilinear compressor cascade with a tip gap of less than 2% of chord is investigated using the Shear Blended Eddy Simulation (SBES) hybrid model. In particular the paper focuses on the low-flow rate operating conditions where unsteady flow features near the tip region occur. The Reynolds number is fixed at 4.4 × 10⁵ while the operating condition is controlled by changing the inlet flow angle. The SBES approach allows resolving the turbulent structures of the tip gap flow while efficiently capturing the blade loading with the wall blended Reynolds-Averaged Navier-Stokes modeling. The results show the drastic change of the tip gap flow at the three selected operating conditions. The tip leakage flow analysis shows the vortex breakdown and the vortex disappearance phenomena that contribute to the unsteadiness and the blockage effects close to the clearance region. At near stall conditions, the tip leakage vortex experiences wandering. The flow along the pressure side of the blade a convective modal structures. At partial stall (PS), the tip leakage vortex core disappears, and the modal structure, in the passage along the pressure side, increases in intensity, excited by fluctuations at the leading edge of the blade.