Transient throttling and prestall disturbances: insights from scale-resolving simulations

When approaching stall, rotating disturbances close to the blade tip clearance arise in many turbomachines. As a clear precursor to stalled operation, this regime imparts additional unsteady loading on the blades, possibly inducing noise and non-synchronous vibrations. Advancing beyond the current limited comprehension of this flow condition, this work employs full-annulus, scale-resolving simulations of a single-stage axial compressor to elucidate the initiation mechanisms of prestall disturbances and rotating stall. For that, transient throttling is implemented at the channel outlet. The numerical setup is validated by unsteady experimental results. The findings are contrasted with previous scale-resolving assessments with constant throttling levels. Testing the previously proposed hypothesis correlating time-average blockage and leading edge spillage with the onset of prestall disturbances, the present study confirms its validity under transient boundary conditions.