ON THE MECHANISM OF UNSTEADY TIP FLOW EVOLUTION ACROSS ROTATIONAL SPEEDS IN A TRANSONIC AXIAL FAN

Abstract This study explores the evolution of unsteady tip flow structures in a transonic axial fan rotor across 70%, 80%, 90%, and 100% of design rotational speed, via experiment-validated unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations and detailed flow diagnostics. Unlike prior work focused on near-stall behavior only at design speed, this research clarifies how rotational speed modulates the onset and characteristics of unsteady leakage vortex dynamics—key to fan stability margin and loss generation. Stall inception at 70% and 100% design speeds is primarily associated with suction surface separation; for the latter operating condition, such separation is predominantly induced by passage shocks. In contrast, stall mechanisms at 80% and 90% design speeds are dominated by TLV-shock interactions. Findings reveal a transition from shock-vortex-dominated tip flow unsteadiness at high speeds to shear-layer instability-driven oscillations at low speeds, highlighting competing centrifugal stabilization and compressibility effects. This work advances understanding of speed-dependent stability boundaries, emphasizing rotational effects for physics-based transonic fan stall control.