Transonic Shock Buffet in an Axial Flow Fan

Abstract

Transonic shock buffet, a self-sustained shock oscillation resulting from shock-boundary layer interaction, is observed across a range of operating points on the performance map of a transonic axial flow fan. Shock oscillations impart time-varying air loads on fan blades with the potential of leading to fatigue-induced structural failure. Accurate estimations of shock buffet onset, shock displacement, and buffet frequency are critical to lifing assessment of turbomachinery blades. This study focuses on predicting transonic shock buffet in a transonic axial flow fan using high-fidelity numerical simulations, followed by investigation of its underlying mechanisms through wave propagation analysis and modal analysis of buffet flow. Steady flow solutions obtained using a RANS solver predict performance characteristics and capture key features of the fan’s shock structure in conformation with experimental and numerical results from the literature. Unsteady flow simulations on a full-annulus model using URANS successfully capture shock buffet and its salient attributes at two operating points—near design mass flow and near stall. Wave propagation analysis and spectral proper orthogonal decomposition of buffet flow reveal a feedback loop of upstream and downstream propagating pressure perturbation waves driving shock buffet. A subtle modification to Lee’s buffet model is proposed for accurately predicting buffet frequency in a turbomachinery context. Buffet flow is characterized by circumferential, radial, and stream-wise pressure perturbation waves, with circumferential flow periodicity breaking down during buffet. A global stability analysis framework is presented and its prognostic potential for predicting shock buffet in turbomachinery is evaluated. The global stability analysis framework enables accurate prediction of buffet frequencies and associated modes with drastically reduced computational cost compared to that required for unsteady simulations. Finally, the aeromechanical response of the fan to buffet-induced unsteady air loads is assessed. The buffet frequencies do not excite resonant blade vibrations or buffeting but induce an alternating pitch-varying structural response associated with an alternating mis-staggering in the fan blades due to aerodynamic mistuning arising of buffet flow. In summary, we have shown, for the first time, transonic shock buffet in an axial flow fan can be captured using a full-annulus simulation. Further, this study advances the understanding of transonic shock buffet mechanisms, demonstrating robust methodologies for predicting shock buffet, and assessing its aeromechanical implications in turbomachinery.