On the Nature of Transonic Shock Buffet in an Axial-Flow Fan

Transonic buffet or self-sustained shock oscillation in transonic flow is not well-understood in fans and compressors. The present work is a numerical study of transonic buffet flow using a full-annulus model of a transonic fan-the NASA rotor 67. Firstly, results obtained with steady simulations are validated with experimental and numerical data available, and shock structures from these simulations are found to exhibit distinct features as seen in experiments. Unsteady Reynolds-averaged Navier-Stokes simulations are then carried out at two operating points-near design mass flow rate and toward stall-on the fan characteristic to capture transonic shock buffet. The unsteady simulations are able to capture necessary attributes of transonic buffet flow. Further, wave propagation in an unsteady flow solution featuring transonic buffet is investigated to understand the mechanism driving shock buffet in axial turbomachines. Upstream- and downstream-propagating pressure perturbation waves are observed on the blade suction surface within a buffet cycle, while only an upstream-propagating wave is seen on the blade pressure surface. The driving mechanism of transonic buffet in this turbomachine is elucidated through a feedback loop of the upstream- and downstream-propagating waves-qualitatively and quantitatively. Besides the streamwise pressure waves, circumferential pressure waves propagating at part speed of the rotor in the spinning direction and radially outward moving pressure waves of similar speeds on the blade suction and pressure surfaces are also detected as distinct features of buffet flow for this turbomachine.