Computational prediction of stall flutter in cascaded airfoils

At off-design conditions turbomachines may experience flow separation from the cascaded airfoils and the circumferential pattern of stalled and unstalled airfoils propagates about the flow annul us. The unsteady periodic nature of propagating stall will force blade vibration; stall flutter of the blades may occur if stall synchronization occurs and the propagation frequency is entrained by the blade natural frequency. In this study a computational scheme based on a modified form of the vortex method is used to simulate the flow over an infinite linear cascade of airfoils. The structural model is based on a two-dimensional characteristic section with one degree of freedom in either torsion or bending. The aerodynamic loading and the structural displacements are solved simultaneously by a time-marching technique. Results are presented for an airfoil cascade with constant geometry, onset flow, and stall cell wavelength as a function of the reduced frequency. Over an appreciable finite interval of blade frequency, the stall frequency becomes entrained (i.e., departs from the value it would have in the presence of completely rigid blades) and synchronization of the two frequencies occurs. This is the region of stall flutter and the details of entrainment, or synchronization, are studied in detail. Implications concerning future computational studies and the potential impact on design are presented.