Part-speed flutter analysis of a wide-chord fan blade

Experimental and theoretical results are reported in this paper for a flutter research rig with 26 wide-chord fan blades. The structural model was based on a 3D finite element eigensolution while two different 3D unsteady aerodynamic codes were used for the fluid. The first one is a thin-layer Reynolds-averaged Navier-Stokes solver using structured meshes and it was used for specified blade motion and interblade phase angle. The second code is an implicit upwind solver that uses unstructured meshes of tetrahedra elements. An inviscid flow representation, together with a viscous loss model, was used for whole annulus calculations with flexible blades. The second approach is an integrated aeroelasticity analysis in the sense that the structural and fluid equations are solved in a coupled fashion by exchanging data at each step of the time integration. Flutter predictions were attempted at 75%, 80% and 85% speeds and the least stable vibration modes were found to lie within the 0-3 nodal diameter range, in agreement with the experimental observations. The predicted variation of flutter margins with respect to speed was found to exhibit fair correlation with available experimental data. A detailed investigation of the blade's surface pressure fluctuations revealed a linear perturbation trend from the mean flow solution and highlighted the importance of getting the correct shock position in the steady flow calculations. Also, the results provided support for an inviscid flutter mechanism which is discussed in some detail.