Aeroelasticity analysis of a bird-damaged fan assembly using a large numerical model

Bird strike is a major consideration when designing fans for civil aero eng ines. Current methods rely on impact tests and structural optimisation but it is highly desirable to have predictive numerical models to assess the ae rodynamic and aeroelastic stability of bird-damaged fan assemblies. A detai led feasibility study towards such a prediction capability, consisting of a CFD solver coupled to a finite element representation of the structure, is reported in this paper. A whole-assembly model was used for both the fluid and the structural domains, an approach necessitated by the loss of cyclic symmetry due to one or more blades undergoing plastic deformation under th e effect of the bird impact. It was assumed that two consecutive blades had suffered unequal amount of bird damage, the so-called heavy-and medium-dam aged blades. A viscous steady-state solution of the bird-damaged assembly w as computed first. The results indicated the formation of a strong wake fro m the heavy-damaged blade onto the downstream medium-damaged blade. It was also found that the mass flow had reduced considerably due to the blockage effect of the damaged region. A structural analysis of the fan assembly sho wed that the vibration modes were significantly different from those of the corresponding undamaged assembly. Viscous unsteady flow calculations with blade motion were performed for the whole assembly and the results indicate d vibration instability in a torsional mode and the possibility of rotating stall, both being due to flow separation behind the heavy-damaged blade. T he aeroelasticity computations required about 1Gb of memory and took about 50 days on a fast single-CPU workstation. The predictions were found to be in good agreement with available experimental data.