Aeroelastic stability analysis of a bird-damaged aeroengine fan assembly

Bird strike is a major consideration when designing fan blades for large-di ameter aeroengines. Current methods rely on impact tests and structural opt imisation but it is highly desirable to have predictive numerical models to assess the aerodynamic and aeroelastic stability of bird-damaged fan assem blies. The aim of this paper is to present such a methodology and to study a representative case. The particular fan assembly under investigation cont ained two consecutive blades with unequal impact damage, the so-called heav y-damage and medium-damage blades. A detailed finite element analysis of th e dynamic behaviour revealed that the vibration modes were significantly di fferent from those of the tuned assembly. The twin modes were found to be s plit into single modes, some with highly distorted modeshapes, the so-calle d rogue modes. A nonlinear viscous flow analysis revealed truly unsteady ef fects and time-accurate aeroelasticity analyses with vibratory blade motion were undertaken to investigate the flutter stability. The computational do main included both a whole-annulus fan assembly and an intake duct and the resulting mesh contained approximately 2,200,000 grid points. The investiga tion was conducted for two points on the compressor characteristic, the fir st one corresponding to higher mass flow/lower pressure ratio and the secon d one to lower mass flow/higher pressure ratio. At the higher mass flow poi nt, the flow separation was restricted to the immediate surrounding passage s and the forcing onto the downstream blades was relatively small. However, a rotating stall event was observed for the lower mass flow point and the subsequent unsteady aerodynamic forces on the blade were high. At both mass flow settings, the flutter stability of the damaged fan assembly was predi cted to be worse than that of the undamaged reference assembly. (C) 2001 Ed itions scientifiques et medicales Elsevier SAS.