M. Imregun et M. Vahdati, Aeroelasticity analysis of a bird-damaged fan assembly using a large numerical model, AERONAUT J, 103(1030), 1999, pp. 569-578
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.