Multi-bladerow fan forced response predictions using an integrated three-dimensional time-domain aeroelasticity model

A non-linear integrated aeroelasticity system to predict the forced vibrati on response of aero-engine fans is presented in this paper. The computation al fluid dynamics (CFD) solver, which uses Favre-averaged Navier-Stokes equ ations on unstructured grids of mixed elements, is coupled to a modal model of the structure so that the effects of blade flexibility can be accommoda ted. The structural motion due to unsteady fluid forces is computed at ever y time step and the flow mesh is moved to follow the structure so that the resulting flow unsteadiness is determined in a time-accurate fashion. Two f an forced response case studies are reported in detail. The first one deals with a high-pressure ratio fan. the excitation being due to the upstream v ariable-angle inlet guide vanes (VIGVs). The unsteady flow analysis with bl ade motion was conducted using a sector of three VIGVs and four rotor blade s. The wake predictions were found to be in good agreement with the corresp onding laser measurements. The flow was observed to be completely separated for high VIGV angles and the excitation encompassed several harmonics. The predicted rotor blade vibration levels were generally found to be within 3 0 per cent of the measured values. The forced response to upstream obstruct ions was studied in the next fan case study. Three whole bladerows, consist ing of 11 struts, 33 VIGVs and 26 rotor blades, were modelled in full. The model also included a prescribed inlet distortion pattern so that the combi ned effects of stator wakes and inlet distortion on the response of the rot or blades could be studied. The unsteady flow calculations were conducted u sing a time-accurate non-linear viscous flow representation. Blade motion w as also included. Such an undertaking required about 4.2 million grid point s to include all three bladerows in a complete stage calculation. To reduce the computational effort, a number of smaller computations were conducted by considering the stator and rotor domains separately: the outflow solutio n of the stator domain was used as an inflow boundary condition to the roto r domain. The results indicated that such isolated bladerow computations we re likely to underpredict the response levels because of neglecting rotor-s tator interactions. A number of low engine order (LEO) harmonics were ident ified from an inspection of the unsteady forcing created by the inlet disto rtions. Good agreement was obtained for cases where experimental data were available.