Acoustic liner design for FENESTRON(r) noise reduction
The paper presents work done on the design of two different liners for FENESTRON(r) noise reduction on a 0.7:1 scaled H135 test rig. A short presentation of typical H135 spectral noise emission is given which forms the basis for the liner design regarding its spectral absorption domain. The existing H135 test rig is described, the modification according to the BLUECOPTERTM design of Airbus Helicopters as shroud design, stator design and fan blade design are presented. After describing test procedure and test parameter the design of the liner is given. According to tonal BPF noise components resulting from turbulent inflow conditions and rotor stator interaction and stochastic broadband components due to high vorticity flow at the fan blade tips two kind of liner haven been designed. An acoustic, double degree of freedom (DDOF) liner was studied both, numerically and by experiments and integrated behind the fan plane. His absorption efficiency was simulated and tested. In addition an aerodynamic liner was integrated for vorticity reduction in the vicinity of the fan blade tips by allowing flow penetration into a partially permeable shroud surface. Stochastic noise reduction was expected due to vorticity reduction. The efficiency of both types of liner could be demonstrated by measurements. Up to 3 dB(A) OASPL reduction was demonstrated.