Numerical investigation of the HQ-liner system for inlet fan noise reduction Romain Mar´ echal, Emmanuel Perrey-Debain, Jean-Michel Ville Universit´ e de Technologie de Compi` egne Laboratoire Roberval UMR CNRS 6253 BP 20529 - 60205 Compi` egne C´ edex France Over the last decade, the concept of Herschel-Quincke (HQ) waveguide resonators has been the subject of intensive research for fan noise control in aircraft turbofan engines. The approach has been numerically and experimentally tested in combination with typical acoustic liners for both inlet and aft fan noise reduction (see for instance [1, 2] and references therein). In particular, results have demonstrated potential of the HQ tubes for the Blade Passing Frequency (BPF) tone attenuation whereas the liner is effective to reduce broadband noise. More recently, adaptive HQ tubes have been developed in order to target the BPF tone for different operating conditions [3]. Despite the progress being made, it is thought that additional research effort should be carried out to evaluate and optimize such an acoustic system. To this end, a new hybrid numerical technique for the computation of the scattering matrix has been devised by the present authors [4]. The technique which is inspired from the numerical model developed in [2] offers two major improvements: (i) the lined portion of the duct is of finite extent and (ii) the exact shape of the HQ tube is taken into account. The HQ-liner system consists of a regular array of HQ tubes installed around the circumference A + A - B + 300 400 500 600 700 800 900 1000 1100 1200 84 86 88 90 92 94 96 98 100 102 SPL (dB) Frequency (Hz) Figure 1: Left: Schematic of the HQ-liner system with 36 tubes. Right: Acoustic performance: rigid duct (straight line), liner (dotted), HQ-liner (dashed). of the inlet duct which acts as circular acoustic waveguide as shown in Fig.1 (left). The cylindrical area (in grey) corresponds to the acoustically treated portion of the duct whereas the rest of the duct is rigid. The incident sound field generated by the fan is decomposed in terms of propagative modes with different amplitudes (vector A + ) and similarly for the transmitted and reflected fields. The key ingredients of our hybrid model rely on the use of the Green’s function formalism for the lined duct and the notion of impedance matrix that connects the pressure to 1