This paper presents experimental and numerical results from the European Union Clean Sky funded ALLEGRA (Advanced Low Noise Landing (Main and Nose) Gear for Regional Aircraft) project. This project was developed in order to assess low noise technologies applied to a full scale nose landing gear model of a regional aircraft. The novel aspect of this campaign was that it was both full scale and focused on a high fidelity detailed model that included a significant portion of the fuselage, belly fairing and wheel bay. This means that, in addition to being able to evaluate the aerodynamic noise generated by the landing gear itself, it was also possible to assess the contributions of the wheel bay cavity, door and fuselage to the total acoustic output. In particular, the fact that the model includes the bay cavity within the fuselage section, allows for an investigation of the contribution of the bay cavity modes to the overall noise emission of the model. It is of interest in this paper to identify the potential for any high order cavity modes that might make a contribution to the overall noise emission. An examination of these modes is necessary if a comprehensive understanding of the acoustic behavior of landing gear bay cavities is to be reached. The noise emissions are assessed through narrowband spectra and spectrograms, the latter generated as a function of frequency and emission angle. A comparison between the results obtained using far field sensors and local sensors mounted in the bay is also presented, in order to characterize the bay cavity tones directivity and understand their contribution to the far field noise. The low and higher order cavity resonance, as well as the Helmholtz resonance, are calculated numerically, and compared with the experimental data from the full scale testing of the wheel bay cavity. The numerical method used is the wave expansion method (WEM), a highly efficient finite difference method that uses wave functions which are exact solutions of the governing differential equation. Results show that the techniques used, successfully quantify the landing gear noise in different frequency ranges. The landing gear noise was found to be up to 15 dB and dominant in the frequency range between 160 Hz and 1000 Hz, with significant contributions between 180-216 Hz and 320-348 Hz. The wheel bay noise is found to contribute tones of up to 12 dB with the Helmholtz resonance at 32 Hz and three other empty wheel bay resonant modes (1,0,0), (3,1,0) and (2,2,0) at 108Hz, 241Hz and 361 Hz respectively radiating to the far-field. A form of the well known Rossiter equation is successfully used to explain the shear layer excitation of these modes as a function of windtunnel velocity The numerical study performed on the bay matches the theoretical results and the experimental results extremely well and plots of the pressure field provide useful insight into the shape of the duct modes. With the addition of the landing gear it is found that the shear layer is disrupted and the modes no longer propagate to the far field. By adding components of the landing gear to the model piece by piece it was found that the leg itself and the doors have the greatest impact to the disruption. It is expected that a similar analysis could be performed on Main Landing Gear wheel bays, and that being larger, duct mode tones may radiate to the far field.
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