Validation of an in-duct to far-field beamformer method for predicting far-field fan broadband noise


Tester B. J. , ÖZYÖRÜK Y. , Sutliff D. L. , Bozak R. F.

22nd AIAA/CEAS Aeroacoustics Conference, 2016, Lyon, France, 30 May - 01 June 2016 identifier

  • Publication Type: Conference Paper / Full Text
  • Volume:
  • Doi Number: 10.2514/6.2016-2894
  • City: Lyon
  • Country: France

Abstract

© 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.Far-field inlet fan noise can be measured under anechoic conditions in some model scale fan rig test facilities but far-field aft fan noise measurements are often not possible because the bypass flow is ducted away through a throttle into an exhaust stack. A beamformer-based technique has been developed for processing measurements taken with an in-duct axial (‘phased’) array in the bypass duct, which with certain modelling assumptions, enables the fan broadband noise level and directivity to be predicted in the far-field. Validation with a realistic fan noise source has been partially achieved by using experimental data from the NASA ANCF low-speed fan rig with flow up to Mach 0.17. The modal transfer functions are computed using a ‘plug’ flow exhaust model based upon a well-established Wiener-Hopf far-field technique (‘GXMunt’) but since the measured far-field data is located at three diameters, modal transfer functions are also computed at that distance and with a spreading jet model using a linearised Euler code (‘FLESTURN’). Both models yield predictions that agree reasonably well with measured data but the latter is more accurate at small angles to the jet axis. However, there are some significant deviations at certain frequencies. This may be due to the original ANCF in-duct axial array being designed for tone measurements. Those measurements exhibited variations in broadband SPL across the array, which is almost certainly some kind of flow-microphone effect, it being absent in the no-flow test data. A new array has now been designed, tested and installed in the ANCF intake, as part of a new phase of the work, to validate the method for fan inlet noise. Preliminary results from this new array, included here, are encouraging. To complete the validation of this method, data is required at the higher Mach numbers corresponding to the aircraft approach, cut-back and sideline conditions, with realistic bypass nozzle and inlet geometries.