A New Modular Fan Rig Noise Test and Radial Mode Detection Capability

A new major large-scale fan rig test facility, UFFA (Universal Fan Facility for Acoustic), has been designed with the objective to allow test bed changes for engine representative OGVs and bypass duct annulus and liners, for reduced build times, and higher fidelity investigation of aft fan noise technologies. An important enhancement consisted in the implementation of three Radial Mode Detection (RMD) devices in the bypass duct and further downstream in the nozzle equivalent plane. High effort was spend on the realisation of a wall-flush mounted sensor array, which has the advantage not to disturb the flow and the acoustic field. However, the separation of different radial mode contributions is realised only implicitly by the analysis of the axial wave number spectrum, which is particularly challenging if sensors are installed only at the outer duct wall. More robust from the numerical point of view is the established technique to directly measure the radial structure of the sound field with sensor rakes. It is one of the main objectives of this paper to verify whether both techniques deliver the same experimental results also at the high targeted frequencies up to kR=75. As the examination of recently obtained data revealed, the sensor rake measurements were influenced by aerodynamic perturbations originating from the fan rotor wakes. The radial mode analysis could be significantly improved by incorporation of appropriate aerodynamic eigenfunctions. Further investigated was the sensitivity of mode detection with sensor rakes against manufacturing and installation tolerances. I. Introduction A major new large scale fan rig test facility, UFFA (Universal Fan Facility for Acoustic), has been designed, manufactured and commissioned by AneCom AeroTest GmbH, Wildau, Germany. The new modular fan rig test facility builds on previous Rolls-Royce large scale fan rig design experience 1 but extends the experimental capability to allow test bed changes for engine representative OGVs and bypass duct annulus and liners, for reduced build times, and higher fidelity investigation of aft fan noise technologies. In order to meet the challenging noise targets in a timely manner for future aircraft, the industry requires engine and nacelle representative high Technology Readiness Level validation test vehicles to deliver the technology ready for implementation in to the full scale product. An important enhancement consisted in the implementation of three Radial Mode Detection (RMD) devices in the bypass duct, i.e. radial sensor rakes directly at OGV exit and radial sensor rakes respectively a wall-flush mounted sensor array further downstream in the nozzle equivalent plane of the UFFA test facility. The sensor arrays were developed by DLR within the frame of the EU FP 6 project VITAL 2 , based on predicted sound field characteristics provided by Rolls-Royce. At the maximum targeted frequency of kR=75 more than 2500 modes of azimuthal mode orders up to m=85 and radial mode orders up to n=10 should be resolved. Major objectives of future RMD measurements are (1) the accurate assessment of fan rig design changes, (2) the provision of high quality data