BTGNX: An Acoustic Benchmark Wind TunnelExperiment for Tip-Gap Noise

One of the main noise contributions in enclosed fans is the tip-gap noise originating from a vortex system developing in the gap between the tip of the rotor blade and the housing. While there are extensive data on the aerodynamics in the tip-gap, acoustic investigations are scarce. The Braunschweig Tip-Gap-Noise eXperiment (BTGNX) aims to close this gap with a wind tunnel experiment that addresses the acoustic shortcomings of previous - aerodynamics focused - experiments and employs extensive near- and far-field measurement techniques to record a broad acoustic database. In addition to varying the inflow velocity, angle of attack, and gap height, interchangeable wingtips were used to investigate the influence of four different geometries. In this work, the experimental set-up is presented and compared to previous tip-gap noise experiments, highlighting modifications made to limit the influence of secondary sound sources. The selected measurement techniques are introduced, consisting of a MEMS-Array (MEMS = Micro Electro Mechanical Systems) with 256 microphones placed on the housing side of the tip-gap, a phased microphone array and ten single microphones. Furthermore, results of a far-field acoustics analysis are shown. The focus is on the gap height variation, where two frequency ranges between 1 and 3 kHz and 6 and 12 kHz were identified in which a change of the radiated noise is clearly visible. It is suspected that these stem from two different sound generation mechanisms resulting from the separation of the Tip Separation Vortex (TSV) on the pressure side of the airfoil on the one hand and the convection of the Tip Leakage Vortex (TLV) past the trailing edge on the other. This conclusion is supported by the results for the investigated geometry variations, which include modifications addressing these sound generation mechanisms. Finally, an application of the experimental database for a numerical investigation running in parallel is demonstrated. The primary aim of the numerical study was to accurately capture the relations between parameter variations for a subsequent wingtip geometry optimization. To support this, the measured parameter variations were ordered with regards to the recorded far-field Sound Pressure Level (SPL). The results also give a concise overview of the measurement results.