A remote microphone technique for aeroacoustic measurements in large wind tunnels

The present study was devoted to the development and application of a remote microphone technique for aeroacoustic measurements in large aerodynamic wind tunnels. In this technique, the microphone and its connecting line to the sensing port are fitted within an aerodynamically streamlined fairing. A model-based calibration method was applied to account for the phase lag and dissipation within the microphone connecting line. The proposed system permits to carry out acoustic measurements when the wind tunnel surface cannot be altered, since the fairing can be simply glued on the wall. The fairing is streamlined to minimize flow disturbances as well as the risk of separation which would otherwise contaminate the wind tunnel flow quality and acoustic measurements. It was furthermore hoped that the acceleration of the flow over the fairing surface might reduce the turbulence fluctuations and thereby increase the signal-to-noise ratio of the acoustic measurements. Those aerodynamic effects were investigated with a set of hot-wire measurements performed in the subsonic L2B small wind tunnel of von Karman Institute (VKI). The mean velocity and the turbulence intensity profile were analyzed in the presence of different inflow turbulence characteristics at 5, 10 and 15 m/s freestream velocity. The effect of incident turbulence on the acoustic measurement was investigated applying the calibration procedure and comparing the measurements with a wall-mounted reference microphone signal in similar flow conditions. The results indicate that the wished beneficial effects of the flow acceleration are not significant. As an application, this technique is used to measure the noise emitted from the contra-rotating fans of the large L1 subsonic wind tunnel of the von Karman Institute, where the microphone fairing is placed inside the diffuser and thus subjected to a thick turbulent boundary layer. The results indicate that in spite of the mitigated turbulence-reduction performance of the fairing, it provides a measurement solution that is suitable for large wind tunnels provided the signals are compensated using the measured dynamic calibration. While it should be stressed that the calibration procedure itself is not new, the originality of the present paper stands in the proposed fairing design, and in the aerodynamic/acoustic investigation of its effect on the flow.