Wall Pressure Fluctuation on the Aircraft Fuselage: Part 2 Impact of Sensor Wake on the Spectrum

One of the main source driving the cabin sound level of an aircraft is the turbulent boundary layer (TBL), specially during cruise condition. The pressure fluctuations underneath the TBL along the external fuselage of aircraft have been studied extensively. Several engineering models have been proposed, mainly semi-empirical ones for zero pressure gradient but yet reliable models are still needed. Flight test measurements are mostly performed at the window location where complete flushness is not guaranteed. Other protrusions (sensors, steps,…) are also prone to affect the TBL but its influence in the downstream region is not known to the knowledge of the authors. The authors present the impact of the sensor wake on the wall pressure fluctuation spectrum. CFD-RANS is performed using several round of adaptation using feature detection based on the normalized Q-criteria. The investigated approach models the change in TBL characteristics at the microphone location far downstream. The TBL characteristics are locally affected by the tip-vortex and are injected into semi-empirical TBL spectrum models. The difference in sound pressure show that the noise level does indeed increase and the noise level can change suddenly around the maximum. Although the tip-vortex is well captured, and the outcome of this investigation clearly demonstrates the impact of protrusion, the results are under-predicted.