Validation of a Hybrid Simulation Method for Flow Noise Prediction

In this paper a joint experimental-numerical study which validates a novel hybrid method for computational aeroacoustics (CAA) with experimental results is presented. To this end an experimental investigation of the noise generated by the flow through an air outlet of the HVAC system of the AUDI TT at different flow rates and flap positions was conducted. The generated sound was recorded at thirty different positions in the near field. Of this configuration, a CFD and CAA analysis with a novel method, where a Large-EddySimulation (LES) is coupled with the acoustic-perturbation-equations (APE) in one solver, was conducted. The comparison of the flow variables show a good agreement between the measured and simulated flow fields. The numerically predicted flow noise is compared by means of the total sound pressure levels (SPL) and spectral analysis at each microphone position. The comparison show an excellent agreement between computed and measured signal, the method being able to predict the shape of the spectra, distinct peaks of the spectra, the overal SPL, the directional pattern and the scaling of the generated noise with the flow rates and flap positions correctly. We can show that the acoustic fluctuations in the nozzle can become as large as the hydrodynamic fluctuations and have to be taken into account when comparing simulation results with measured pressure signals. In the employed method a strong emphasis is put on a good computational performance, so that an industrial application of this method is feasible.