The present study focuses on the noise generation mechanisms in outflow valves, which are commonly used in airplanes in order to regulate the cabin pressure and maintain a safe environment for the passengers. In order to make a detailed flow and acoustic investigation, experimental and numerical data are collected on a simplified 2D butterfly valve installed in a rectangular transparent channel. Two operating points are investigated : (a) a transient operating point at a flight altitude of 17 000 feet, where a high level tonal noise has been reported and creates acoustic nuisances in the cabin ; (b) the cruise condition operating point, that corresponds to a flight altitude of 40 000 feet, and where broadband noise may be reduced. Measurements of the far field noise and the wall static pressure as well as high speed Schlieren flow visualization are performed. They hightlight the very complex structure of the flow. Expansion waves and shock cells are observed at the upstream and downstream edges of the seal step, which are possible sources of tonal noise. Vortex shedding have also been observed for the transonic case, and might also generate tonal noise. In order to get a better understanding of the noise generation mechanisms, three-dimensional simulations of the experimental set-up are performed. They rely on steady or unsteady Reynolds-Averaged Navier-Stokes (Scale-Adaptive Simulations) computations and Large Eddy Simulations. All numerical results compare well with experiments on the mean flow. The unsteady simulations reveal strong shock oscillations on the forward-facing corner of the valve step and consequent strong vortex shedding when the jet shear layer interacts with this shock. The observed tonal noise is related to these two mechanisms and the shear layer instability upstream of the step, and the expansion fan on the backward-facing corner of the step. The LES also shows strong higher frequency peaks not yet available in the measurements that are clearly seen as the dominant noise-radiation mechanisms upstream of the valve in the dilatation field.

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