Impact of Turbulent Inflow on the Far-Field Noise Generated by a Propeller Operating at Low Reynolds Number

This study analyzes the alteration undergone by turbulent eddies as they approach a propeller operating at low Reynolds number, with the purpose of investigating the resulting effects on the noise emitted by the propeller. The two mechanisms affecting turbulence distortion, the streamtube contraction and the interaction between the turbulent structures and the blade, have been investigated experimentally. The set-up consists of a propeller with a diameter of 30 cm operating at a 75% chord-based Reynolds number of 10.8*10^4 interacting with the turbulence produced by a rectangular grid. The flow behavior has been studied by particle image velocimetry (PIV) and hot-wire anemometry (HWA), while a microphone arc was installed for the acoustic analysis. The results reveal that the interaction between incoming turbulence and the propeller plays a dominant role in the alteration of turbulence with respect to the streamtube contraction. This is due to the relatively low contraction ratio of the propeller at this regime, equal, in this case, to C.R.=1.3. Turbulence characteristics are used as input for two different analytical noise-prediction models, both based on Amiet's theory for turbulence-impingement noise. The first implements the original formulation of Amiet for propeller noise, which requires a position along the blade to be specified to define all the inputs. The second has been developed in the present work to account for the variations of the blade geometry and turbulence conditions in the radial direction. The comparison between the noise predictions and the experimental measurements shows that a better agreement can be obtained with the second model. This reveals that noise generation is strongly dependent on the variation of the flow conditions and propeller geometry along the radial direction, confirming that the description of these characteristics can enhance the accuracy of low-fidelity noise-prediction methods.