Numerical investigation of rod-airfoil interaction noise reduction using cylindrical collars.

Noise reduction by collars applied to rod-airfoil was studied numerically. The flow field and acoustic far-field are predicted using a large eddy simulation and the Ffowcs Williams and Hawking acoustic analogy. The present numerical method is first validated by existing experimental and numerical results for the baseline case. Then, to reduce interaction noise, a rod with collars is designed (denoted as the Col case). The main noise reduction mechanisms of the collars are investigated in detail. The numerical results show that the collars reduce the noise in the low- and medium-frequency bands of the rod, for which the tonal noise is reduced by 24.83 dB. The airfoil noise throughout the frequency band is thereby reduced as the main sound source. The upstream wake is regularized, and vortex shedding is suppressed. The surface pressure fluctuations along the rod, leading edge, and trailing edge of airfoil exhibit an obvious attenuation in the Col case compared with the baseline, which leads to a decrease in the sound source strength. It is also found that there exist spanwise decorrelation and decoherence effects along the rod with collars, which means the evolution of the turbulent vortices is regularized and the physical size of eddies is minified.