Control of fan broadband noise through wavy leading and trailing edge serrations

Abstract

The present study provides comprehensive experimental investigations into the use of wavy leading edge (LE) and trailing edge (TE) serrations as a passive means for augmenting the reductions of fan broadband noise. The findings clearly indicate that the wavy LE – TE serrated fan could yield greater noise reduction performance than the un-serrated and wavy LE serrated fan, over a wide range of frequencies. In general, the wavy LE-TE serrated fans offer a maximum noise reduction of about 10 - 6 dB and an average reduction of about 4 - 5 dB, over a broad range of frequencies. For the range of frequencies from about 3 to 8 kHz, the wavy LE – TE serrated fan delivers a notable additional noise reductions of about 1-2 dB as compared to LE serrated ones, which is observed for all rpm values. For all rpm values, the un-serrated and serrated fans exhibit maximum / minimum directivity at an emission angle of about 127.5^o / 77.5^o. The lower far-field acoustic emissions (i.e., interaction noise + self-noise) offered by the wavy LE-TE serrated fans arises due to intense far-field destructive interference as a result of the faster spanwise phase variation of the velocity/pressure as well as the higher spanwise de-coherence. Further, the wavy LE-TE serrations mitigate direct scattering at both the LE and TE by dispersing sound energy over a wider area, which results in less intense noise signature in the far field. Thus, it clearly demonstrates that the second generation wavy LE-TE serrations could provide the substantial reduction of the far-field noise as compared to first-generation wavy serrations over a broad range of frequencies.