Aerodynamic noise characteristics of non-circular cylinders in subcritical flow regime

The present study experimentally investigates the aerodynamic noise from the flow around cylinders of square and equilateral triangle cross-sections at different angles of incidence (alpha). The cylinder models have a side dimension of 10 mm and a span of 300 mm. The free stream velocity (U- 0 ) is in the range of 12-36 m/s, and the corresponding Reynolds numbers are 7.8 x 10(3) to 2.3 x 10(4), which is in the subcritical flow regime. The characteristic acoustic tones are generated at alpha = 30 degrees and 45 degrees for square and triangular cylinders. The frequency of acoustic tones linearly increases with the free stream velocity, and the corresponding Strouhal numbers are found to be in the range of 0.13-0.16. Depending on the angle of incidence, the overall sound pressure level is higher than the background noise by 4-24 dB for the square cylinder and 3-15 dB for the triangular cylinder at U- 0 = 36 m/s. The highest noise level of the square cylinder is 90 dB at alpha = 45 degrees and 79 dB at alpha = 30 degrees for the triangular cylinder. The spectral scaling with the sixth power of the free stream velocity indicates the dipole behaviour of the acoustic tones. The mean and root-mean-square velocity profiles in the wake region characterise the noise emissions at different angles of incidence. The comparative acoustic study of the non-circular cylinders with a circular counterpart showed that the highest noise level is from the square cylinder at alpha = 45 degrees. The directivity study shows that the noise level of the square cylinder at alpha = 45 degrees at 90 degrees angular location (theta) is higher by 6.5 dB than that at theta = 30 degrees.