Aerodynamic noise characteristics of tandem cylinders in a subcritical flow regime

The present experimental study investigates the noise generated from the subcritical flow past two tandem circular cylinders of different downstream-to-upstream diameter ratios ( d 2 /d 1 ). The configurations considered for the study are (A) d 2 /d 1 = 1, (B) d 2 /d 1 = 2.5 and (C) d 2 /d 1 = 0.4. The gap between cylinders has spacing to diameter ratios ( s/d) of 2 and 4, and the free stream Reynolds numbers vary from 8.4 × 10 3 to 3.6 × 10 4 . At s/d = 2, acoustic tones from configurations A and B are associated with vortex shedding from the downstream cylinder, and configuration-C has broadband spectral variations. At s/d = 4, configuration-B has two acoustic tones corresponding to vortex shedding from both cylinders, with dominant higher-frequency tones from the smaller upstream cylinder, and Configuration-C has narrowband tones from the larger upstream cylinder. The mean wake velocities of configurations at s/d = 4 are lower, with relatively higher gradients than at s/d = 2. The corresponding turbulence levels are 8.3% and 6.5%, respectively. Peak tonal frequencies have a linear variation with free stream velocity, and the respective Strouhal numbers estimated are in the range from 0.17 to 0.20. Scaling of the spectra with sixth power of the free stream velocity shows the dipole behavior of noise sources. The tandem configurations have higher overall noise levels than the background noise by up to 26 dB at s/d = 2 and 21 dB at s/d = 4. Configuration-A at s/d = 2 has the highest noise level of 91 dB.