In this paper, the flow-excited acoustic resonance of an in-line row of cylinders ranging from one to five is investigated. Cylinders of three different diameters of 12.7 mm, 19.1 mm, and 25.4 mm are tested in cross flow with flow speeds up to 160 m/s. Two different tube lengths of 76.2 mm and 127 mm are used to investigate the effect of the cylinder’s aspect ratio at a given diameter on the excitation mechanism of acoustic resonance. A fixed spacing ratio of L/D = 2 is used for all cases. For more than one cylinder of the larger diameter, the self-excitation of resonance occurs at two discrete flow velocity regions that are generally wider than the case of a single cylinder. A larger diameter does not only trigger the excitation of the pre-coincidence resonance region, but also increases the normalized acoustic pressure of this pre-coincidence resonance. On the contrary, the cylinder’s aspect ratio does not have a similar effect on the pre-coincidence and coincidence resonance regions. Therefore, it is important that the effect of diameter should be included in formulas predicting the occurrence of resonance for in-line tube bundles. In addition, the Strouhal number related to the coincidence resonance decreases with the increase in the number of cylinders. The coincidence resonance is related to the vortex shedding in the wake of the last cylinder, while the pre-coincidence resonance is related to the shear layer in the gap between successive cylinders.

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