Effect of tube pitch ratio of in-line tube banks on acoustic resonance and vortex shedding

Abstract

In the present paper the attention is focused on effects of tube pitch ratios of in-line tube banks on acoustic resonance and vortex shedding. We examined the characteristics of the acoustic resonance in the tube banks with tube pitch ratios ranging from 4.0 to 1.4 in the flow direction. Acoustic pressures were measured on the two sidewalls of the experimental apparatus. The tube surface pressure fluctuations due to the vortex shedding without the acoustic pressure were measured at the acoustic pressure nodes at which the acoustic resonance occurred. Acoustic resonance of low-order modes in the transverse direction occurred at tube pitch ratios of 4.0–2.8 in the flow direction. Alternative vortices were noticeably formed in the tube banks. As the tube pitch ratio in the flow direction of the in-line tube banks increased, the Strouhal number decreased, and as the former decreased, acoustic resonance of higher-order longitudinal modes occurred. The alternative vortices were formed in the tube banks at low gap velocities with no resonance. However, if acoustic resonance of a longitudinal mode occurred, symmetric vortices were formed in the tube banks. The onset velocity of a longitudinal mode was lower than that of a transverse mode, at small tube pitch ratios. As the number of rows of tubes increased, the acoustic resonance of transverse mode occurred at small tube pitch ratios. We discussed the prediction formula of Strouhal number at the tube pitch ratio in the flow direction of 2.0-1.4.