Flow Excited Acoustic Resonance of Two Side-by-Side Cylinders in Cross Flow

The aeroacoustic response of two side-by-side cylinders in cross-flow is investigated experimentally. In order to investigate the effect of the gap between the cylinders on the acoustic resonance mechanism, six spacing ratios between the cylinders have been investigated. These spacing ratios are in the range of T/D = 1.25 to 3, where D is the diameter of the cylinders and T is the center-to-center distance between them. Special attention is given to the bi-stable flow regime, which is reported in the literature for intermediate spacing ratios. During the tests, the acoustic cross-modes of the duct housing the cylinders are self-excited. For the intermediate spacing ratios, T/D = 1.25, 1.35, 1.46 and 1.75, two distinct vortex shedding frequencies at the off-resonance conditions are observed. These are associated with the wider and narrower wakes of the cylinders, as described in the literature. In this case, acoustic resonance occurs at a Strouhal number that is between those observed before the onset of resonance. In addition, the acoustic resonance synchronizes vortex shedding in the two wakes and thereby eliminates the bi-stable flow phenomenon. For large spacing ratios, T/D = 2.5 and 3, vortex shedding occurs at a single Strouhal number at which the acoustic resonance is initiated.