Experimental study of the self-excited resonance effect on the dynamic lift and flow structure around inline cylinders

Abstract

The excitation of acoustic resonance by air flow around a row of inline cylinders is investigated experimentally using phase-locked particle image velocimetry and direct measurements of the dynamic lift forces. The experiments are performed during self-excited acoustic resonance for arrangements of two, three, four, and five inline cylinders with a spacing ratio L∕D=2.0, where L is the center-to-center distance between the cylinders and D is the cylinder diameter, to clarify the effect of increasing the number of cylinders on the resonance mechanism and the characteristics of the periodic flow cycle. The PIV measurements show that increasing the number of cylinders results in weaker and distorted periodic flow structures around the most downstream cylinders. However, the phase and strength of the flow structures within the most upstream gap are minimally affected. Intrinsic flow features, such as gap flow that is observed in some cases, lead to the disruption of the shear layer impingement on downstream cylinders. Moreover, direct measurements of the dynamic lift forces on all cylinders simultaneously identified the upstream gap as the location where most of the energy exchange between the flow and the acoustic fields occurs. Decomposition of the dynamic lift force with respect to the acoustic pressure revealed that the distorted flow structures were accompanied by weaker, sometimes negative, contribution to the acoustic resonance excitation. The findings presented in this article explain the observation that for inline configurations of cylinders, as well as inline tube bundles, increasing the number of cylinders in the flow direction does not result in stronger resonance, and demonstrate that countermeasures for resonance excitation should target the most upstream rows of cylinders.