Flow -sound interaction mechanisms of a single and two tandem cylinders in cross-flow

The flow-sound interaction mechanisms of a single cylinder and two tandem cylinders in cross-flow are investigated by means of model tests and numerical simulations. An experimental set-up is designed, which is conducive to self-generation of acoustic resonance. One of the objectives of this research is to investigate the effect of the gap between the cylinders on the acoustic resonance mechanism. Special attention is given to the proximity interference region in which the spacing between the cylinders is relatively small, as encountered in industrial applications.

The aeroacoustic response of the tandem cylinders is found to be considerably different from that of isolated cylinders. For isolated cylinders, acoustic resonance of a given mode occurs over a single range of flow velocity and is excited by the natural vortex shedding process observed in the absence of acoustic resonance. However, for the two tandem cylinders, the resonance occurs over two different ranges of flow velocity. One of these ranges, the coincidence acoustic resonance, is similar to that observed for isolated cylinders and the other range, the pre-coincidence acoustic resonance, occurs at much lower flow velocities than that corresponding to the frequency coincidence. The excitation mechanism of the pre-coincidence acoustic resonance is remarkably similar to that observed for in-line tube bundles.

Additional experiments are performed to better understand the mechanism generating the pre-coincidence acoustic resonance. In these experiments, the cylinder diameter is varied independently of the spacing ratio. It is found that the pre-coincidence acoustic resonance occurs only when the diameter of the two tandem cylinders increases and exceeds a certain value. For tests with small cylinder diameter, the pre-coincidence acoustic resonance does not occur.

To understand the coupling and energy transfer mechanism between the flow field and the sound field, an innovative force transducer is developed to measure directly the dynamic lift force acting on cylinders exposed to acoustic resonance and cross-flow. By means of this force transducer, the effect of acoustic resonance on the dynamic lift coefficient is examined for the first time. When the acoustic resonance is initiated, a drastic increase in the dynamic lift coefficient is observed. This is associated with abrupt changes in the phase between the lift forces and the acoustic pressure. Decomposition of the lift force into in-phase and out-of-phase components is, with respect to the sound field, found to provide a phenomenological model which explains the mechanism of energy transfer between the flow and the sound field.

Finally, a numerical simulation of the flow-sound interaction mechanism is performed for the case of a single and two tandem cylinders in cross-flow. The simulation includes the effect of sound on vortex shedding but does not consider the effect of flow on the acoustic field. Despite this simplification, the simulation provides new insights into the flow-sound interaction mechanisms, including the nature and the locations of the aeroacoustic sources in the flow field. It also identifies the differences between the coincidence and the pre-coincidence resonances for the tandem cylinders case.