Effect of acoustic resonance on the dynamic lift of tube arrays

The effect of acoustic resonance on the dynamic lift force acting on the central tube in square and normal triangle tube arrays is investigated experimentally. For each array pattern three different tube spacing ratios, corresponding to small, intermediate and large spacing ratios, are tested. The resonant sound field in the tube array is found to cause two main effects. First, it generates a “sound-induced” dynamic lift due to the resonant acoustic pressure distribution on the surface of the tube, and secondly, it synchronizes vorticity shedding from the tubes and thereby enhances the hydrodynamic lift force due to vortex shedding. The combined effect of these two unsteady lift forces depends on the phase shift between them, which is dictated by the frequency ratio of the acoustic mode to the natural vortex shedding frequencies. When the flow velocity is increased during the coincidence resonance range, the phase shift increases rapidly and therefore the effects of the two lift components change from reinforcing to counteracting each other. For the pre-coincidence lock-on range, the frequency ratio remains larger than unity and the two lift components always reinforce each other. Numerical simulations are also performed to compute the sound-induced lift force, and sound-enhancement coefficients are developed to estimate the effect of sound on the vortex shedding forces. The simulation and experimental results are implemented in a simplified design guide, which can be used to evaluate the dynamic lift forces acting on the tubes during acoustic resonances.