Phase-resolved PIV measurements of flow over three unevenly spaced cylinders and its coupling with acoustic resonance

The characteristics of acoustic resonance excitation by flow over the arrangement of three unevenly spaced inline cylinders in cross flow were experimentally investigated. Phase-resolved particle image velocimetry (PIV) measurements were conducted to demonstrate the role of the separated shear layers around the cylinders in the excitation mechanism of acoustic resonance. The Strouhal number of self-sustained flow oscillations around the investigated arrangement is presented. Before the onset of acoustic resonance excitation, the location of the middle cylinder has a significant effect on the shear layer separation and impingement mechanism. At flow velocities that caused coincidence between an acoustic mode frequency and the intrinsic vortex shedding frequency that would occur under free-field condition, severe acoustic resonance corresponding to acoustic particle velocities of up to one-tenth of the main flow velocity was observed. For certain arrangements, acoustic resonance was detected at lower flow velocities than necessary for frequency coincidence. The Strouhal number of these pre-coincidence oscillations corresponds to that of a cavity formed between two successive cylinders. Phase-resolved PIV measurements show significant differences between flow field during and in the absence of acoustic resonance. Most importantly, acoustic resonance is excited when vortices roll up and impinge on the middle cylinder and the downstream cylinder, and no flow passes through the two gaps. The acoustic mode frequency and the Strouhal number of the Rossiter-like modes are decreased when resonance takes place at a higher Mach number. The aerodynamic interference between the two successive gaps formed by the three cylinders seems essential to explain the variation in the amplitude of resonance excitation.