Flow structures behind a vibrissa-shaped cylinder at different angles of attack: Complication on vortex-induced vibration

Abstract

Complementary experimental studies have been conducted with a vibrissa-shaped cylinder at different angles of attack, through vortex-induced vibration (VIV) test in a wind tunnel, along with extensive measurements of wake dynamics in a water channel using time-resolved particle image velocimetry (TR-PIV). The VIV responses of an elastically mounted vibrissa-shaped cylinder are experimentally compared at various angles of attack in the range of θ = 0°–90°. At the reduced velocity of U0/f0Dh = 3–10 (f0 being the system's natural frequency), the cross-flow displacement of the cylinders convincingly demonstrates that the vibrissa-shaped cylinder at a small angle of attack (θ ≤ 30°) is stable, and without appreciable displacement. Beyond θ = 30°, a prominent three-branched VIV response is readily identified, and increasing the angle of attack results in an upward shift of the synchronized region and a considerable intensification of the peak amplitude. Subsequently, TR-PIV measurements are made of the wake flow behind the vibrissa-shaped cylinder, to determine the spatio-temporally varying flow fields in two spanwise planes, i.e., the saddle and the nodal planes. Four systems with different angles of attack are chosen for comparison at ReD = 1.8 × 103, i.e., θ = 0°, 30°, 60° and 90°. In the two systems with θ = 0° and 30°, the wake regions feature weak velocity fluctuations in highly limited areas. However, increasing the angles of attack (to θ = 60° and 90°) gives rise to expanded recirculation zones, highly unstable flow reversals immediately behind the cylinder, and strengthened velocity fluctuations in the bulk wake regions. Cross-correlation of the fluctuating longitudinal velocities shows that at θ = 60° and 90° the energetic large-scale vortical structures form earlier, and they exert considerable influence on the near-wake fluid behind the cylinder. Finally, a sophisticated data-driven dynamic mode decomposition (DMD) process is used to extract the dominant unsteady structures in the four systems with different angles of attack. In the system with θ = 0°, two dominant DMD modes at frequencies St = 0.23 and 0.30 are identified in the saddle and the nodal planes, respectively, and those frequencies are St = 0.18 and 0.19 in the system with θ = 30°. The interaction between these dominant events at different frequencies tends to disrupt the formation of a strong vortex-shedding process. Therefore, the hydrodynamic force on the cylinder does not make a concerted contribution to suppressing the VIV behavior along the spanwise direction. In the systems with θ = 60° and 90°, the corresponding DMD modes exhibit much more synchronous, organized characteristics in the saddle and nodal planes, and unsteady events at the same frequencies are detected in both planes, reaching St = 0.14 (for θ = 60°) and 0.12 (for θ = 90°). These effects, along with the intensified vortex-shedding processes in the saddle and nodal planes, exert a concerted hydrodynamic force on the cylinder, causing it to start with an oscillatory state.