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

摘要

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 theta = 0 degrees-90 degrees. At the reduced velocity of U-0/f(0)D(h) = 3-10 (f(0) 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 (theta <= 30 degrees) is stable, and without appreciable displacement. Beyond theta = 30 degrees, 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 Re-D = 1.8 x 10(3), i.e., theta = 0 degrees, 30 degrees, 60 degrees and 90 degrees. In the two systems with theta = 0 degrees and 30 degrees, the wake regions feature weak velocity fluctuations in highly limited areas. However, increasing the angles of attack (to theta = 60 degrees and 90 degrees) 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 theta = 60 degrees and 90 degrees 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 theta = 0 degrees, 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 theta = 30 degrees. 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 theta = 60 degrees and 90 degrees, 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 theta = 60 degrees) and 0.12 (for theta = 90 degrees). 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.