Vortex induced coupled vibration of an elastically mounted square cylinder with a detached solid and flexible plate

Abstract

The vortex induced coupled vibration of an elastically mounted square cylinder with a detached solid/flexible plate at Re=150 is numerically investigated in the present work. Influences of non-dimensional plate length L*, gap distance G* and plate flexibility ω* on vibration responses, flow physics and filament motion are analyzed. For a square cylinder-detached solid plate system, a short plate (L*= 0.25, 0.5) can suppress vortex induced vibration (VIV) in a wide range of G*(0≤G*≤2). In the optimal situation, the vibration can be suppressed almost completely (98% reduction in Ay*). A long solid plate suppresses VIV at a small G*. Nonetheless, it enhances vibration at a large G*. The effective range of G* (range of G* enables VIV suppression) shrinks fast with an increasing plate length. Two mechanisms for an effective VIV suppression are summarized by examining the transient flow fields. On the other hand, a square cylinder-detached flexible plate system maintains a more stable VIV suppression performance within a wide range of G* (0.5≤G*≤2.0) and ω* (0.5≤ω*≤3.0). A more compelling advantage is that, flexibility turns VIV enhancing to VIV suppression at large gap distance (G*=2.0). Although varying flexibility in a wide range gives marginal effect on response statistics, the filament motion undergoes drastic changes. Three types of flapping motion are observed. They are symmetric flapping, asymmetric flapping, and a most interesting one, flow induced reversed flapping (FIRF), which occurs at large gap distance and high flexibility (G*=2.0 and ω*=3.0). The necessary conditions and the onset of reversed flapping are also analyzed. FIRF can be viewed as an automatic gap distance adjusting mechanism. Through it, a shortened gap distance prevents strong and immediate vortex shedding behind the square cylinder, and a good VIV suppression performance can be obtained. This work can help in improving understanding of complex multi-physics coupling FSI problems. Moreover, analyses on VIV responses of a square cylinder-detached plate system can also provide guidance in passive flow control of VIV of a square cylinder.