Vortex-induced vibrations of a circular cylinder with two symmetrically arranged control rods

In this paper, the vortex-induced vibration (VIV) of a main cylinder with two symmetrically arranged control rods at Reynolds number of 500 is numerically simulated. The control rods and the main cylinder are connected rigidly and can move synchronously, which is modeled as a spring oscillator with double degree of freedom. Seven gap ratios (G/D= 0.1 similar to 2) and sixteen angles (alpha- 15 degrees similar to 165 degrees) are used to model the cylinder system. Thirteen flow patterns are identified, and the VIV of the cylinder system in every flow pattern is discussed separately. It is found that the characteristics of the VIV of the cylinder system are similar in the same flow pattern, which may give a way to understand the VIV of the multiple cylinders further. Based on the classification in previous researches, some advanced insights into the passive control are achieved. The control rods located in the boundary layer (G/D= 0.1) can change the VIV of the main cylinder most effectively, and the effect decreases with increasing gap ratio. The control rods immerged in the wake of the main cylinder may result in sub-harmonic resonance, while the control rods located centrally in front of the main cylinder does not obviously affect the VIV of the main cylinder. As G/D= 0.1 similar to 0.7, overall, the control rods located nearly side-by-side with main cylinder can amplify the VIV, while the control rods located downstream of the main cylinder can suppress the VIV. As G/D- 1 similar to 2, the control rods can suppress the VIV slightly, and the effect increases with increasing gap ratio and decreasing angle. For G/D= 0.1 & alpha= 105 degrees, the VIV is magnified for maximum effect, and the peak amplitude is about twice that of the single cylinder. For G/D- 0.2 & alpha- 45 degrees, the VIV is suppressed for maximum effect, and the peak amplitude is about 0.88% of that of the single cylinder.