Vortex-induced vibration (VIV) of a circular cylinder in combined steady and oscillatory flow

Vortex-induced vibration (VIV) of a circular cylinder in the combined steady and oscillatory flow is investigated numerically by solving the two-dimensional Reynolds-Averaged Navier-Stokes equations. The focus of the study is to investigate the effects of flow ratio, a, on the response of the cylinder. The flow ratio is defined as the percentage of the steady flow velocity component in the total fluid velocity. Simulations are carried out for a constant Keulegan-Carpenter (KC) number of 10 and flow ratios ranging from 0 to 1 with an increment of 0.2. The reduced velocities for each flow ratio range from 2 to 25 to ensure that the whole lock-in regime is covered. In the resonance regime, the frequency of the cross-flow vibration component may lock onto the oscillatory flow frequency or the natural frequency of the system, depending on the flow ratio. It locks onto twice the oscillatory flow frequency if the flow ratio a≤0.2, and locks onto the natural frequency of the system if a≥0.6. It is found that the lock-in regime in the combined steady and oscillatory flow is wider than both the one in the pure steady flow and the one in the pure oscillatory flow. The widest lock-in regime occurs as the flow ratio is 0.4 and 0.6 and it is about twice as wide as that in the pure oscillatory or pure steady flow. The response at a=0.2 is very similar to that in the pure oscillatory flow case (a=0), while at a=0.8 the response is very similar to that in the pure steady flow case. At a=0.8, the amplitude of the cylinder in the cross-flow direction reaches as high as 1.5 diameters in the "Super upper" branch because of the slowly increasing fluid velocity. It is also found that the vortex shedding goes through 2S, 2P and 2T modes in one period of oscillatory flow at a=0.8 and Vr=7, where the cross-flow amplitude is the maximum