INVESTIGATION ON SUPPRESSION OF VORTEX-INDUCED VIBRATION USING HELICAL STRAKES

While the effect of helical strakes on suppression of Vortex-Induced Vibrations (VIV) has been studied extensively, the mechanism of VIV mitigation using helical strakes is less well documented in the literature. In the present study, experiments were conducted in a wind tunnel at four velocities, i.e. 1.92, 3.83, 5.74 and 7.65 m/s. Strakes with a dimension of 10d in pitch and 0.12d in height were fitted onto a rigid cylinder of diameter d = 80mm, and subjected to a transverse air flow. Hotwire techniques were used to measure the instantaneous velocity fluctuations at various locations to explore the mechanism for VIV mitigation by using helical strakes. It was found that the helical strakes reduce VIV by about 98%. Unlike the bare cylinder which experiences lock-in over the reduced velocity of 5 similar to 9, the straked cylinder does not show any lock-in region. In exploring the mechanism of VIV reduction by helical strakes, it was found that vortices shed from the straked cylinder are weakened significantly. The dominant frequency of the vortex structures along the spanwise direction of the bare cylinder was very stable. This is not the case for the straked cylinder wake, which differs by about 36% of the averaged peak frequency over the length of 3.125d along the cylinder axial direction, indicating that the vortex shed from the latter is out-of-phase and mismatching. This is supported by the phase shift between the velocity signals measured at two locations separated in the spanwise direction. The cross-correlation coefficients in the bare cylinder wake were much larger than that obtained in the straked cylinder wake, indicating that the correlation length of the former is much larger than that of the latter.