In suppressing vortex-induced vibration (VIV) of flexible risers in deep sea, a new device based on the Bernoulli equation and effects of geometric disturbance on flows around a bluff body is proposed. The mechanism is the disturbance in a radial-spanwise plane on fluid flows around the body and spanwise uniformity of vortex shedding. In present experiments, two types of disturbance, harmony and cone-like, and different waviness (ratio of wave height to wavelength) of 0.025, 0.05 and 0.1 are investigated. Experimental results have shown that oscillating amplitudes with such disturbance are reduced at mild and high waviness at occurrence of synchronization of a circular cylinder without disturbance. Reduction of RMS values reach up to 48.5% for harmonic disturbance and 53% for cone-like disturbance at the waviness of 0.1.
In many engineering, a great deal of bluff structures are applied, such as bridges, buildings, risers and heat exchange tubes. Such bluff body is usually subjected to a fluid flow at sufficiently higher Reynolds numbers. The flow separates from bluff body giving rise to vortex shedding periodically from either sides of body. This alternating vortex shedding is accompanied with the occurrence of fluctuating pressure forces on structural surfaces. Therefore, the body undergoes vibrations if it is light and flexibly mounted, called vortex-induced vibrations (VIV). These vibrations could be some times large enough to cause structural fatigue damage, even catastrophe, which in turn endangers manufacture and people's living safety. In half a century, many control methods and devices have been proposed and applied, such as radial fins, strake, fairing and control rods. More details can be referenced in reviews by Sarpkaya and Isaacson (1981), and Kumar-Sohn and Gowda (2008). Generally, all control methods can be classified as two kinds, active and passive control, from the point of view of control theory.