To explore the active control method of the square cylinder wake field modification, a self-developed piston-driven synthetic jet actuator is used to generate periodic blowing and suction jets through the slits to affect the external flow field. The flow characteristics around the square cylinder controlled by synthetic jet is experimentally investigated with the force measuring system and Particle Image Velocity (PIV) technique. The statistical fluid forces, the frequency characteristics, and the time-averaged flow flied are compared with different control parameters, i.e. jet exit angle (β), the momentum coefficient (Cμ), and the excitation frequency (f*). With control, dramatic reduction in CLrms (up to 61%) is attained and the corresponding value of (equation) is 12% at β =45°. Narrower and longer backflow region represents a decrease in flow-induced forces.
Periodic wake vortex shedding alternately behind offshore platforms when the structures encounter currents with certain velocities in the flow field, inevitably causing vibratory hydrodynamic forces. Under the excitation of fluctuation fluid forces, vortex-induced motions (VIMs) (Liu et al., 2017) has been observed on large rigid structures, however, VIM response exacerbates the fatigue damage of moorings and risers and seriously affects the production safety (Stappenbelt, 2010). Therefore, actively seeking solutions to suppress VIM is of great importance to optimizing platform performance and safe operations. VIM is essentially a multi-cylinder flow problem, which can be simplified to flow around square cylinder commonly found in offshore platforms, meanwhile the optimization problem accordingly transformed into exploring the square cylinder wake field control methods.
Synthetic jet controlled by actuators, such as the reciprocating motion of a piston to achieve periodic injection and suction of fluid through a slit/orifice, has a rapid development in the last twenty years. Compared with the traditional continuous and pulsed jets, the synthetic jet can provide momentum for the flow filed without external mass injection, which is known as zero-net-mass jet. So far, a series of experimental and numerically studies have been performed on the flow around the circular cylinder with synthetic jets and have made some interesting discoveries: Béra et al. (2000) studied the influence of jet momentum with different values on the flow field around the circular cylinder in a wind tunnel. The result shows that when the jet exit angle is 110°, the lift force on the cylinder increases, but correspondingly, the drag also increases slightly. In the experiments (Fujisawa and Takeda, 2003; Tensi et al., 2002), the jet exit angle was near the flow separation point, and they found that the synthetic jet could effectively delay the separation of the cylinder and reduce or even disappear the backflow region behind the circular cylinder, thus achieving the drag reduction. However, the lift force increases slightly due to the flow field asymmetry caused by single silt. With a synthetic jet deployed on the front surface, Feng and Wang (2014) (Re=800) found that the synthetic jet vortex pair endured the impingement effect of free-stream and finally generated an envelope acting as a virtual aero-shaping modification. When the synthetic jets were positioned at the separation point, the vortex lock-on phenomenon and symmetric shedding modes which bring the complete suppression of lift fluctuations can be concluded in Liu and Feng (2015) (Re=500).