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An active control method for suppressing the response of an elastically mounted cylinder by forcing rotary oscillation is presented. Vortex-induced vibration (VIV) of structures is related to the interaction between body and shedding vortex. In the synchronization/lock-in regime, when the vortex shedding frequency f s matches the natural frequency f N of the spring-mass system, large displacement amplitude in the transverse direction is observed. The effect of rotary oscillation on unsteady laminar flow past a freely vibrating cylinder has been investigated. In this study, the cylinder has two degrees of freedom: forced rotary oscillation and vortex induced vibration. The investigation is based on the solutions of flow equations by using the immersed boundary method at moderate Reynolds number. The present computational results indicate the rotary oscillation control can be implemented to suppress the response amplitude of VIV by locking the vortex shedding frequency f s at the forcing frequency f r in the "lock-on" region. The "lock-on" phenomenon occurs in the wake of a rotationally oscillating cylinder, which is free to vibrate in the transverse direction. The essence of the present active control method is to change the frequency of the vortex shedding, rather than suppress it. The response of an elastically mounted cylinder is drastically suppressed to less than 1% of the cylinder diameter, when proper frequency ratio f r / f N and rotational velocity are imposed. Detailed analyses of aerodynamic performance are given to interpret the mechanism of the suppression of response caused by forced rotary oscillation. The effects of mass ratio and velocity rate of rotary oscillation are also found to play an important role in the spring-mass system. The efficiency of the present method increases with Reynolds number. C 2015 AIP Publishing LLC. [http://dx.

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