Lock-on phenomenon in the wake of a circular cylinder is investigated at the Reynolds number of 220 (Amode instability regime) using direct numerical simulation. For lock-on, the sinusoidal perturbation, whose frequency is near twice the natural shedd...
Lock-on phenomenon in the wake of a circular cylinder is investigated at the Reynolds number of 220 (Amode instability regime) using direct numerical simulation. For lock-on, the sinusoidal perturbation, whose frequency is near twice the natural shedding frequency, is superimposed on the free stream velocity. In order to analyze the vortex structure, the power spectra of the streamwise velocity signal and orthogonal enstrophies are investigated. It is observed that the three-dimensional vortex structure behind the circular cylinder turns into two-dimensional one due to the perturbed velocity. And this change causes the jump of the Strouhal number from the stable regime (A-mode instability regime) to the unstable regime (extrapolated from twodimensional shedding regime) in the Strouhal-Reynolds number relationship. As a result, the vortex shedding frequency is locked on the perturbation frequency depending not on the natural shedding frequency but on the modified shedding frequency.