Vortex formation and evolution for flow over a circular cylinder excited by symmetric synthetic jets

The flow around a circular cylinder is controlled by a pair of synthetic jets symmetrically positioned near the separation points at a Reynolds number of 1200. The flow field in the near wake is measured by a collaboration of the flow visualization and the particle image velocimetry (PIV) techniques. The experiment is conducted under six different momentum coefficients C_μ, ranging from 0.007 to 0.421, while seven excitation frequencies are selected as harmonics of the dominant frequency. Various wake patterns appeared during this study, among which are the 2S asymmetric mode, the 2(P + S) asymmetric mode, the 2P symmetric mode, the 2S symmetric mode and the bistable state where vortices shed with both symmetric and asymmetric modes. The phase averaging method, the proper orthogonal decomposition (POD) based vorticity reconstruction and Fourier analysis are adopted to study the vortex dynamics. Particular attention is given to the vortex-formation mechanism for different modes under C_μ = 0.187, when all of the typical modes emerged. Vortex behaviors under symmetric excitation can be interpreted by looking into the processes of flow separation and vortex shedding. It is shown that synthetic jets can lead to unsteady flow separation and various vortex shedding modes are influenced by the periodic vorticity concertation induced by the synthetic jets.