Modification of a circular cylinder wake with synthetic jet: Vortex shedding modes and mechanism

The wake behind a circular cylinder is modified by a synthetic jet positioned at the front stagnation point. The flow field is measured with a time-resolved particle image velocimetry (PIV) system, and the proper orthogonal decomposition (POD) and λci methods are used to analyze the vortex dynamics. The synthetic jet vortex pair is induced near the exit orifice periodically and then moves upstream. The interaction between the synthetic jet and the oncoming flow gives rise to an envelope formed upstream of the circular cylinder, which acts as a virtual aerodynamic shape. It is found that the envelope can be categorized into the periodic closed envelope and the quasi-steady open envelope, leading to different shedding modes for the wake around the circular cylinder. In the present investigation, six kinds of vortex shedding modes under synthetic jet control have been classified as natural Karman vortex mode, bistable state mode I, symmetric mode, bistable state mode II, antisymmetric mode with shortened vortex formation length, vortex generation close to the rear stagnation point. The vortex dynamics analysis indicates that the wake vortex trajectory, vortex circulation, and convection velocity at the vortex core all exhibit regular variations for these typical shedding modes. The formation mechanisms for these shedding modes have been further revealed, which present some novel formation processes in comparison with the natural Karman vortex street. Moreover, the effects of the synthetic jet momentum coefficient and excitation frequency on the control are also compared, which suggests that the type of the front envelope is most important for the vortex shedding modes.