Experimental investigation on control of vortex shedding mode of a circular cylinder using synthetic jets placed at stagnation points

Control of flow around a circular cylinder by synthetic jets has been experimentally investigated in a water tunnel with particle image velocimetry (PIV) technique. The synthetic jets are positioned at both the front and rear stagnation points. With power spectrum analysis, proper orthogonal decomposition (POD) method and other techniques for data processing, particular attention is paid to the control of vortical structures around the circular cylinder, in which the excitation frequency f _e is one to three times of the natural frequency f ₀ and the cylinder Reynolds number and the excitation amplitude are fixed. The influenced-scope of the synthetic jet enlarges as the excitation frequency increases, and thus the synthetic jet dominates the global flow field gradually. For the natural case and the control case at f _e/f ₀ = 1, the distributions of the first two POD modes and the power spectra for their POD coefficients all exhibit the characteristics of the natural shedding with antisymmetric mode. For f _e/f ₀ = 2 and f _e/f ₀ = 3, the variations in the third and fourth POD modes and the corresponding power spectra indicate that the wake vortex shedding mode changes and the dominant frequency becomes the excitation frequency. For f _e/f ₀ = 2, the wake vortex sheds downstream with either the antisymmetric or the symmetric mode; for f _e/f ₀ = 3, the synthetic jet vortex pair interacts with the near wake shear layers from both sides to induce a pair of the symmetric wake vortices, which is gradually converted into an antisymmetric mode when shedding downstream.