CONTROL OF VON KÁRMÁN VORTEX SHEDDING THROUGH SYNTHETIC JET TECHNOLOGY: MOMENTUM COEFFICIENT AND NON-DIMENSIONAL FREQUENCY EFFECTS

Synthetic jets have been recently employed in several fields of application, such as flow control and thermal management. A typical synthetic jet device consists of a cavity bounded by an orifice plate and an oscillating membrane. The oscillation of this membrane allows synthetizing the jet from the ambient in which the device is embedded, without the need of an external piping. This feature involves simple design and small size and, thus, makes synthetic jet devices very appealing in flow control applications. The present work reports on the experimental investigation of the flow field over a circular cylinder controlled by a two-dimensional synthetic jet located at the rear stagnation point. Time-resolved particle image velocimetry (PIV) measurements are carried out in an aspirated wind tunnel at a cylinder Reynolds number equal to about 4500 in order to assess the influence of the momentum coefficient Cμ and the ratio of the excitation frequency to the natural shedding frequency f+ on the wake flow structure. Cμ is varied between 0 (case with no flow control) and 20%, while f+ is varied between 0.5 and 2, thus covering a wide range of operating conditions. The time evolution of the velocity fields is studied by means of an innovative decomposition, based on Fourier analysis and relying exclusively on the PIV data, which allows separating the periodic coherent fluctuations at different frequencies (namely, synthetic jet and shedding frequencies) from the uncoherent turbulent one. The present results show that the flow topology of the cylinder wake is highly sensitive to the variations of Cμ and f+. The increase of both Cμ and f+ leads to a reduction of the cylinder wake extension, although with different mechanisms. The increase of Cμ at low values of f+ results in a longer fluid stroke during the synthetic jet ejection, which momentarily interrupts the vortex shedding; in the following suction phase, the wake reverts to the shedding mode, which is observed to occur in a symmetrical configuration at high values of Cμ. At the highest investigated values of f+, the synthetic jet consists essentially of discrete vortex pairs (not followed by trailing jets) and the increase of Cμ results in a stronger interaction of these structures with the separated shear layers and the von Kármán vortices. In these operating conditions (high values of Cμ and f+), the vortex shedding is observed to synchronize with the synthetic jet, i.e., the shedding frequency is shifted to a subharmonic of the excitation frequency. Also the velocity fluctuations strictly depend on the two investigated control parameters: they are observed to increase with Cμ, while the increase of f+ causes strong fluctuations in both the coherent and the uncoherent components of the velocity field within the very near wake region.