Vortex shedding suppression and wake control: A review

Suppression of vortex-induced vibration of a circular cylinder using suction-based flow control

Vortex dynamics of a circular cylinder controlled by a synthetic jet positioned at the back stagnation point is experimentally investigated using particle image velocimetry (PIV) technique. The proper orthogonal decomposition (POD) method is adopted to present the variations of the POD energy, mode, coefficient, corresponding dominant frequency, and the reconstructed spanwise vorticity. It is found that the dominant dimensionless control parameters should be the synthetic jet stroke length L0/D, where D is the diameter of the experimental circular cylinder, and the equivalent momentum coefficient Cμ. For the same stroke length L0/D=3.3, the states of the wake vortex shedding are determined by the momentum coefficient. They can be categorized into three groups summarizing all the parameters tested: antisymmetric Karman vortex shedding mode (Cμ≤0.027), vortex synchronization with shedding modes varying between the symmetric and antisymmetric ones (0.061≤Cμ≤0.109), and vortex synchronization with symmetric s...

Proper orthogonal decomposition analysis of vortex dynamics of a circular cylinder under synthetic jet control

The flow over a circular cylinder controlled by a two-dimensional synthetic jet positioned at the mean rear stagnation point has been experimentally investigated in a water channel at the cylinder Reynolds number Re = 950. This is an innovative arrangement and the particle-image-velocimetry measurement indicates that it can lead to a novel and interesting phenomenon. The synthetic-jet vortex pairs induced near the exit convect downstream and interact with the vorticity shear layers behind both sides of the cylinder, resulting in the formation of new induced wake vortices. The present vortex synchronization occurs when the excitation frequency of the synthetic jet is between 1.67 and 5.00 times the natural shedding frequency at the dimensionless stroke length 99.5. However, it is suggested that the strength of the synthetic-jet vortex pair plays a more essential role in the occurrence of vortex synchronization than the excitation frequency. In addition, the wake-vortex shedding is converted into a symmetric mode from its original antisymmetric mode. The symmetric shedding mode weakens the interaction between the upper and lower wake vortices, resulting in a decrease in the turbulent kinetic energy produced by them. It also has a significant influence on the global flow field, including the velocity fluctuations, Reynolds stresses and flow topology. However, their distributions are still dominated by the large-scale coherent structures.

Circular cylinder vortex-synchronization control with a synthetic jet positioned at the rear stagnation point

In contrast to usual synthetic jets, the “hybrid-synthetic jets” of non-zero time-mean nozzle mass flow rate are increasingly often considered for control of flow separation and/or transition to turbulence as well as heat and mass transfer. The paper describes tests of a scaled-up laboratory model of a new actuator version, generating the hybrid-synthetic jets without any moving components. Self-excited flow oscillation is produced by aerodynamic instability in fixed-wall cavities. The return flow in the exit nozzles is generated by jet-pumping effect. Elimination of the delicate and easily damaged moving parts in the actuator simplifies its manufacture and assembly. Operating frequency is adjusted by the length of feedback loop path. Laboratory investigations concentrated on the propagation processes taking place in the loop.

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No-moving-part hybrid-synthetic jet actuator

Experimental tests were conducted to control the flow around a cylinder by means of unsteady blowing (synthetic jet) through a single slot disposed on the wall of the model. The flow Reynolds number (based on the diameter of the model) was R_D = 10^5. The efficiency of the synthetic jet is quantified in terms of delaying separation and modifying the drag coefficient. The investigations were of three types: measurements of the mean pressure distribution, wall visualizations of the separation line position and measurements of the mean flow-field in the wake.

J. Tensi

Papers

Modification of the Wake behind a Circular Cylinder by Using Synthetic Jets