Coherent structure in a two-dimensional jet passively controlled by wake of a cylinder installed in nozzle contraction

It is possible to control the spread of a plane jet passively by the vortices from a two-dimensional cylinder installed in the nozzle contraction. In this study, the wake characteristics of a cylinder and the coherent structure of a jet by this passive control method were examined by flow visualization and quantitative velocity measurements. In wide range of strouhal number based on vortex shedding frequency and jet exit velocity, the spreads of the jet were found to be suppressed. This suppression is caused by the high frequency vortices from the control cylinder which inhibits growth and pairing of the low frequency vortices. On the other hand, in certain cases of the strouhal number being 0.05-0.10, the spread of the jet was remarkably enhanced associated with distinctive peaks of spectrum at higher order of vortex shedding frequency caused by the deformation of the vortices in flow contraction nozzle. In the case of enhanced mode, vortex from the cylinder moves to the opposite side shear layer with the same vorticity sign as the vortex, and it enhances roll up of the shear layer.