Proper orthogonal decomposition analysis of the flow field in a plane jet

An experimental study was performed for the developing structural characteristics of a plane jet at Re = 3000. The velocity field measurements were made using particle image velocimetry (PIV) in a water jet facility. The proper orthogonal decomposition (POD) method was applied to the two-dimensional PIV data to reveal large-scale vortical structures in the jet flow. The symmetrical counter-rotating vortices that have been discussed in previous jet studies were confirmed in the initial region. It was found that these vortices were generated as a result of the first vortex merging at the subharmonic sideband frequency, (fo±fc)/2, where fo was the initial jet shear instability frequency and fc was the jet column frequency. Moving downstream, their characteristic frequency evolved into fo/2-3fc/4 through nonlinear interaction. In the interaction region, symmetrical vortices were gradually displaced with each other in the streamwise direction and antisymmetrical vortices were eventually formed. The negative correlation between streamwise velocity fluctuations at two points on opposite sides of the jet centreline was caused by the passage of vortical structures.