Coherent Streamwise Vortex Structures in the Near Field of the Three Dimensional Wall Jet

Abstract

A turbulent threedimensional wall jet with an exit Reynolds number of 250,000 was investigated using stereoscopic particle image velocimetry (PIV) in the nearfield region (x/D = 5). The proper orthogonal decomposition (POD) was applied to all three components of the velocity field to investigate the underlying coherent structures in the flow. A lowdimensional reconstruction of the turbulent velocity field using the first five POD modes showed the presence of coherent streamwise vortex structures formed in the outer shearlayers of the wall jet, not unlike those found in the nearfield of free jets. The instantaneous streamwise vorticity reconstructed from the lowdimensional reconstructed velocity field indicates the presence of a persistent vortex pair close to the wall and on either side of the jet centerline that appear similar to the mean streamwise vorticity. These regions do not appear to be directly related to the positioning of the streamwise vortex structures in the outer shearlayer.