Free surface effects on spanwise turbulent structure in the far-field of submerged jets

The flow structure in the cross-sectional and spanwise planes of a turbulent round jet interacting with a free surface has been studied using planar particle image velocimetry. The submerged jet was positioned at an offset height of 5d below the free surface, where d is the nozzle diameter. Measurements were conducted in both streamwise-surface-normal (x-y) and nine streamwise-spanwise (x-z) planes (-4 <= y/d <= 4) located in the far-field (x/d = 42-62) at an exit Reynolds number of 28 000. To highlight the effects of the free surface, measurements were also performed to evaluate the characteristics of a reference free jet at similar initial conditions. For each jet, the spanwise planes were used to reconstruct the three-dimensional (3D) averaged velocity field to reveal the salient flow features in the cross section of the jet. The reconstruction shows the presence of the well-known surface current, which is usually prominent in the far-field of surface jets. The spanwise development of the surface current is found to reduce the streamwise and spanwise Reynolds normal stresses in the upper shear layer of the surface jet, but the Reynolds shear stress and its associated dominant quadrant motions in the spanwise plane are enhanced near the free surface. As the free surface is approached, the spanwise spread rate increases, but the local vorticity thickness decreases due to the enhancement of the mean shear in the spanwise direction. Two-point spatial correlations are used to show that the large-scale structures near the free surface undergo oblique stretching in the spanwise plane to augment the wider spanwise growth of the surface current. The spatial distributions of the energetic modes based on proper orthogonal decomposition also reveal interesting features near the free surface that are consistent with the inclination of the turbulent structures relative to the flow direction.