In this study, the spatial structures of a submerged turbulent jet interacting with a free surface were investigated experimentally. The jet axis was located at three different depths (H/D = 2, H/D = 4 and H/D = 6) beneath the free surface and the Reynolds number was fixed as 3480. Laser-induced fluorescence technique was used for qualitative visualization and the time-resolved particle image velocimetry technique was used for the quantitative measurements. The dynamics of the flow structures were examined further using the proper orthogonal decomposition analysis technique. The results revealed that the dynamic characteristics of large-scale turbulent motions were significantly different with the submerged depths. In case of H/D = 2, the dominant spatial structures displayed a surface vibration induced reverse flow along the boundary, and its subsequent deflection changed the flow structures in the horizontal center plane. The violent free surface vibration caused an unsteady up-and-down motion of the flow structures and had a “squeeze effect” on the flow structures. In case of H/D = 4, the upwelling motion of some vortices in the jet and their subsequently downward entrainment motion significantly changed the dominant spatial structures both in the vertical and horizontal central planes. When the jet was fully attached to the free surface, the vortical structures underwent a merging and restructuring process due to the vertical confinement of the free surface. In case of H/D = 6, the dominant spatial structures both in the vertical and horizontal central planes showed an approximately symmetric pattern, indicating that the dominant structures were not changed by the free surface. After attached to the free surface, the jet did not undergo a merging and restructuring process as shown in case of H/D = 4.

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