Experimental Study And POD Analysis Of A Submerged Jet Interacting With A Free Surface

In the present study, the flow field of a jet interacting with a free surfacewas investigated experimentally. Shadowgraph and laser-inducedfluorescence techniques were used for qualitative visualization, and thetime-resolved PIV technique was used for the quantitative measurements.The dynamics of the flow structures were examined further using the properorthogonal decomposition (POD) analysis technique. The effects of theReynolds number and submerged depth were studied respectively. Theinteraction characteristics and dynamic structures of the jet with the freesurface were reported in this paper.A close-loop jet system was designed and developed for the experiments.Shadowgraph and LIF flow visualization system, and the time-resolved PIVsystem were set up. To obtain a comprehensive understanding of theinteraction of the jet with the free surface, the present study was conductedfrom two aspects.First, the effects of the Reynolds number were investigated. Twodifferent Reynolds numbers (1920,3480) were examined for a configuration,in which the depth of the jet beneath the free surface was fixed to H=2D.Shadowgraph results showed the surface disturbance of the two jets, and theLIF images revealed the subsurface evolution characteristics of the two jetflows. An understanding of the interaction characteristics of two submergedjets with the free surface was gained by examining the time-averagedvelocity field, stream-wise turbulent intensity profiles and the jet half-width.Dynamic structures of the two jets were compared by implementing the time-resolved proper orthogonal decomposition. The large scale vortex in thereconstructed flow field was identified by swirling strength method, and theirevolution characteristics were further determined.Second, the effects of submerged depth were investigated. The jet axiswas located at three different depths (H/D=2, H/D=4and H/D=6) below thefree surface and the Reynolds number was fixed to3480. The LIF imagesshowed the subsurface evolution characteristics of the large scale structuresin the three jets. The time-averaged velocity field revealed the submergeddepth effect on the velocity field. The dominant dynamic structures of thethree jets were compared using proper orthogonal decomposition technique.The large scale vortex was identified by swirling strength method, and theirevolution characteristics were further compared.