Entrainment in a free surface plunging jet
Ambient fluid entrainment and near-field flow characteristics of a free surface plunging jet are investigated using time resolved particle image velocimetry. The plunging height is about twice the nozzle diameter and we study five different Reynolds numbers ranging from ReFS 5000 to ReFS 13,070. We find that the near-field entrainment in the ReFS 5000 and Re FS 6086 jets is enhanced significantly than the latter cases due to mixing transition that occurs at about 2-3DN below the free surface resulting in the breakdown of laminar vortices into smaller secondary structures. With an increase in the Reynolds number, mixing transition is declined in the latter three cases as they have a more homogenous and turbulent flow structure. We also study the statistical flow properties of plunging jets and compare our results with similar studies performed on free jets. Plunging jets show a reduction in the penetration depth of about 20-30% and the length of Zone of Flow Establishment (ZFE) of about 40-60% when compared to free jets at the same Reynolds numbers. Reduction in the length of ZFE allows plunging jets to achieve self-similarity earlier than the corresponding free jets. However, the velocity fluctuations in the shear layer show a good agreement with a recent study performed on a free jet at Re 5000 indicating that free surface interactions have little effect on the velocity fluctuations in the shear layer. We further investigate the unsteady flow behaviour and mixing transition in the near-field of plunging jets using a vortex identification and tracking scheme (λCI) and elaborate on events such as vortex pairing and breakdown. We also look into the distribution of coherent structures in the near-field over time in order to find the depth of mixing transition. The depth where primary structures breakdown into smaller secondary structures decreases as the Reynolds number is increased and is consistent with the reduction in the length of ZFE.
Vlachos, Purdue University.
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