Pinch-off and formation number of negatively buoyant jets

Ruo Qian Wang, Adrian Wing-Keung Law, E. Eric Adams

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


Previous investigations of starting buoyant jets are extended towards negative buoyancy to address key issues in the formation processes. A series of large-eddy simulations (LES) is performed to identify whether an optimal vortex can be formed with negative buoyancy and if so what the corresponding formation number would be. The numerical code was previously validated for non-buoyant and positively buoyant jets and is further validated here for negatively buoyant jets using literature data on submerged fountains. Subsequently, jets with a range of negative buoyancies are simulated using source Reynolds numbers of 2000 < Re < 3000. A revised, directional form of Richardson number (Rid) is proposed to accommodate the entire range of buoyancy, with Rid > 0 for positively buoyant jets, Rid = 0 for non-buoyant jets, and Rid < 0 for negatively buoyant jets. Simulations identify two ranges of negative buoyancy. For weakly negatively buoyant starting jets (- 0.05 <≲Rid < 0), the pinch-off and formation of an optimal starting vortex occur, and the corresponding formation number can be determined using a revised algorithm, catering to negative buoyancy, based on the observed step jump in the vortex ring's circulation. This algorithm suggests a continuation of the declining trend in formation number with declining Rid identified previously for positively buoyant jets. For strongly negatively buoyant jets (Rid < -0.05), the starting vortex falls back onto the stem after the initial roll-up for -2.0 <≲ Rid < -0.05, and the pinch-off process does not exist for Rid<≲ -2.0 with the starting vortex and stem connected at all time. A plot of time-varying vortex circulation and penetration for starting plumes with different Rid unifies our understanding of buoyant vortex dynamics.

Original languageEnglish (US)
Article number052101
JournalPhysics of Fluids
Issue number5
StatePublished - May 16 2011
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Computational Mechanics
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering
  • Fluid Flow and Transfer Processes


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