Lagrangian statistics across the turbulent-nonturbulent interface in a turbulent plane jet

摘要

Lagrangian statistics from millions of particles are used to study the turbulent entrainment mechanismnin a direct numerical simulation of a turbulent plane jet at Reλ ≈ 110. The particles (tracers) are initiallynseeded at the irrotational region of the jet near the turbulent shear layer and are followed as they arendrawn into the turbulent region across the turbulent-nonturbulent interface (TNTI), allowing the study ofnthe enstrophy buildup and thereby characterizing the turbulent entrainment mechanism in the jet. Thenuse of Lagrangian statistics following fluid particles gives a more correct description of the entrainmentnmechanism than in previous works since the statistics in relation to the TNTI position involve data fromnthe trajectories of the entraining fluid particles. The Lagrangian statistics for the particles show the existencenof a velocity jump and a characteristic vorticity jump (with a thickness which is one order of magnitudengreater than the Kolmogorov microscale), in agreement with previous results using Eulerian statistics. Thenparticles initially acquire enstrophy by viscous diffusion and later by enstrophy production, which becomesn“active” only deep inside the turbulent region. Both enstrophy diffusion and production near the TNTIndiffer substantially from inside the turbulent region. Only about 1% of all particles find their way intonpockets of irrotational flow engulfed into the turbulent shear layer region, indicating that “engulfment”nis not significant for the present flow, indirectly suggesting that the entrainment is largely due to “nibbling”nsmall-scale mechanisms acting along the entire TNTI surface. Probability density functions of particle positionsnsuggests that the particles spend more time crossing the region near the TNTI than traveling inside the turbulentnregion, consistent with the particles moving tangent to the interface around the time they cross it.