The dynamics of an axisymmetric turbulent jet in ambient turbulence interpreted from the passive scalar field statistics.

The effect of approximately homogeneous isotropic turbulence on the dynamics of an axisymmetric turbulent jet (Re = 10 600 and 5800) in an ambient with a negligible mean flow is interpreted from the statistics of the passive scalar field. The ambient turbulence is generated by a random jet array and scalar concentrations are measured in orthogonal cross sections of the jet using planar laser-induced fluorescence. Statistics of the scalar field of the jet in a turbulent ambient are compared to those in a quiescent ambient, using classical Eulerian averages and those conditioned on the jet centroid. A two-region model for the jet structure in ambient turbulence is proposed based on the centroidal statistics. Following the developing region of the jet, the ambient turbulence disrupts the jet structure, due to modulation of the jet interface, meandering of the jet by large eddies, and entrainment of the turbulent ambient fluid, resulting in a faster concentration decay and reduced entrainment compared to the quiescent ambient. Further downstream, once the ambient turbulence has destroyed the jet, only molecular and turbulent diffusion modify the scalar concentrations. The regions' relative lengths depend primarily on the relative turbulence intensity (ξ) between the ambient and the jet, as assessed using centroidal analysis, which removes the effect of the relative length scale (l) on the jet behavior in the turbulent ambient. The centroidal scalar statistics reveal self-similarity and self-preservation in mean scalar properties before jet breakup, which occurs abruptly once ξ > 0.5. The smaller scales of ambient turbulence modulate the jet boundary and, when entrained, lead to a wider range of centerline concentrations and root mean square concentrations, which are hypothesized to increase local concentration gradients within the jet and reduce jet mixing. [ABSTRACT FROM AUTHOR]