Book Description

"The passive scalar field of an axisymmetric turbulent jet and an isokinetic jet in an approximately homogeneous isotropic turbulence (HIT) with negligible mean flow is studied experimentally. The present research builds on that of Khorsandi et al. 2013 and Perez-Alvarado 2016, who studied the velocity field and the passive scalar field of an axisymmetric turbulent jet in a turbulent ambient, respectively. The primary objective is to deduce the jet structure, and to study the jet mixing in the HIT ambient by following the meandering path of the jet, i.e. conditional on the jet centroid. The secondary objective, complementing the first, is to study the diffusion of a momentumless patch of a passive scalar in the HIT ambient.The effect of a turbulent ambient on the dynamics and mixing of the passive scalar field of an axisymmetric turbulent jet is investigated. The experiments were conducted either in a quiescent or a turbulent ambient. The turbulent ambient was generated by a random jet array to achieve an approximately zero-mean-flow HIT ambient in the measurement plane. Two jet Reynolds numbers of Re = 5800 and 10600 were studied. Planar laser-induced fluorescence was used to measure the concentrations of the passive scalar dye (Sc = 2000) at orthogonal cross-sections of the jet at axial distances of x/d = 20, 30, 40, 50, 60. The statistics of the passive scalar field were conditioned on the jet centroid and were compared to the Eulerian statistics and to those of the jet in a quiescent ambient. The use of the centroidal analysis allowed the structure of the jet in the HIT ambient to be deduced, for which a two-region model was proposed. In the first region, following the developing region of the jet, the ambient turbulence progressively disrupts the jet structure and results in a faster concentration decay compared to the quiescent ambient. At a critical downstream distance, where the relative turbulence intensity between the ambient and the jet (ξ = urms,HIT/ urms,jet ) exceeds 0.5, the HIT ambient has destroyed the jet structure and the second region starts. In the second region, the turbulent diffusion is the only mechanism to transport the passive scalar field. The first-order centroidal statistics of the scalar field show self-similarity and self-preservation before jet break-up. The width of the jet is larger in the HIT ambient compared to that in a quiescent ambient and grows with axial distance but remains unchanged beyond jet break-up. Using the present passive scalar data and the velocity data from Khorsandi et al. 2013, it is argued that the momentum-driven entrainment of the jet in the HIT ambient is reduced compared to that in a quiescent ambient, and that the entrainment ceases beyond the jet break-up. The entrainment of the smaller scales of the ambient turbulence leads to a wider range of centerline concentrations and rms concentrations within the jet, and they are hypothesized to increase local concentration gradients and reduce the jet mixing.Diffusion of a patch of a passive scalar in the HIT ambient is studied. A high-Sc number passive scalar dye (Sc = 2500) is released isokinetically from a large diameter jet (d = 29.97 mm), and an orthogonal view of the passive scalar field is obtained using planar laser-induced fluorescence. The temporal evolution of the scalar patch is due to molecular diffusion and to turbulent diffusion in a quiescent ambient and in the HIT ambient, respectively. Time-averaged statistics of the passive scalar field are assessed at t = 0.2, 1, 1.8, 2.6, 3.4 s using a centroidal analysis. The mean concentration decays quickly and the rms concentration increases within the scalar patch. Compared to the quiescent ambient case, a wider range of the concentrations is present at the centroid of the scalar field. The size of the scalar patch increases with time, which is attributed to an increasing turbulent diffusivity for times shorter than the integral time scale of the turbulence"--