The effect of a zero-mean-flow turbulent ambient on the geometry of the turbulent/turbulent interface (TTI) of an axisymmetric jet is investigated and compared with the traditional turbulent/non-turbulent interface (TNTI). The ambient turbulence is generated using a random jet array. Orthogonal cross-sections of the jet subjected to different levels of ambient turbulence intensities and length scales are captured using planar laser-induced fluorescence. The enhanced radial transport of concentration from the jet core towards the interface and the increased scalar fluctuations within the jet caused by the ambient turbulence result in steeper mean and root-mean-square scalar conditional jumps across the TTI layer compared with those of the TNTI, respectively. When compared with the quiescent ambient, the mean effect of background turbulence is to stretch and corrugate the interface surface area as evident from the wider probability density of the interface radial position, the lowered occurrence of zero-curvature surface elements, the greater misalignment between the radial and normal unit vectors of the interface, the increased tortuosity and the increased magnitude of the fractal exponent.

The effect of the geometric features of the turbulent/non-turbulent interface on the entrainment of a passive scalar into a jet

The effect of approximately homogeneous isotropic turbulence on the dynamics of an axisymmetric turbulent jet (Re = 10600 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 the 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 the 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 the mean scalar properties before jet break-up, which occurs abruptly once ξ > 0.5. The smaller scales of the ambient turbulence modulate the jet boundary and, when entrained lead to a wider range of centerline concentrations and, also rms concentrations, which are hypothesized to increase local concentration gradients within the jet and reduce the jet mixing. 42 velocity and the concentration. The rate of width growth is 43 increased (but less than the increase in the rate of decay of the 44 mean centerline properties) 8,[10][11][12]16 , while the mass flow rate 45 decreases. 11 Similarly, the velocity deficit in a wake decays 46 more quickly 17,18 and its width growth increases once the in-47 tegral scale of the ambient turbulence is larger than the width 48 of the wake. 19 49 The effect of the relative turbulence intensity and the rel-50 ative length scale between the ambient and the flow was ini-51 tially observed for boundary layers. 20 In wakes, the ambient 52 turbulence intensity is the dominant factor. 21,22 There is hy-53 pothesized to be little effect of the ambient turbulence on the 54 jet or plume flow close to the exit, where it is less intense 55 than the jet turbulence. However, its increasing impact with 56 downstream distance due to an increasing relative turbulence 57 intensity will result, at some point, in the break-up of the jet 58 and destruction of its structure. 9 After the jet structure is de-59 stroyed, a passive scalar will then only disperse due to detrain-60

Planar laser-induced fluorescence is employed to investigate the concentration field in the developing region of a coaxial jet with an annular to inner jet velocity of one. Seeding the annular flow with a scalar shows that prior to the erosion of the inner scalar core, an inner interface exists, across which the scalar quantities exhibit sharp gradients akin to the traditional outer interface. Coarse grain filtering revealed that the outer interface establishes the universal fractal scaling before the onset of self-similarity, while the inner interface does not develop sufficient scale separation to do so due to the lack of shear.

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