GRADIENT TRAJECTORY ANALYSIS OF THE TURBULENT/NON-TURBULENT INTERFACE IN A JET FLOW

Based on planar high-speed Rayleigh scattering measurements of the mixture fraction Z of propane discharging from a turbulent round jet into co-flowing carbon dioxide at nozzle based Reynolds numbers Re0=3,000-8,600, we use scalar gradient trajectories to investigate the local structure of the turbulent scalar field with a focus on the scalar turbulent/non-turbulent interface. The latter is located between the fully turbulent part of the jet and the outer flow. Using scalar gradient trajectories, we partition the turbulent scalar field into these three regions according to an approach developed by Mellado et al. (2009). Based on these different regions, we investigate in a next step zonal statistics of the scalar pdf P(Z) as well as the scalar difference along the trajectory ∆Z and its mean scalar value Zm, where the latter two quantites are used to parameterize the scalar profile along gradient trajectories. We show that the scalar probability density function P(Z) can be reconstructed from zonal gradient trajectory statistics of the joint probability density function P(Zm,∆Z). Furthermore, we relate our results for the scalar turbulent/non-turbulent interface on the one hand to the findings made in other experimental and numerical studies of the turbulent/non-turbulent interface and discuss them on the other hand in the context of the flamelet approach and the modelling of probability density functions in turbulent non-premixed combustion.