Experiments have been conducted to investigate turbulent mixing and the dynamics
of outer fluid interfaces, i.e. the interfaces between mixed fluid and pure ambient
fluid. A novel six-foot-diameter octagonal-tank flow facility was developed to enable
the optical imaging of fluid interfaces above the mixing transition, corresponding to
fully developed turbulence. Approximately 10003 whole-field three-dimensional space–
time measurements of the concentration field were recorded using laser-induced-
fluorescence digital-imaging techniques in turbulent jets at a Reynolds number of
Re ∼ 20 000, Schmidt number of Sc ∼ 2000, and downstream distance of
∼ 500 nozzle
diameters. Multiple large-scale regions of spatially nearly uniform-concentration fluid
are evident in instantaneous visualizations, in agreement with previous findings above
the mixing transition. The ensemble-averaged probability density function of concentration is found to exhibit linear dependence over a wide range of concentration
thresholds. This can be accounted for in terms of the dynamics of large-scale well-
mixed regions. Visualization of the three-dimensional space–time concentration field
indicates that molecular mixing of entrained pure ambient fluid is dynamically initiated and accomplished in the vicinity of the unsteady large scales. Examination
of the outer interfaces shows that they are dynamically confined primarily near the
instantaneous large-scale boundaries of the flow. This behaviour is quantified in terms
of the probability density of the location of the outer interfaces relative to the flow
centreline and the probability of pure ambient fluid as a function of distance from
the centreline. The current measurements show that the dynamics of outer interfaces
above the mixing transition is significantly different from the behaviour below the
transition, where previous studies have shown that unmixed ambient fluid can extend
across a wide range of transverse locations in the flow interior. The present observations of dynamical confinement of the outer interfaces to the unsteady large scales,
and considerations of entrainment, suggest that the mechanism responsible for this
behaviour must be the coupling of large-scale flow dynamics with the presence of
small-scale structures internal to the large-scale structures, above the mixing transition. The dynamics and structure of the outer interfaces across the entire range of
space–time scales are quantified in terms of a distribution of generalized level-crossing
scales. The outer-interface behaviour determines the mixing efficiency of the flow, i.e.
fraction of mixed fluid. The present findings indicate that the large-scale dynamics of
the outer interfaces above the mixing transition provides the dominant contribution
to the mixing efficiency. This suggests a new way to quantify the mixing efficiency of
turbulent flows at high Reynolds numbers.
First application of large reactivity measurement through rod drop based on three-dimensional space–time dynamics
