Investigation Of Universal Small-Scale Structures In Turbulence Using Shake-The-Box Lagrangian Particle Tracking And FlowFit

A characteristic property of turbulent flows is the presence of a universal small-scale structure consisting of a shear layer separated by two stretched vortices. This pattern becomes visible by averaging the velocity sampled in the eigenframe of the local strain rate tensor. In this contribution, the structure is detected and tracked over time to investigate its formation and decay. For this purpose, experimental data of a von Kármán flow at Re_λ = 370 is used, which is analyzed with Shake-The-Box (STB) Lagrangian Particle Tracking (LPT) and the data assimilation method FlowFit. The universal structure is characterized by the time-resolved mean distribution of dissipation and pressure in the strain rate eigenframe. Using the average power balance terms along all particle trajectories passing through the direct vicinity of conditioned high-dissipation (> 7.5 < ε >) and enstrophy (> 7.5 < νω^2>) events during the time-span from -4 τ_η < t < 4 τ_η the Lagrangian energy transport mechanism through such intermittent events can be discovered, at least in a mean sense. It is shown how the impact of the particles in a high dissipative event leads to a part of the energy being converted into heat and another part into rotation in a time period of approximately 2τ_η . This finding is consistent with the existence of the universal structure. Thus, a connection of Eulerian universal structures with their underlying energy exchange processes is proposed. Furthermore, instantaneous high dissipative events and the Lagrangian tracks that constitute them are shown and related to the statistical results.