Lagrangian Turbulence at Unprecedented Reynolds Numbers

The Lagrangian reference frame, in which turbulence is viewed by tracking fluid elements over time, is the natural framework for studying transport and mixing phenomena (Sawford (2001)) and previously unexplored properties of turbulence (Toschi & Bodenschatz (2009)). Particularly important Lagrangian dynamics such as the formation of clouds occur at large Reynolds numbers. To our knowledge, the Variable Density Turbulence Tunnel (Bodenschatz et al. (2014)) is the only apparatus capable of generating turbulence at Taylor-scale Reynolds numbers up to 6000, while permitting Lagrangian measurements. The turbulence generation is highly adjustable through a uniquely flexible active grid (Griffin et al. (2019)) and by tuning the pressure of the working fluid SF6 up to 15 bar. Here we present the first Lagrangian particle tracking measurements in this high-pressure wind tunnel. We present the particle properties and the particle injection device. We further describe the laser illumination and the vibration-damped high-speed imaging setup. We present statistics of particle accelerations at Reynolds numbers greater than 3000, marking the highest Reynolds numbers at which such statistics have ever been recorded, as well as radial distribution functions between Stokes Numbers 0.02 and 0.2.