Lagrangian Particle Tracking at Large Reynolds Numbers

Particle tracking in turbulent flows is fundamental to the study of the transport of tracers, inertial particles or even active objects in space and time, i.e. the Lagrangian reference frame. It provides experimental tests of theoretical predictionscite{Toschi2009} (e.g. for the statistics of fluid accelerations and particle dispersion) and helps to understand important natural processes where the inertia of particles is important (e.g. cloud microphysicscite{Bertens2021}). While the spatial (Eulerian) features of turbulent flows have been studied for high, atmospheric Reynolds numbers ($R_lambda > 10^4$)cite{Tsuji2004}, the profound difficulties in accurately tracking particles in turbulent flows have limited the Reynolds numbers in the Lagrangian reference frame to Taylor scale Reynolds number $R_lambda lesssim 10^3$. Here we describe a setup that offers Lagrangian particle tracking at $R_lambda$ between 100 and 6000 in the Max Planck Variable Density Turbulence Tunnel (VDTT). We describe the imaging setup within the pressurised facility, the laser illumination, the particles and the particle dispersion mechanism. We verify that the particles are monodisperse, carry negligible charges and are good tracers for the whole range of experimental conditions. We detail the challenges and the constraints of the setup and we present typical data from the experiment.