Lagrangian and Eulerian views into a turbulent boundary layer flow using time-resolved tomographic PIV

Abstract

The momentum exchange mechanisms in a turbulent boundary layer flow are of manifold temporal and spatial scales and governed by the organization of self-sustaining coherent flow structures driven by entrained high momentum fluid. Generic flow structures such as hairpin-like vortices and spanwise alternating wall bounded low- and high-speed streaks have been observed and extensively analyzed with both experimental and numerical methods, e.g. by means of PIV [1, 2] or DNS [3]. In many studies the role of these structures for the wall normal and spanwise fluid exchange has been highlighted mostly within an Eulerian reference frame. But for a full understanding of the momentum exchange in turbulent flows a step to a spatially resolved Lagrangian frame of reference would be advantageous. However, experiments measuring turbulent flows in a Lagrangian reference frame need techniques which allow following the trajectory of (single) particles in a high temporal resolution comparable to the smallest turbulence time scales and at the same experiment also for a long observation time in order to cover also the integral time scales. Up to now the Lagrangian turbulence characterization has been performed by statistical analysis over several spatially independent single particles measured by tracking methods [4, 5]. Using these techniques the achieved spatial resolution did not allow describing the coherent structures. One of the challenges of experimental turbulence research is therefore the combination of both viewpoints in a single experiment.