Recent Developments in Measuring the Smallest Velocity Scales

Many important flow phenomena occur near surfaces; they include skin friction, turbulence generation, heat and mass transfers, etc. The length scales that govern these processes are some of the smallest encountered in classical fluid mechanics. For example, in high-Reynolds number wall-bounded flows, the viscous wall unit can be on the order of a micrometer or more than one order of magnitude smaller than the Kolmogorov scale. In large experimental facilities, probing flows at such scales is a challenge; very few datasets exist, especially near walls, and many outstanding questions remain.
This seminar will present recent developments in optical diagnostics to push velocimetry to micrometer scales in three dimensions (3D). The developments include 1- the extension of molecular tagging velocimetry to 3D by adopting Fourier light-field microscopy (FLFM); 2- the generalization of FLFM to plenoptic 3.0 and its application to Lagrangian particle tracking, 3- the development of polymers that have a refractive index matched to that of water. High-order statistics and instantaneous flow fields acquired in turbulent flows will demonstrate the state of the techniques.