Direct measurement of vorticity using tracer particles with internal markers

TL;DR

This paper introduces a novel digital inline holography method for direct vorticity measurement in fluid flows, utilizing tracers with internal markers and 3D tracking to achieve high spatial resolution of small-scale turbulence dynamics.

Contribution

The study develops a new DIH-based technique with internal marker tracers and a rotation tracking algorithm for precise vorticity measurement at small scales.

Findings

Successfully measured tracer rotation rates from 0.3 to 0.7 rad/frame.

Demonstrated simultaneous measurement of rotation and translation of multiple tracers.

Achieved vorticity measurement in regions smaller than 100 μm, suitable for Kolmogorov-scale turbulence.

Abstract

Current experiment techniques for vorticity measurement suffer from limited spatial and temporal resolution to resolve the small-scale eddy dynamics in turbulence. In this study, we develop a new method for direct vorticity measurement in fluid flows based on digital inline holography (DIH). The DIH system utilizes a collimated laser beam to illuminate the tracers with internal markers and a digital sensor to record the generated holograms. The tracers made of the polydimethylsiloxane (PDMS) prepolymer mixed with internal markers are fabricated using a standard microfluidic droplet generator. A rotation measurement algorithm is developed based on the 3D location reconstruction and tracking of the internal markers and is assessed through synthetic holograms to identify the optimal parameter settings and measurement range (e.g., rotation rate from 0.3 to 0.7 rad/frame under numerical aperture of imaging of 0.25). Our proposed method based on DIH is evaluated by a calibration experiment of single tracer rotation, which yields the same optimal measurement range. Using von K\'arm\'an swirling flow setup, we further demonstrate the capability of the approach to simultaneously measure the Lagrangian rotation and translation of multiple tracers. Our method can measure vorticity in a small region on the order of 100 ${\mu}$m or less and can be potentially used to quantify the Kolmogorov-scale vorticity field in turbulent flows.