Abstract
We introduce a new method to measure Lagrangian vorticity and the rotational dynamics of anisotropic particles in a turbulent fluid flow. We use 3D printing technology to fabricate crosses (two perpendicular rods) and jacks (three mutually perpendicular rods). Time-resolved measurements of their orientation and solid-body rotation rate are obtained from four video images of their motion in a turbulent flow between oscillating grids with = 91. The advected particles have a largest dimension of 6 times the Kolmogorov length, making them a good approximation to anisotropic tracer particles. Crosses rotate like disks and jacks rotate like spheres, so these measurements, combined with previous measurements of tracer rods, allow experimental study of axisymmetric ellipsoids across the full range of aspect ratios. The measured mean square tumbling rate, , confirms previous direct numerical simulations that indicate that disks tumble much more rapidly than rods. Measurements of the alignment of a unit vector defining the orientation of crosses with the direction of their solid-body rotation rate vector provide the first direct observation of the alignment of anisotropic particles by the velocity gradients in a turbulent flow.
Highlights
The motion of particulate matter in fluid flows has long been a central problem in both fundamental and applied fluid mechanics
We introduce a new way to measure the orientations and rotational dynamics of anisotropic particles that behave like ellipsoids
Shown is the probability density function (PDF) obtained from direct numerical simulations of spheres [21]
Summary
The motion of particulate matter in fluid flows has long been a central problem in both fundamental and applied fluid mechanics. Analytic work [15, 16] and numerical simulations [17, 18, 19, 20] have made significant progress extending studies of non-spherical particles in complex flows, but experimental measurements of the dynamics of anisotropic particles have lagged far behind due to the difficulty of measuring their time-dependent orientation in three dimensions. Methods have been developed for measuring time-resolved orientation and position of thin rods in 3D turbulence with stereoscopic optical imaging [21]. Another technique uses large transparent anisotropic particles with tracer particles inside and measures their rotations with particle image velocimetry [22]. Our technique of measuring the solid-body rotation of small jacks allows us to accurately measure the fluid vorticity using straightforward imaging methods
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