Measuring vorticity vector from the spinning of micro-sized mirror-encapsulated spherical particles in the flow

Huixuan Wu; Haitao Xu; Eberhard Bodenschatz

doi:10.1063/1.5121016

Measuring vorticity vector from the spinning of micro-sized mirror-encapsulated spherical particles in the flow

Review of Scientific Instruments ◽

10.1063/1.5121016 ◽

2019 ◽

Vol 90 (11) ◽

pp. 115111 ◽

Cited By ~ 1

Author(s):

Huixuan Wu ◽

Haitao Xu ◽

Eberhard Bodenschatz

Keyword(s):

Spherical Particles ◽

Vorticity Vector

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Why spheroids orient preferentially in near-wall turbulence

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.619 ◽

2016 ◽

Vol 807 ◽

pp. 221-234 ◽

Cited By ~ 18

Author(s):

Lihao Zhao ◽

Helge I. Andersson

Keyword(s):

Channel Flow ◽

Symmetry Axis ◽

Isotropic Turbulence ◽

Strain Tensor ◽

Flow Simulation ◽

Wall Turbulence ◽

Spherical Particles ◽

Vorticity Vector ◽

Near Wall Turbulence ◽

Near Wall

Non-spherical particles are known to orient preferentially in near-wall turbulence, but rod-like and disk-like particles align themselves differently relative to the mean vorticity direction. To uncover the mechanism that gives rise to such preferential particle orientations in anisotropic turbulence, Lagrangian statistics from a channel-flow simulation have been analysed. Ni et al. (J. Fluid Mech., vol. 743, 2014, R3) showed that the fluid vorticity and long rods independently aligned with the Lagrangian fluid stretching direction in isotropic turbulence. Following their approach, we deduced the left Cauchy–Green strain tensor along Lagrangian trajectories of tracer spheroids in channel-flow turbulence. The results showed that the alignment of the fluid vorticity vector with the strongest Lagrangian stretching direction in the channel centre, just as in isotropic turbulence, vanished in the vicinity of the walls. The analysis revealed that the directions of the strongest Lagrangian stretching and compression in near-wall turbulence are in the streamwise and wall-normal directions, respectively. All over the channel we found that the symmetry axis of prolate spheroids aligned with the direction of strongest Lagrangian stretching whereas oblate spheroids oriented with the direction of Lagrangian compression. This finding is apparently universal since the same trends were found in highly anisotropic wall turbulence as well as in isotropic turbulence. Contrary to the prevailing view, we have shown for the first time that the preferential orientation of the symmetry axis of long rods in the streamwise direction and of flat disks in the wall-normal direction is caused by Lagrangian stretching and not by fluid rotation. This finding fills a gap in our understanding of orientation and rotation of tracer spheroids in anisotropic wall turbulence.

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STEM Study of Surface Plasmon Excitation in Small Spherical Particles

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100133801 ◽

1990 ◽

Vol 48 (2) ◽

pp. 42-43

Author(s):

Daniel UGARTE

Keyword(s):

Energy Loss ◽

Spherical Particles ◽

Small Particles ◽

Bulk Materials ◽

Low Loss ◽

Plasmon Excitation ◽

Surface Modes ◽

Surface Plasmon Excitation ◽

Analytical Electron ◽

Analytical Electron Microscope

Small particles exhibit chemical and physical behaviors substantially different from bulk materials. This is due to the fact that boundary conditions can induce specific constraints on the observed properties. As an example, energy loss experiments carried out in an analytical electron microscope, constitute a powerful technique to investigate the excitation of collective surface modes (plasmons), which are modified in a limited size medium. In this work a STEM VG HB501 has been used to study the low energy loss spectrum (1-40 eV) of silicon spherical particles [1], and the spatial localization of the different modes has been analyzed through digitally acquired energy filtered images. This material and its oxides have been extensively studied and are very well characterized, because of their applications in microelectronics. These particles are thus ideal objects to test the validity of theories developed up to now.Typical EELS spectra in the low loss region are shown in fig. 2 and energy filtered images for the main spectral features in fig. 3.

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