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Kelken Chang

Publications and source records attributed to Kelken Chang.

4 recordsLinked to original sources
arXiv2019

Juggling with light

We discovered that when a pair of small particles is optically levitated, the particles execute a dance whose motion resembles the orbits of balls being juggled. This motion lies in a plane perpendicular to the polarization of the incident light. We ascribe the dance to a mechanism by which the dominant force on each particle cyclically alternates between radiation pressure and gravity as each particle takes turns eclipsing the other. We explain the plane of motion by considering the anisotropic scattering of polarized light at a curved interface.

physics.optics↗
arXiv2012

Explorations into the inertial and integral scales of homogeneous axisymmetric turbulence

A flow generator is described in which homogeneous axisymmetric turbulent air flows with varying and fully controllable degrees of anisotropy, including the much studied isotropic case, are generated by the combined agitations produced by 32 acoustic mixers focusing at the center of the system. The axisymmetric turbulence in a central volume of the size of the inertial scale is shown to have negligible mean and shear. The Taylor Reynolds number is about 480. The influence of large scale anisotropy on the turbulence is examined from three aspects, namely the velocity structure functions, the velocity correlation functions, and the integral lengths. The directional dependence of two different second order transverse structure functions, in which one of them has separations stretched along the axis of symmetry of the turbulence and the other one normal to it, is studied. It is shown that the inertial range scaling exponents, determined using the extended-self-similarity procedure, and the Kolmogorov constants of the two structure functions are unaffected by the direction in which the structure functions are measured. Finally, it is found that, except in the isotropic case, the second order transverse velocity correlation functions deviate from each other at the large scale with increasing anisotropy. A self-similarity argument similar to one found in the study of critical phenomena is proposed. It is shown that the argument leads to a power-law relationship between the large scale velocity fluctuation and the correlation length, with an exponent that depends on the inertial range scaling exponent of the turbulence. The data collapse predicted by the self-similarity hypothesis is verified. It is demonstrated that the value of the power-law exponent is consistent with the value of the inertial range scaling exponent.

physics.flu-dyn↗
arXiv2011

Experimental study of the influence of anisotropy on the inertial scales of turbulence

We ask whether the scaling exponents or the Kolmogorov constants depend on the anisotropy of the velocity fluctuations in a turbulent flow with no shear. According to our experiment, the answer is no for the Eulerian second-order transverse velocity structure function. The experiment consisted of 32 loudspeaker-driven jets pointed toward the centre of a spherical chamber. We generated anisotropy by controlling the strengths of the jets. We found that the form of the anisotropy of the velocity fluctuations was the same as that in the strength of the jets. We then varied the anisotropy, as measured by the ratio of axial to radial root-mean-square (RMS) velocity fluctuations, between 0.6 and 2.3. The Reynolds number was approximately constant at around $R_λ$ = 481. In a central volume with a radius of 50 mm, the turbulence was approximately homogeneous, axisymmetric, and had no shear and no mean flow. We observed that the scaling exponent of the structure function was $0.70 \pm 0.03$, independent of the anisotropy and regardless of the direction in which we measured it. The Kolmogorov constant, $C_2$, was also independent of direction and anisotropy to within the experimental error of 4%.

physics.flu-dyn↗