arXiv subjects

Gregory P. Bewley

Publications and source records attributed to Gregory P. Bewley.

21 records · Page 2Linked to original sources

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↗

arXiv2008

Characterization of reconnecting vortices in superfluid helium

When two vortices cross, each of them breaks into two parts and exchanges part of itself for part of the other. This process, called vortex reconnection, occurs in classical as well as superfluids, and in magnetized plasmas and superconductors. We present the first experimental observations of reconnection between quantized vortices in superfluid helium. We do so by imaging micron-sized solid hydrogen particles trapped on quantized vortex cores (Bewley GP, Lathrop DP, Sreenivasan KR, 2006, Nature, 441:588), and by inferring the occurrence of reconnection from the motions of groups of recoiling particles. We show the distance separating particles on the just-reconnected vortex lines grows as a power law in time. The average value of the scaling exponent is approximately 1/2, consistent with the scale-invariant evolution of the vortices.

cond-mat.other↗

arXiv2005

Characterisation of superfluid vortices in helium II

Matter at low temperatures exhibits unusual properties such as superfluidity, superconductivity, Bose-Einstein condensation, and supersolidity. These states display quantum mechanical behaviours at scales much larger than atomic dimensions. As in many phase transitions, defects can occur during the transition to the low temperature state. The study of these defects yields useful information about the nature of the transitions and of the macroscopic states themselves. When cooled below the lambda temperature (~2.172 K), liquid helium acquires superfluid properties, and the defects in the superfluid take the form of line vortices with quantized circulation. The formation of these vortices has been suggested as a model for cosmological structure formation. Here we visualize these superfluid (or quantized) vortices by suspending micron-sized solid particles of hydrogen in the fluid. The particles at low concentrations arrange themselves with nearly equal spacing along vortex lines. The number density of the vortex lines in a steadily rotating state compares well with Feynman's prediction. For the first time, it is possible to observe superfluid turbulence in which the superfluid vortices interact, and exist in networks with complex branching. Our method makes it possible to characterize the vortices during the transition across the lambda point and in the turbulent state itself.

cond-mat.stat-mech↗