Detection of vortex coherent structures in superfluid turbulence

Abstract

Filamentary regions of high vorticity irregularly form and disappear in the turbulent flows of classical fluids. We report an experimental comparative study of these so-called "coherent structures" in a classical versus quantum fluid, using liquid helium with a superfluid fraction varied from 0% up to 83%. The low pressure core of the vorticity filaments is detected by pressure probes located on the sidewall of a 78-cm-diameter Von Kármán cell driven up to record turbulent intensity (R λ ∼ √ Re 10000 ). The statistics of occurrence, magnitude and relative distribution of the filaments in a classical fluid are found indistinguishable from their superfluid counterpart, namely the bundles of quantized vortex lines. This suggest that the internal structure of vortex filaments, as well as their dissipative properties have a negligible impact on their macroscopic dynamics, such as lifetime and intermittent properties.

Coarse-grained pressure dynamics in superfluid turbulence

“…In this work we performed detailed numerical simulations of superfluid turbulence at the level of individual quantized vortices through the vortex filament model. Through course graining we find compelling evidence supporting the conclusions of [1]. Elementary simulations of an isolated bundle show that the number of vortices in a bundle can be directly inferred from the size of the pressure dip, with good agreement between numerics and the HVBK equations.…”

“…This is an important observation when analyzing experimental data, and suggests that the data presented in Ref. [1] indicates that the embedded coherent structures in the flow are larger than the probe scale. If we chose the filtering scale to be significantly larger than the average bundle size, then all coherence of the flow would be removed due to averaging.…”

“…The authors of Ref. [1] conjectured that coherent vortex bundles arise as extreme negative pressure fluctuations exceeding −3σ press . In Fig.…”

“…Despite this it has been observed that turbulence in superfluids can, in a statistical sense, share many of the properties of its classical brethren; coherent bundles of superfluid vortices are often invoked as an import feature leading to this quasi-classical behaviour. A recent experimental study [1] inferred the presence of these bundles through intermittency in the pressure field, however direct visualization of the quantized vortices to corroborate this finding was not possible. In this work we performed detailed numerical simulations of superfluid turbulence at the level of individual quantized vortices through the vortex filament model.…”

“…The most common measurement technique is to measure pressure fluctuations and to then infer the dynamics of the velocity field [3,[6][7][8]. Inspired by the group of Roche [1] who recently studied the statistics of pressure fluctuations as a route for detection of coherent structures, we perform a series of numerical simulations of superfluid turbulence in the zero-temperature limit to investigate the dynamics and statistics of coarsegrained fields and examine the relationship between the pressure and velocity in a variety of superfluid turbulent regimes. This is crucial, as while significant advances in the visualization of quantum turbulence have been made in the last decade [9,10], we are yet to have a direct visualization of coherent structures in quantum turbulence.…”