A study shows the difference between conventional flows and superfluid helium in the 3-D countercurrent

Researchers from the Weizmann Institute of Science, the University of Rome, the CNRS and the University of Helsinki recently conducted a study on the difference between 3D anisotropic turbulence in fluids classical and superfluid ones such as helium. Their conclusions, published in Letters of physical examination (PRL), are supported by both theory and experimental evidence.

"This research was initiated by our group at the Weizmann Institute in Israel, consisting of Victor L 'vov, Itamar Procaccia and Anna Pomyalov, who were trying to understand new experimental observations made by Professor Wei's groups. Guo from Florida State University, Tallahassee and Professor Ladislav Skrbek from Charles University in Prague, "said Phys.org Itamar Procaccia, one of the researchers who conducted the study. "Our main goal was to understand a surprising apparent difference in the energy distribution between turbulent vortices of different scales in classical viscous fluids such as air and water and superfluids. like helium at low temperatures. "

All turbulent flows, whether natural or laboratory, are anisotropic to energy injection scales, which means that the energy is distributed differently between their turbulent eddies. Previous studies have shown that the homogeneous and isotropic turbulence model (HIT) is particularly effective in predicting the statistical properties of turbulence at scales much smaller than stirring scales, but larger than dissipation scales.

In conventional fluids, three-dimensional anisotropic turbulence tends to isotropy and homogeneity with decreasing scales, so it is possible to apply the HIT model to them. In their study, however, Procaccia and his colleagues have shown that the opposite is true for superfluids. ⁴It has a turbulence in the three-dimensional geometry of the countercurrent channel, which becomes less isotropic as the scales diminish to the point of becoming almost two-dimensional.

The approach they use involves a so-called "two-fluid model" of superfluid helium. This model is based on the early works of Laszlo Tisza and Lev Landau in 1940-1941, which were later improved by H. Hall, W. F. Vinen, I. M. Khalatnikov and I. L. Bekarevich.

"The model describes superfluid helium as an interpenetrating mixture of two fluids: a superfluid that moves without friction and a normal viscous fluid that are coupled by mutual friction," explained Procaccia.

Previous studies conducted by two teams of researchers in Tallahasse, Florida and Prague examined superfluid helium under a temperature gradient, creating what is called the "countercurrent". As its name indicates, against the current, different components of a fluid flow in opposite directions; the superfluid flows from the cold to the hot and the normal fluid from the hot to the cold.

"Our model rationalized some of these experimental observations and predicted new features that were later experimentally confirmed," Procaccia said. "The main result of our study is that, unlike classical turbulent flows that are becoming increasingly isotropic at smaller scales, the flow we examined becomes less and less isotropic as scales decrease. "

Before conducting their study, Procaccia and his colleagues had theoretically predicted that their experiments would lead to the observations they then collected. However, the strength of the effect they observed has become evident only after conducting direct numerical simulations on a supercomputer of the EU, in collaboration with a team of researchers led by Luca Biferale. According to Procaccia, their theoretical and numerical discoveries have already prompted other experimental groups to continue their research on countercurrent turbulence.

"At the Weizmann Institute, we are now developing our theory, being attentive to new experimental techniques that allow in-depth studies of turbulence in superfluid helium," Procaccia said. "Our group continues to participate in the analysis of new experimental data, in the hope of contributing to a better understanding of superfluid flows from laboratory experiments to cosmological realization, such as neutron stars."

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