# Vortex bundle collapse and Kolmogorov spectrum. Talk given at the Low Temperature Conference, Kazan, 2015

## Abstract

The statement of problem is motivated by the idea of modeling the classical turbulence with a set of chaotic quantized vortex filaments in superfluids. Among various arguments supporting the idea of quasi-classic behavior of quantum turbulence, the strongest, probably, is the dependence of the spectra of energy, obtained in numerical simulations and experiments. At the same time the mechanism of classical vs quantum turbulence (QT) is not clarified and the source of the dependence is unclear. In this work we concentrated on the nonuniform vortex vortex bundles. This choice is related to actively discussed question concerning a role of collapses in the vortex dynamics in formation of turbulent spectra. We demonstrate that the nonuniform vortex vortex bundles, which appear in result of nonlinear vortex dynamics generates the energy spectrum, which close to the Kolmogorov dependence .

## I Introduction and scientific background

The problem of modeling classical turbulence with a set of chaotic
quantized vortices is undoubtedly in the mainstream of modern studies of
vortex states in quantum fluids (see, e.g., Vinen2000 (),Skrbek2012 (),Nemirovskii2013 ()).

One of the evidences of the quasi-classical behavior of QT is the -dependence of the spectra of energy, obtained in numerical simulations
and experiments, and their comparison with the Kolmogorov law . The experimental observation of the Kolmogorov
law is only obtained in presence of the normal component–see Maurer1998 (). There is as yet no direct experimental evidence relating to
the spectrum of the turbulent energy at very low temperatures (see Vinen2002 ()). Measuring the fluctuation of the VLD (see, Roche2007 (),
Bradley2008 ()) gave results, which are, probably, inconsistent with the
quasi-classical behavior of QT. As for numerical results, there are several works, which
demonstrate the dependence . There are
both the works, based on the vortex filament methods (VFM) Araki2002 (); Kivotides2002 (); Kivotides2001c ()
and works using GPE Nore1997 (); Nore1997a (), Kobayashi2005 (),Sasa2011 ().

The most common view of quasi-classical turbulence is the model of vortex bundles. The point is that the quantized vortices have a fixed core radius, so they don’t possess the very important property of classical turbulence – stretching vortex tubes with a decrease in the core size. The latter is responsible for the turbulence energy cascade from large scales to the small scales. Collections of near-parallel quantized vortices (vortex bundles) do possess this property, so the idea that the quasi-classical turbulence in quantum fluids is realized via vortex bundles of different sizes and intensities (number of threads ) seems quite natural. However the concept of the bundle structure does not explain appearance of Kolmogorov type spectrum , since the usual uniform vortex array just generates the coarse grained solid body rotation.

In the work we study nonuniform vortex arrays, whose structure is determined by the collapsing vortex dynamics.

## Ii Uniform vortex array

The energy of the vortex tangle, expressed via vortex filaments elements ( is the label variable of - loop ) is defined as (see Nemirovskii1998 (),Nemirovskii2002 (),Nemirovskii2013a ())

(1) |

For anisotropic situations, formula (1) is understood as an angular average, but one has to treat this formula with precaution (see Nemirovskii2015 ()). Thus, for calculation of the energy spectrum of the 3D velocity field, induced by the vortex filament we need to know the exact configuration of vortex lines.

Let’s study the question, what is the energy spectrum of 3D flow induced by the array of vortex filaments, imitating the bundle. First we consider a set of straight vortex filaments forming the square lattice . Points are coordinates for vortices on the -plane, indices runs from to . The neighboring lines are separated by distance , i.e., . In case of different straight lines we have to perform integration between different lines and where distances between vortices on the -plane. Then equation (1) can be rewritten as

(2) |

Integral over is in the table by Ryzhik & Gradshtein (3.876) (see Gradshteyn1980 ())

(3) |

Thus, determination of the spectrum on the basis (3) should be done with the use of the quadruple summation (over ), which requires large computing resources. Clear, however, that for very small , which corresponds to very large distance, the whole array can be considered as large single vortex with the circulation . Accordingly, the spectrum (per unit height) should be (. For large , which corresponds to very small distance from each line, the spectrum (per unit height) should be as for the single straight vortex filament. In the intermediate region , and (this condition implies that inverse wave number is larger than the intervortex space between neighboring lines, but smaller then the size of the whole array ), we can replace the quadruple summation by the quadruple integration with infinite limits. This procedure corresponds that we exclude the fine-scale motion near each of vortex, and are interested in the only large-scale, coarse-grained motion. After obvious change of variables etc. we get that the whole integral should scale as , and accordingly (compare with Nowak2012 ()). As it is shown in Nemirovskii2013a (), the velocity scales as . Thus, the uniform vortex array creates the course-grained motion, which is rotation (velocity is proportional to the distance from center), as it should be. Moreover, the coefficient is proportional to , which corresponds to the Feynman rule. Concluding this subsection we state that the uniform vortex bundles do not generate the Kolmogorov spectra.

## Iii Vortex lines breaking

Currently, in classic hydrodynamics, the highly important topic - the role of hydrodynamic collapses in the formation of turbulent spectra - is being intensively discussed (See e.g., Kuznetsov2000 (), Kerr2013 (), Nemirovskii1982 ()). Briefly, this phenomenon can be described as spontaneous infinite growth of the vorticity field with formation of singularity in . In particular, in the continuously distributed vortex field the vortex lines (not quantized vortex filaments, just hydrodynamic vortex lines!) start to accumulate at some points forming singular distribution as it is illustrated in Fig. 1. The latter results in the increment for velocity field , which, in turn results it to the Kolmogorov spectrum . In classical hydrodynamics this scenario is known as the vortex breaking. This phenomenon was analyzed in series of papers by Kuznetsov with coauthors ( see/ e.g. Kuznetsov2000 (),Agafontsev2015 () and references therein)in the framework of the integrable incompressible hydrodynamic model with the Hamiltonian . Studying Euler equations in terms of vorticity (also known as Helmholtz’s vorticity equations)

(4) |

Kuznetsov concluded that in the vicinity of the touching point the maximum value of vorticity develops in the blow-up manner

(5) |

with the approaching the infinity at some time . The domain of vorticity is not isotropic, it has a pancake structure. The main dependence of vorticity field is connected with the transverse to the bundle direction and scales as

(6) |

The similar consideration can be applied for quantum vortices. It, however, can be done only in particular case of vortex bundles, when the quantum vortex filaments form a near-parallel structure. In this case The coarse-grained hydrodynamic equations for the superfluid vorticity are obtained from the Euler equation for the superfluid velocity after averaging over the vortex lines . The coarse-grained hydrodynamic of the vortex bundles is studied by many authors (see e.g., Sonin1987 (),HENDERSON2000 (),Holm2001 (),Jou2011 ()), but the basis of these studies was the hydrodynamics of rotating superfluids, or the Hall-Vinen-Bekarevich-Khalatnikov (HVBK) model (see e.g., book Khalatnikov1965 ()). In the vortex bundles, the coarse-grained vorticity field of , and the 2D vortex line density (which coincides with the areal density in plane perpendicular to the bundle) are related to each other by means of the Feynman’s rule, . In terms HVBK the dynamics of this vorticity obeys the following equation (see Khalatnikov1965 ())

(7) |

where is the velocity of lines,

(8) |

It is easy to see that in case of zero temperature, when mutual friction vanishes , and taking into account that vortex lines move with the averaged velocity the dynamics of macroscopic (or the coarse-grained) vorticity is identical to the dynamics of classical field, therefore all, stated above conclusions concerning the collapse of vorticity are valid for quantum fluids.

## Iv Noninform lattice

Let’s now consider the nonuniform vortex bundle. To model this situation we just can choose that the distance between lattice points (see Sec. 2) is not constant, but depends on the numbers of the cell nodes. We have to realize that the problem of the spontaneous formation of vortex bundles is only on the stage of discussion so far, and there is no ideas concerning an exact arrangement of these bundles. We will choose the power law dependence for the distance between the lattice points.

(9) |

We do not ascertain the quantity , it is free parameter of our approach. Under condition (9) the expression (3) turns into

(10) |

That means that while changing the summation by integration we have to put (instead of the change of variables made in Sec. 2). As a result we get, that the whole integral should be scaled as . It is easy to see that when , the spectrum .

Let’s find the density of vortices on the plane under condition (9), or, according to the Feynman rule, the distribution of vorticity . In the ”space” of indices vortices are distributed uniformly (one vortex per lattice site ), but since the distances between the sites vary, the distribution of vortices in the real space is nonuniform. Let us consider ”the ring” of radius from to in . Then, the number of points in ring is just , the radius of ring in real space is , and the thickness of ring is . From these relations it follows that the real scales with as

(11) |

If , then , as it should be for the classical turbulence Kuznetsov2000 (),Agafontsev2015 ().

## V Conclusions

Summarizing, it can be concluded that the 3D energy spectrum close to the Kolmogorov dependence which was observed in many numerical simulations on the superfluid turbulence Araki2002 ()-Sasa2011 (), can appear from the collapsing vortex bundle.

The work was supported by grant 15-02-05366 from RFBR (Russian Foundation of Fundamental Research)

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