arXiv subjects

Sergey S. Vergeles

Publications and source records attributed to Sergey S. Vergeles.

11 recordsLinked to original sources

arXiv2026

Maintenance of a columnar vortex by inertial waves in rotational turbulence

We develop a theory that determines the radial profile of the mean velocity in a coherent geostrophic vortex forming in a turbulent rapidly rotating fluid. Following the conditions of our experiments, we assume that the flow in the vortex is sustained by the absorption of short-wavelength inertial waves arriving from the vortex periphery. The nonlinear interaction between waves is assumed negligible in the theory. Comparison with experimental data supports the validity of the developed model.

physics.flu-dyn↗

arXiv2025

Absorption and reflection of inertial waves by a geostrophic vortex

We study interaction of inertial waves with geostrophic flow in a rapidly rotating fluid system. In accordance with experimental conditions in [1, 2], we consider inertial waves, which were excited by a source being near side boundary of the flow and enter the region where geostrophic vortex flow is present. The wave equation is derived and analyzed in the paper that describes the propagation of convergent and divergent cylindrical waves on the background of mean vortex flow, which is considered as an axisymmetric differential rotation. We show that a monochromatic wave does not exert any torque on the vortex flow in the inviscid limit until it is absorbed inside the critical layer. Among convergent waves those only are absorbed which carries angular momentum of the same sign as one's of the rotation in the vortex. Convergent waves with the opposite sign of angular momentum are just reflected from the vortex. The absorption of a wave is possible only if the vortex flow is characterized by fast enough angular velocity there. The behavior of the wave near the critical layer can be described by the well-known model where the mean flow is rectilinear shear flow. We show that the conventional wave train approximation for the short-wave limit is not applicable in the vicinity of the layer and revise it, deriving the proper equation and reformulating the conservation law of wave action. For the vicinity of critical layer, a model which accounts for the viscous dissipation is derived; viscous effects are studied for absorption both of monochromatic wave and wave train.

physics.flu-dyn↗

arXiv2023

Alpha-effect in three-dimensional coherent vortex of conducting rotating liquid

We study one-point statistics of spiral turbulent pulsations on the background of three-dimensional large-scale vortex in a rotating fluid. Assuming that the helical flow is created by a statistically axially symmetric random force with broken mirror symmetry, we analytically calculate velocity-vorticity mean including its magnitude and the anisotropy. For electrically conducting liquid, we examine the $α$-effect in the system. The found elements of $α$-matrix strongly depend on relation between Rossby $\mathrm{Ro}$ and magnetic Prandtl $\mathrm{Pr}_{\mathrm{m}}$ numbers in considered region $\mathrm{Ro}\lesssim 1$, $\mathrm{Pr}_{\mathrm{m}} \lesssim 1$. We establish criterion for the numbers when the $α$-effect leads to instability of large-scale magnetic field

physics.flu-dyn↗

arXiv2023

Mixing in two-dimensional shear flow with smooth fluctuations

Chaotic variations in flow speed up mixing of scalar fields via intensified stirring. This paper addresses the statistical properties of a passive scalar field mixing in a regular shear flow with random fluctuations against its background. We consider two-dimensional flow with shear component dominating over smooth fluctuations. Such flow is supposed to model passive scalar mixing e.g. inside a large-scale coherent vortex forming in two-dimensional turbulence or in elastic turbulence in a micro-channel. We examine both the decaying case and the case of the continuous forcing of the scalar variances. In both cases dynamics possesses strong intermittency, that can be characterized via the single-point moments and correlation functions calculated in our work. We present general qualitative properties of pair correlation function as well as certain quantitative results obtained in the framework of the model with fluctuations that are short correlated in time.

physics.flu-dyn↗

arXiv2021

Velocity structure function in a geostrophic coherent vortex under strong rotation

We consider analytically pair structure function of turbulent pulsations on the background of a coherent geostrophic vortex in a fast rotating fluid. The statistics of the turbulent pulsation is determined by their dynamics which is the dynamics of inertia waves affected by the differential rotation in the vortex and weak viscous damping. Our consideration is restricted by the smallest scales, where the velocity field remains smooth. We establish the anisotropy of the structure function. The velocity gradient of the turbulent pulsations achieves its largest value for the radial direction and its smallest value in the streamwise direction, resembling its behaviour in turbulent flow with mean shear component without imposed rotation.

physics.flu-dyn↗

arXiv2021

Velocity profiles of cyclones and anticyclones in a rotating turbulent flow

Strong rotation makes an underlying turbulent flow quasi-two-dimensional that leads to the upscale energy transfer. Recent numerical simulations show that under certain conditions, the energy is accumulated at the largest scales of the system, forming coherent vortex structures known as condensates. We analytically describe the interaction of a strong condensate with weak small-scale turbulent pulsations and obtain an equation that allows us to determine the radial velocity profile $U(r)$ of a coherent vortex. When external rotation is fast, the velocity profiles of cyclones and anticyclones are identical to each other and are well described by the dependence $U(r) \propto \pm r \ln (R/r)$, where $R$ is the transverse size of the vortex. As the external rotation decreases, this symmetry disappears: the maximum velocity in cyclones is greater and the position of the maximum is closer to the axis of the vortex in comparison with anticyclones. Besides, our analysis shows that the size $R$ of the anticyclone cannot exceed a certain critical value, which depends on the Rossby and Reynolds numbers. The maximum size of the cyclones is limited only by the system size under the same conditions. Our predictions are based on the linear evolution of turbulent pulsations on the background of the coherent vortex flow and are accompanied by estimates following from the nonlinear Navier-Stokes equation.

physics.flu-dyn↗

arXiv2020

Virtual wave stress in deep-water crossed surface waves

Waves excited on the surface of deep water decay in time and/or space due to the fluid viscosity, and the momentum associated with the wave motion is transferred from the waves to Eulerian slow currents by the action of the virtual wave stress. Here, based on the conservation of the total momentum, we found the virtual wave stress produced by calm gravity waves under the assumption that the slow Eulerian currents are weak in the sense that the Froude number is small and the scattering of surface waves by slow flow inhomogeneities can be neglected. In particular, we calculated the virtual wave stress generated by a propagating wave and two orthogonal standing waves. The obtained results possess Euler invariance, are consistent with previously known ones, and generalize them to the case of the excitation of almost monochromatic waves propagating in arbitrary directions.

physics.flu-dyn↗

arXiv2019

Parametric behavior of diffraction on guided-mode resonant grating of subwavelength thickness

We consider Wood anomalies in diffraction spectrum from two-dimensional dielectric periodic grid embedded in a surrounding media. The grid is of subwavelength thickness, and diffraction of wave having S-polarization is investigated in the vicinity of the emergence of first diffraction maximum. We reduce Maxwell equations to coupled-mode theory with three parameters, which are integral characteristics of material arrangement in the grid. In particular, we show that such grids are capable to have full reflectance in parametrically narrow frequency bandwidth. The effect is accompanied by a parametric evanescent field enhancement in the region near the grid. In particular, we consider grids with sinusoidal profile and show that this type of grids possesses unique diffraction properties due to absence of the guided mode coupling. For such grids, there is a thin transparency window at a background of near to zero transmission at slightly nonnormal incidence. We estimate what are enough grid size and the incident beam flatness to resolve the singularities.

physics.optics↗

arXiv2018

Influence of a thin compressible insoluble liquid film on the eddy currents generated by interacting surface waves

Recently the generation of eddy currents by interacting surface waves was observed experimentally. The phenomenon provides the possibility for manipulation of particles which are immersed in the fluid. The analysis shows that the amplitude of the established eddy currents produced by stationary surface waves does not depend on the fluid viscosity in the free surface case. The currents become parametrically larger being inversely proportional to the square root of the fluid viscosity in the case when the fluid surface is covered by an almost incompressible thin liquid (i.e. shear elasticity is zero) film formed by an insoluble agent with negligible internal viscous losses as compared to the dissipation in the fluid bulk. Here we extend the theory for a thin insoluble film with zero shear elasticity and small shear and dilational viscosities on the case of an arbitrary elastic compression modulus. We find both contributions into the Lagrangian motion of passive tracers, which are the advection by the Eulerian vertical vorticity and the Stokes drift. Whereas the Stokes drift contribution preserves its value for the free surface case outside a thin viscous sublayer, the Eulerian vertical vorticity strongly depends on the fluid viscosity at high values of the film compression modulus. The Stokes drift acquires a strong dependence on the fluid viscosity inside the viscous sublayer, however, the change is compensated by an opposite change in the Eulerian vertical vorticity. As a result, the vertical dependence of the intensity of eddy currents is given by a sum of two decaying exponents with both decrements being of the order of the wave number. The decrements are numerically different, so the Eulerian contribution becomes dominant at some depth for the surface film with any compression modulus.

physics.flu-dyn↗

arXiv2014

Quantum theory of the spaser-based nanolaser

We present a quantum theory of a spaser-based nanolaser, under the bad-cavity approximation. We find first- and second-order correlation functions $g^{(1)} (τ)$ and $g^{(2)}(τ)$ below and above the generation threshold, and obtain the average number of plasmons in the cavity. The latter is shown to be of the order of unity near the generation threshold, where the spectral line narrows considerably. In this case the coherence is preserved in a state of active atoms in contradiction to the good-cavity lasers, where the coherence is preserved in a state of photons. The damped oscillations in $g^{(2)}(τ)$ above the generation threshold indicate the unusual character of amplitude fluctuations of polarization and population, which become interconnected in this case. Obtained results allow to understand the fundamental principles of operation of nanolasers.

cond-mat.mes-hall↗

arXiv2012

Localized surface plasmon modes in a system of two interacting metallic cylinders

We study an optical response of a system of two parallel close metallic cylinders having nanoscale dimensions. Surface plasmon excitation in the gap between the cylinders are specifically analyzed. In particular, resonance frequencies and field enhancement were investigated as functions of geometrical characteristics of the system and Ohmic losses in the metal. The results of numerical simulations were systematically compared with the analytical theory, obtained in the quasi-static limit. The analytical method was generalized in order to take into account the retardation effects. We also present the physical qualitative picture of the plasmon modes, which is validated by numerical simulations and analytical theory.

physics.optics↗