arXiv subjects

O. M. Podvigina

Publications and source records attributed to O. M. Podvigina.

6 recordsLinked to original sources

arXiv2021

Impact of a moon on the evolution of a planet's obliquity: a non-resonant case

We investigate how the variation of the obliquity (the axial tilt) of a hypothetical exo-Earth is effected by the presence of a satellite, an exo-Moon. Namely, we study analytically and numerically how the range of obliquity of the exo-Earth changes if an exo-Moon is added to a system comprised of exo-Sun, exo-Earth and exo-planets. We say that the impact of the exo-Moon is stabilising if upon the addition of the exo-Moon the range of obliquity decreases, while we call the impact destabilising if the range increases as the exo-Moon is added to the system. The problem is considered in a general setup. The exo-Earth is assumed to be rigid, axially symmetric and almost spherical, the difference between the largest and the smallest principal moments of inertia being a small parameter of the problem. Assuming the orbits of the celestial bodies to be quasiperiodic, we apply time averaging to study rotation of the exo-Earth at times large compared to the respective periods. Non-resonant frequencies are assumed. We identify a class of systems for which we prove analytically that the impact of the exo-Moon is stabilising and a class where it is destabilising. We also investigate numerically how the impact of the exo-Moon in a particular system comprised of a star and two planets varies on modifying the geometry of the orbits of the exo-Moon and the second planet and the initial obliquity.

astro-ph.EP↗

arXiv2006

The center manifold theorem for center eigenvalues with non-zero real parts

We define center manifold as usual as an invariant manifold, tangent to the invariant subspace of the linearization of the mapping defining a continuous dynamical system, but the center subspace that we consider is associated with eigenvalues with small but not necessarily zero} real parts. We prove existence and smoothness of such center manifold assuming that certain inequalities between the center eigenvalues and the rest of the spectrum hold. The theorem is valid for finite-dimensional systems, as well as for infinite-dimensional systems provided they satisfy an additional condition. We show that the condition holds for the Navier-Stokes equation subject to appropriate boundary conditions.

physics.flu-dyn↗

arXiv2006

Instability of small-amplitude convective flows in a rotating layer with stress-free boundaries

We consider stability of steady convective flows in a horizontal layer with stress-free boundaries, heated below and rotating about the vertical axis, in the Boussinesq approximation (the Rayleigh-Benard convection). The flows under consideration are convective rolls or square cells, the latter being asymptotically equal to the sum of two orthogonal rolls of the same wave number k. We assume, that the Rayleigh number R is close to the critical one, R_c(k), for the onset of convective flows of this wave number: R=R_c(k)+epsilon^2; the amplitude of the flows is of the order of epsilon. We show that the flows are always unstable to perturbations, which are a sum of a large-scale mode not involving small scales, and two large-scale modes, modulated by the original rolls rotated by equal small angles in the opposite directions. The maximal growth rate of the instability is of the order of max(epsilon^{8/5},(k-k_c)^2), where k_c is the critical wave number for the onset of convection.

physics.flu-dyn↗

arXiv2003

Generation of multiscale magnetic field by parity-invariant time-periodic flows

We study generation of magnetic fields involving large spatial scales by time- and space-periodic small-scale parity-invariant flows. The anisotropic magnetic eddy diffusivity tensor is calculated by the standard procedure involving expansion of magnetic modes and their growth rates in power series in the scale ratio. Our simulations, conducted for flows with random harmonic composition and exponentially decaying energy spectra, demonstrate that enlargement of the spatial scale of magnetic field is beneficial for generation by time-periodic flows. However, they turn out, in general, to be less efficient dynamos, than steady flows.

physics.flu-dyn↗

arXiv2001

Numerical evidence of breaking of vortex lines in an ideal fluid

Emergence of singularity of vorticity at a single point, not related to any symmetry of the initial distribution, has been demonstrated numerically for the first time. Behavior of the maximum of vorticity near the point of collapse closely follows the dependence 1/(t0-t), where t0 is the time of collapse. This agrees with the interpretation of collapse in an ideal incompressible fluid as of the process of vortex lines breaking.

physics.flu-dyn↗

arXiv2001

Dynamo effect in parity-invariant flow with large and moderate separation of scales

It is shown that non-helical (more precisely, parity-invariant) flows capable of sustaining a large-scale dynamo by the negative magnetic eddy diffusivity effect are quite common. This conclusion is based on numerical examination of a large number of randomly selected flows. Few outliers with strongly negative eddy diffusivities are also found, and they are interpreted in terms of the closeness of the control parameter to a critical value for generation of a small-scale magnetic field. Furthermore, it is shown that, for parity-invariant flows, a moderate separation of scales between the basic flow and the magnetic field often significantly reduces the critical magnetic Reynolds number for the onset of dynamo action.

nlin.CD↗