arXiv subjects

Alexandros Alexakis

Publications and source records attributed to Alexandros Alexakis.

At least 73 records · Page 4Linked to original sources

arXiv2013

Symmetry breaking and universality of decaying MHD Taylor-Green flows

We investigate the evolution and stability of a decaying magnetohydrodynamic (MHD) Taylor-Green flow. The chosen flow has been shown to result in a steep total energy spectrum with power law behaviour $k^{-2}$. We investigate the symmetry breaking of this flow by exciting perturbations of different amplitudes. It is shown that for any finite amplitude perturbation there is a high enough Reynolds number for which the perturbation will grow enough at the peak of dissipation rate resulting to a non-linear feedback in the flow and subsequently break the Taylor-Green symmetries. In particular, we show that symmetry breaking at large scales occurs if the amplitude of the perturbation is $ρ_{crit} \sim Re^{-1}$ and at small scales occurs if $ρ_{crit} \sim Re^{-3/2}$. This symmetry breaking modifies the scaling laws of the energy spectra at the peak of dissipation rate away from the $k^{-2}$ scaling and towards the classical $k^{-5/3}$ and $k^{-3/2}$ power laws.

physics.flu-dyn↗

arXiv2013

Transition from wave turbulence to dynamical crumpling in vibrated elastic plates

We study the dynamical regime of wave turbulence of a vibrated thin elastic plate based on experimental and numerical observations. We focus our study to the strongly non linear regime described in a previous letter by N. Yokoyama & M. Takaoka. At small forcing, a weakly non linear regime is compatible with the Weak Turbulence Theory when the dissipation is localized at high wavenumber. When the forcing intensity is increased, a strongly non linear regime emerges: singular structures dominate the dynamics at large scale whereas at small scales the weak turbulence is still present. A turbulence of singular structures, with folds and D-cones, develops that alters significantly the energy spectra and causes the emergence of intermittency.

nlin.CD↗

arXiv2013

The origins of $k^{-2}$ spectrum in the decaying Taylor-Green magnetohydrodynamic turbulent flows

We investigate the origins of $k^{-2}$ spectrum in a decaying Taylor-Green magnetohydrodynamic flow with zero large scale magnetic flux that was reported in Lee et al. (2010). A possible candidate for this scaling exponent has been the weak turbulence phenomenology. From our numerical simulations, we observe that current sheets in the magnetic Taylor-Green flow are formed in regions of magnetic discontinuities. Based on this observation and by studying the influence of the current sheets on the energy spectrum, using a filtering technique, we argue that the discontinuities are responsible for the $-2$ power law scaling of the energy spectra of this flow.

physics.flu-dyn↗

arXiv2013

Structures and dynamics of small scales in decaying magnetohydrodynamic turbulence

The topological and dynamical features of small scales are studied in the context of decaying magnetohydrodynamic turbulent flows using direct numerical simulations. Joint probability density functions (PDFs) of the invariants of gradient quantities related to the velocity and the magnetic fields demonstrate that structures and dynamics at the time of maximum dissipation depend on the large scale initial conditions. This is evident in particular from the fact that each flow has a different shape for the joint PDF of the invariants of the velocity gradient in contrast to the universal teardrop shape of hydrodynamic turbulence. The general picture that emerges from the analysis of the invariants is that regions of high vorticity are correlated with regions of high strain rate $\bm S$ also in contrast to hydrodynamic turbulent flows. Magnetic strain dominated regions are also well correlated with region of high current density $\bm j$. Viscous dissipation ($\propto \bm S^2$) as well as Ohmic dissipation ($\propto \bm j^2$) reside in regions where strain and rotation are locally almost in balance. The structures related to the velocity gradient possess different characteristics than those associated with the magnetic field gradient with the latter being locally more quasi-two dimensional.

physics.flu-dyn↗

arXiv2012

From Weakly to Strongly Magnetized Isotropic MHD Turbulence

High Reynolds number isotropic magneto-hydro-dynamic turbulence in the presence of large scale magnetic fields is investigated as a function of the magnetic field strength. For a variety of flow configurations the energy dissipation rate ε, follows the Kolmogorov scaling ε~ U^3/L even when the large scale magnetic field energy is twenty times larger than the kinetic. Further increase of the magnetic energy showed a transition to the ε~ U^2 B / L scaling implying that magnetic shear becomes more efficient at this point at cascading the energy than the velocity fluctuations. Strongly helical configurations form helicity condensates that deviate from these scalings. Weak turbulence scaling was absent from the investigation. Finally, the magnetic energy spectra showed support for the Kolmogorov spectrum k^{-5/3} while kinetic energy spectra are closer to the Iroshnikov-Kraichnan spectrum k^{-3/2}.

physics.flu-dyn↗

arXiv2012

Critical exponents in zero dimensions

In the vicinity of the onset of an instability, we investigate the effect of colored multiplicative noise on the scaling of the moments of the unstable mode amplitude. We introduce a family of zero dimensional models for which we can calculate the exact value of the critical exponents $β_m$ for all the moments. The results are obtained through asymptotic expansions that use the distance to onset as a small parameter. The examined family displays a variety of behaviors of the critical exponents that includes anomalous exponents: exponents that differ from the deterministic (mean-field) prediction, and multiscaling: non-linear dependence of the exponents on the order of the moment.

cond-mat.stat-mech↗

arXiv2011

Bounding the scalar dissipation scale for mixing flows in the presence of sources

We investigate the mixing properties of scalars stirred by spatially smooth, divergence-free flows and maintained by a steady source-sink distribution. We focus on the spatial variation of the scalar field, described by the {\it dissipation wavenumber}, $k_d$, that we define as a function of the mean variance of the scalar and its gradient. We derive a set of upper bounds that for large Péclet number ($\Pe\gg1$) yield four distinct regimes for the scaling behavior of $k_d$, one of which corresponds to the Batchelor regime. The transition between these regimes is controlled by the value of $\Pe$ and the ratio $ρ=\ell_u/\ell_s$, where $\ell_u$ and $\ell_s$ are respectively, the characteristic lengthscales of the velocity and source fields. A fifth regime is revealed by homogenization theory. These regimes reflect the balance between different processes: scalar injection, molecular diffusion, stirring and bulk transport from the sources to the sinks. We verify the relevance of these bounds by numerical simulations for a {two-dimensional, chaotically mixing} example flow and discuss their relation to previous bounds. Finally, we note some implications for three dimensional turbulent flows.

physics.flu-dyn↗

arXiv2011

On the 2D behavior of 3D MHD with a strong guiding field

The Magneto-hydrodynamic (MHD) equations in the presence of a guiding magnetic field are investigated by means of direct numerical simulations. The basis of the investigation consists of 9 runs forced at the small scales. The results demonstrate that for a large enough uniform magnetic field the large scale flow behaves as a two dimensional (non-MHD) fluid exhibiting an inverse cascade of energy in the direction perpendicular to the magnetic field, while the small scales behave like a three dimensional MHD-fluid cascading the energy forwards. The amplitude of the inverse cascade is sensitive to the magnetic field amplitude, the domain size, the forcing mechanism, and the forcing scale. All these dependencies are demonstrated by the varying parameters of simulations. Furthermore, in the case that the system is forced anisotropically in the small parallel scales an inverse cascade in the parallel direction is observed that is feeding the 2D modes.

physics.flu-dyn↗

arXiv2011

Searching for the fastest dynamo: Laminar ABC flows

The growth rate of the dynamo instability as a function of the magnetic Reynolds number Rm is investigated by means of numerical simulations for the family of the ABC flows and for 2 different forcing scales. For the ABC flows that are driven at the largest available length scale it is found that as the magnetic Reynolds number is increased: (a) The flow that results first in dynamo is the 2.5D flow for which A=B and C=0 (and all permutations). (b) The second type of flow that results in dynamo is the one for which A=B=2C/5 (and permutations). (c) The most symmetric flow A=B=C is the third type of flow that results in dynamo. (d) As Rm is increased, the A=B=C flow stops being a dynamo and transitions from a local maximum to a third-order saddle point. (e) At larger Rm the A=B=C flow re-establishes its self as a dynamo but remains a saddle point. (f) At the largest examined Rm the growth rate of the 2.5D flows starts to decrease, the A=B=C flow comes close to a local maximum again and the flow A=B=2C/5 (and permutations) results in the fastest dynamo with growth rate $γ~0.12$ at the largest examined Rm. For the ABC flows that are driven at the second largest available length scale it is found that (a) the 2.5D flows A=B, C=0 (and permutations) are again the first flows that result in dynamo with a decreased onset. (b) The most symmetric flow A=B=C is the second type of flow that results in dynamo. It is and remains a local maximum. (c) At larger Rm the flow A=B=2C/5 (and permutations) appears as the third type of flow that results in dynamo. As Rm is increased it becomes the flow with the largest growth rate. The growth rates appear to have some correlation with the Lyaponov exponents but constructive re-folding of the field lines appears equally important in determining the fasted dynamo flow.

physics.flu-dyn↗

arXiv2010

Nonlinear dynamos at infinite magnetic Prandtl number

The dynamo instability is investigated in the limit of infinite magnetic Prandtl number. In this limit the fluid is assumed to be very viscous so that the inertial terms can be neglected and the flow is slaved to the forcing. The forcing consist of an external forcing function that drives the dynamo flow and the resulting Lorentz force caused by the back reaction of the magnetic field. The flows under investigation are the Archontis flow, and the ABC flow forced at two different scales. The investigation covers roughly three orders of magnitude of the magnetic Reynolds number above onset. All flows show a weak increase of the averaged magnetic energy as the magnetic Reynolds number is increased. Most of the magnetic energy is concentrated in flat elongated structures that produce a Lorentz force with small solenoidal projection so that the resulting magnetic field configuration was almost force-free. Although the examined system has zero kinetic Reynolds number at sufficiently large magnetic Reynolds number the structures are unstable to small scale fluctuations that result in a chaotic temporal behavior.

physics.flu-dyn↗

arXiv2009

Stratified shear flow instabilities at large Richardson numbers

Numerical simulations of stratified shear flow instabilities are performed in two dimensions in the Boussinesq limit. The density variation length scale is chosen to be four times smaller than the velocity variation length scale so that Holmboe or Kelvin-Helmholtz unstable modes are present depending on the choice of the global Richardson number Ri. Three different values of Ri were examined Ri =0.2, 2, 20. The flows for the three examined values are all unstable due to different modes namely: the Kelvin-Helmholtz mode for Ri=0.2, the first Holmboe mode for Ri=2, and the second Holmboe mode for Ri=20 that has been discovered recently and it is the first time that it is examined in the non-linear stage. It is found that the amplitude of the velocity perturbation of the second Holmboe mode at the non-linear stage is smaller but comparable to first Holmboe mode. The increase of the potential energy however due to the second Holmboe modes is greater than that of the first mode. The Kelvin-Helmholtz mode is larger by two orders of magnitude in kinetic energy than the Holmboe modes and about ten times larger in potential energy than the Holmboe modes. The results in this paper suggest that although mixing is suppressed at large Richardson numbers it is not negligible, and turbulent mixing processes in strongly stratified environments can not be excluded.

physics.flu-dyn↗

arXiv2007

The Lorentz force effect on the On-Off dynamo intermittency

An investigation of the dynamo instability close to the threshold produced by an ABC forced flow is presented. We focus on the on-off intermittency behavior of the dynamo and the counter-effect of the Lorentz force in the non-linear stage of the dynamo. The Lorentz force drastically alters the statistics of the turbulent fluctuations of the flow and reduces their amplitude. As a result much longer burst (on-phases) are observed than what is expected based on the amplitude of the fluctuations in the kinematic regime of the dynamo. For large Reynolds numbers, the duration time of the ``On'' phase follows a power law distribution, while for smaller Reynolds numbers the Lorentz force completely kills the noise and the system transits from a chaotic state into a ``laminar'' time periodic flow. The behavior of the On-Off intermittency as the Reynolds number is increased is also examined. The connections with dynamo experiments and theoretical modeling are discussed.

physics.flu-dyn↗

arXiv2007

Nonlocal Phenomenology for anisotropic MHD turbulence

A non-local cascade model for anisotropic MHD turbulence in the presence of a guiding magnetic field is proposed. The model takes into account that (a) energy cascades in an anisotropic manner and as a result a different estimate for the cascade rate in the direction parallel and perpendicular to the guiding field is made. (b) the interactions that result in the cascade are between different scales. Eddies with wave numbers $k_\|$ and $k_\perp$ interact with eddies with wave numbers $q_\|,q_\perp$ such that a resonance condition between the wave numbers $q_\|,q_\perp$ and $k_\|,k_\perp$ holds. As a consequence energy from the eddy with wave numbers $k_\|$ and $k_\perp$ cascades due to interactions with eddies located in the resonant manifold whose wavenumbers are determined by: $q_\|\simeq ε^{{1}/{3}}k_\perp^{2/3}/B$, $q_\perp=k_\perp$ and energy will cascade along the lines $k_\|\sim C+k_\perp^{2/3} ε^{1/3}/B_0$. For a uniform energy injection rate in the parallel direction the resulting energy spectrum is $E(k_\|,k_\perp)\simeq ε^{2/3}k_\|^{-1}k_\perp^{-5/3}$. For a general forcing however the model suggests a non-universal behavior. The connections with previous models, numerical simulations and weak turbulence theory are discussed.

physics.flu-dyn↗

arXiv2006

Marginally unstable Holmboe modes

Marginally unstable Holmboe modes for smooth density and velocity profiles are studied. For a large family of flows and stratification that exhibit Holmboe instability, we show that the modes with phase velocity equal to the maximum or the minimum velocity of the shear are marginally unstable. This allows us to determine the critical value of the control parameter R (expressing the ratio of the velocity variation length scale to the density variation length scale) that Holmboe instability appears R=2. We then examine systems for which the parameter R is very close to this critical value. For this case we derive an analytical expression for the dispersion relation of the complex phase speed c(k) in the unstable region. The growth rate and the width of the region of unstable wave numbers has a very strong (exponential) dependence on the deviation of R from the critical value. Two specific examples are examined and the implications of the results are discussed.

physics.flu-dyn↗

arXiv2006

Energy and enstrophy dissipation in steady state 2-d turbulence

Upper bounds on the bulk energy dissipation rate $ε$ and enstrophy dissipation rate $χ$ are derived for the statistical steady state of body forced two dimensional turbulence in a periodic domain. For a broad class of externally imposed body forces it is shown that $ε\le k_{f} U^3 Re^{-1/2}(C_1+C_2 Re^{-1})^{1/2}$ and $χ\le k_{f}^{3}U^3 (C_1+C_2 Re^{-1})$ where $U$ is the root-mean-square velocity, $k_f$ is a wavenumber (inverse length scale) related with the forcing function, and $Re = U /νk_f$. The positive coefficients $C_1$ and $C_2$ are uniform in the the kinematic viscosity $ν$, the amplitude of the driving force, and the system size. We compare these results with previously obtained bounds for body forces involving only a single length scale, or for velocity dependent a constant-energy-flux forces acting at finite wavenumbers. Implications of our results are discussed.

physics.flu-dyn↗

arXiv2005

On the inverse cascade of magnetic helicity

We study the inverse cascade of magnetic helicity in conducting fluids by investigating the detailed transfer of helicity between different spherical shells in Fourier space in direct numerical simulations of three-dimensional magnetohydrodynamics (MHD). Two different numerical simulations are used, one where the system is forced with an electromotive force in the induction equation, and one in which the system is forced mechanically with an ABC flow and the magnetic field is solely sustained by a dynamo action. The magnetic helicity cascade at the initial stages of both simulations is observed to be inverse and local (in scale space) in the large scales, and direct and local in the small scales. When saturation is approached most of the helicity is concentrated in the large scales and the cascade is non-local. Helicity is transfered directly from the forced scales to the largest scales. At the same time, a smaller in amplitude direct cascade is observed from the largest scale to small scales.

physics.plasm-ph↗

arXiv2005

On Holmboe's instability for smooth shear and density profiles

The linear stability of a stratified shear flow for smooth density profiles is studied. This work focuses on the nature of the stability boundaries of flows in which both Kelvin-Helmholtz and Holmboe instabilities are present. For a fixed Richardson number the unstable modes are confined to finite bands between a smallest and a largest marginally unstable wavenumber. The results in this paper indicate that the stability boundary for small wavenumbers is comprised of neutral modes with phase velocity equal to the maximum/minimum wind velocity whereas the other stability boundary, for large wavenumbers, is comprised of singular neutral modes with phase velocity in the range of the velocity shear. We show how these stability boundaries can be evaluated without solving for the growth rate over the entire parameter space as was previously done. The results indicate further that there is a new instability domain that has not been previously noted in the literature. The unstable modes, in this new instability domain, appear for larger values of the Richardson number and are related to the higher harmonics of the internal gravity wave spectrum.

physics.flu-dyn↗

arXiv2005

Shell to shell energy transfer in MHD, Part I: steady state turbulence

We investigate the transfer of energy from large scales to small scales in fully developed forced three-dimensional MHD-turbulence by analyzing the results of direct numerical simulations in the absence of an externally imposed uniform magnetic field. Our results show that the transfer of kinetic energy from the large scales to kinetic energy at smaller scales, and the transfer of magnetic energy from the large scales to magnetic energy at smaller scales, are local, as is also found in the case of neutral fluids, and in a way that is compatible with Kolmogorov (1941) theory of turbulence. However, the transfer of energy from the velocity field to the magnetic field is a highly non-local process in Fourier space. Energy from the velocity field at large scales can be transfered directly into small scale magnetic fields without the participation of intermediate scales. Some implications of our results to MHD turbulence modeling are also discussed.

physics.flu-dyn↗