arXiv subjects

Vassilios Dallas

Publications and source records attributed to Vassilios Dallas.

24 records · Page 2Linked 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

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↗

arXiv2011

Strong polymer-turbulence interactions in viscoelastic turbulent channel flow

This paper is focused on the fundamental mechanism(s) of viscoelastic turbulence that lead to polymer induced turbulent drag reduction phenomenon. A great challenge in this problem is the computation of viscoelastic turbulent flows, since the understanding of polymer physics is restricted to mechanical models. An effective state-of-the-art numerical method to solve the governing equation for polymers modelled as non-linear springs, without using any artificial assumptions as usual, was implemented here for the first time on a three-dimensional channel flow geometry. The capability of this algorithm to capture the strong polymer-turbulence dynamical interactions is depicted on the results, which are much closer qualitatively to experimental observations. This allowed a more detailed study of the polymer-turbulence interactions, which yields an enhanced picture on a mechanism resulting from the polymer-turbulence energy transfers.

physics.flu-dyn↗

arXiv2011

Rapid growth of cloud droplets by turbulence

Assuming perfect collision efficiency, we demonstrate that turbulence can initiate and sustain the rapid growth of very small water droplets in air even when these droplets are too small to cluster, and even without having to take gravity and small-scale intermittency into account. This is because the range of local Stokes numbers of identical droplets in the turbulent flow field is broad enough even when small-scale intermittency is neglected. This demonstration is given for turbulence which is one order of magnitude less intense than is typical in warm clouds but with a volume fraction which, even though small, is nevertheless large enough for an estimated a priori frequency of collisions to be ten times larger than in warm clouds. However, the time of growth in these conditions turns out to be one order of magnitude smaller than in warm clouds.

physics.flu-dyn↗

arXiv2010

The stagnation point von Kármán coefficient

On the basis of various DNS of turbulent channel flows the following picture is proposed. (i) At a height y from the y = 0 wall, the Taylor microscale λis proportional to the average distance l_s between stagnation points of the fluctuating velocity field, i.e. λ(y) = B_1 l_s(y) with B_1 constant, for δ_ν<< y \lesssim δ. (ii) The number density n_s of stagnation points varies with height according to n_s = C_s y_+^{-1} / δ_ν^3 where C_s is constant in the range δ_ν<< y \lesssim δ. (iii) In that same range, the kinetic energy dissipation rate per unit mass, ε= 2/3 E_+ u_τ^3 / (κ_s y) where E_+ is the total kinetic energy per unit mass normalised by u_τ^2 and κ_s = B_1^2 / C_s is the stagnation point von Kármán coefficient. (iv) In the limit of exceedingly large Re_τ, large enough for the production to balance dissipation locally and for - ~ u_τ^2 in the range δ_ν<< y << δ, dU_+/dy ~ 2/3 E_+/(κ_s y) in that same range. (v) The von Kármán coefficient κis a meaningful and well-defined coefficient and the log-law holds only if E_+ is independent of y_+ and Re_τin that range, in which case κ~ κ_s. The universality of κ_s = B_1^2 / C_s depends on the universality of the stagnation point structure of the turbulence via B_1 and C_s, which are conceivably not universal. (vi) DNS data of turbulent channel flows which include the highest currently available values of Re_τsuggest E_+ ~ 2/3 B_4 y_+^{-2/15} and dU_+/dy_+ ~ B_4/(κ_s) y_+^{-1 - 2/15} with B_4 independent of y in δ_ν<< y << δif the significant departure from - ~ u_τ^2 is taken into account.

physics.flu-dyn↗