arXiv subjects

Guido Boffetta

Publications and source records attributed to Guido Boffetta.

46 records · Page 3Linked to original sources

arXiv2013

Turbulent fluid acceleration generates clusters of gyrotactic microorganisms

The motility of microorganisms is often biased by gradients in physical and chemical properties of their environment, with myriad implications on their ecology. Here we show that fluid acceleration reorients gyrotactic plankton, triggering small-scale clustering. We experimentally demonstrate this phenomenon by studying the distribution of the phytoplankton Chlamydomonas augustae within a rotating tank and find it to be in good agreement with a new, generalized model of gyrotaxis. When this model is implemented in a direct numerical simulation of turbulent flow, we find that fluid acceleration generates multi-fractal plankton clustering, with faster and more stable cells producing stronger clustering. By producing accumulations in high-vorticity regions, this process is fundamen- tally different from clustering by gravitational acceleration, expanding the range of mechanisms by which turbulent flows can impact the spatial distribution of active suspensions.

physics.flu-dyn↗

arXiv2013

Geotropic tracers in turbulent flows: a proxy for fluid acceleration

We investigate the statistics of orientation of small, neutrally buoyant, spherical tracers whose center of mass is displaced from the geometrical center. If appropriate-sized particles are considered, a linear relation can be derived between the horizontal components of the orientation vector and the same components of acceleration. Direct numerical simulations are carried out, showing that such relation can be used to reconstruct the statistics of acceleration fluctuations up to the order of the gravitational acceleration. Based on such results, we suggest a novel method for the local experimental measurement of accelerations in turbulent flows.

physics.flu-dyn↗

arXiv2012

Clustering of gyrotactic microorganisms in turbulent flows

We study the spatial distribution of gyrotactic microorganisms transported by a three-dimensional turbulent flow generated by direct numerical simulations. We find that gyrotaxis combines with turbulent fluctuations to produce small scales (multi-)fractal clustering. We explain this result by showing that gyrotactic swimming cells behave like tracers in a compressible flow. The effective compressibility is derived in the limits of fluid acceleration much larger and smaller than the gravity.

nlin.CD↗

arXiv2009

Modeling Kelvin Wave Cascades in Superfluid Helium

We study two different types of simplified models for Kelvin wave turbulence on quantized vortex lines in superfluids near zero temperature. Our first model is obtained from a truncated expansion of the Local Induction Approximation (Truncated-LIA) and it is shown to possess the same scalings and the essential behaviour as the full Biot-Savart model, being much simpler than the latter and, therefore, more amenable to theoretical and numerical investigations. The Truncated-LIA model supports six-wave interactions and dual cascades, which are clearly demonstrated via the direct numerical simulation of this model in the present paper. In particular, our simulations confirm presence of the weak turbulence regime and the theoretically predicted spectra for the direct energy cascade and the inverse wave action cascade. The second type of model we study, the Differential Approximation Model (DAM), takes a further drastic simplification by assuming locality of interactions in $k$-space via a differential closure that preserves the main scalings of the Kelvin wave dynamics. DAMs are even more amenable to study and they form a useful tool by providing simple analytical solutions in the cases when extra physical effects are present, e.g. forcing by reconnections, friction dissipation and phonon radiation. We study these models numerically and test their theoretical predictions, in particular the formation of the stationary spectra, and the closeness of the numerics for the higher-order DAM to the analytical predictions for the lower-order DAM .

nlin.CD↗

arXiv2006

Lyapunov exponents of heavy particles in turbulence

Lyapunov exponents of heavy particles and tracers advected by homogeneous and isotropic turbulent flows are investigated by means of direct numerical simulations. For large values of the Stokes number, the main effect of inertia is to reduce the chaoticity with respect to fluid tracers. Conversely, for small inertia, a counter-intuitive increase of the first Lyapunov exponent is observed. The flow intermittency is found to induce a Reynolds number dependency for the statistics of the finite time Lyapunov exponents of tracers. Such intermittency effects are found to persist at increasing inertia.

nlin.CD↗

arXiv2004

Lagrangian tracers on a surface flow: the role of time correlations

Finite time correlations of the velocity in a surface flow are found to be important for the formation of clusters of Lagrangian tracers. The degree of clustering characterized by the Lyapunov spectrum of the flow is numerically shown to be in qualitative agreement with the predictions for the white-in-time compressible Kraichnan flow, but to deviate quantitatively. For intermediate values of compressibility the clustering is surprisingly weakened by time correlations.

nlin.CD↗

arXiv2003

Onset and universality of drag reduction in the turbulent Kolmogorov flow

We investigate the phenomenon of drag reduction in a viscoelastic fluid model of dilute polymer solutions. By means of direct numerical simulations of the three-dimensional turbulent Kolmogorov flow we show that drag reduction takes place above a critical Reynolds number $Re_c$. An explicit expression for the dependence of $Re_c$ on polymer elasticity and diffusivity is derived. The values of the drag coefficient obtained for different fluid parameters collapse onto a universal curve when plotted as a function of the rescaled Reynolds number $Re/Re_c$. The analysis of the momentum budget allows to gain some insight on the physics of drag reduction, and suggests the existence of a maximum drag reduction asymptote.

nlin.CD↗

arXiv2002

Diffusive transport and self-consistent dynamics in coupled maps

The study of diffusion in Hamiltonian systems has been a problem of interest for a number of years. In this paper we explore the influence of self-consistency on the diffusion properties of systems described by coupled symplectic maps. Self-consistency, i.e. the back-influence of the transported quantity on the velocity field of the driving flow, despite of its critical importance, is usually overlooked in the description of realistic systems, for example in plasma physics. We propose a class of self-consistent models consisting of an ensemble of maps globally coupled through a mean field. Depending on the kind of coupling, two different general types of self-consistent maps are considered: maps coupled to the field only through the phase, and fully coupled maps, i.e. through the phase and the amplitude of the external field. The analogies and differences of the diffusion properties of these two kinds of maps are discussed in detail.

nlin.CD↗

arXiv1999

Power Laws in Solar Flares: Self-Organized Criticality or Turbulence?

We study the time evolution of Solar Flares activity by looking at the statistics of quiescent times $τ_{L}$ between successive bursts. The analysis of 20 years of data reveals a power law distribution with exponent $α\simeq 2.4$ which is an indication of complex dynamics with long correlation times. The observed scaling behavior is in contradiction with the Self-Organized Criticality models of Solar Flares which predict Poisson-like statistics. Chaotic models, including the destabilization of the laminar phases and subsequent restabilization due to nonlinear dynamics, are able to reproduce the power law for the quiescent times. In the case of the more realistic Shell Model of MHD turbulence we are able to reproduce all the observed distributions.

chao-dyn↗

arXiv1995

Strong Chaos without Butterfly Effect in Dynamical Systems with Feedback

We discuss the predictability of a conservative system that drives a chaotic system with positive maximum Lyapunov exponent $λ_0$, such as the erratic motion of an asteroid in the gravitational field of two bodies of much larger mass. We consider the case where in absence of feedback (restricted model), the driving system is regular and completely predictable. A small feedback of strength $ε$ still allows a good forecasting in the driving system up to a very long time $T_p \sim ε^{-α}$, where $α$ depends on the details of the system. The most interesting situation happens when the Lyapunov exponent of the total system is strongly chaotic with $λ_{tot} \approx λ_0$ , practically independent of $ε$. Therefore an exponential amplification of a small incertitude on the initial conditions in the driving system for any $ε\neq 0$ coexists with very long predictability times. The paradox stems from saturation effects in the evolution for the growth of the incertitude as illustrated in a simple model of coupled maps and in a system of three point vortices in a disk.

chao-dyn↗