arXiv subjects

Sergio Chibbaro

Publications and source records attributed to Sergio Chibbaro.

At least 37 records · Page 2Linked to original sources

arXiv2021

Three-dimensional turbulence effects on plankton dynamics behind an obstacle

We study a predator-prey model of plankton dynamics in the two and three-dimensional wakes of turbulent flows behind a cylinder, focusing on the impact of the three-dimensional character of the carrying velocity field on population variance spectra and spatial distributions. By means of direct numerical simulations, we find that the qualitative behavior of the biological dynamics is mostly independent of the space dimensionality, which suggests that only the relation between the typical flow and biological timescales is crucial to observe persistent blooms. Similarly, in both cases, we find that the spectral properties of the planktonic populations are essentially indistinguishable from those of an inert tracer. The main difference arising from the comparison of the two and three-dimensional configurations concerns the local spatial distribution of plankton density fields. Indeed, the three-dimensional turbulent dynamics tend to destroy the localized coherent structures characterizing the two-dimensional flow, in which the planktonic species are mostly concentrated, thus reducing the phytoplankton biomass in the system.

physics.flu-dyn↗

arXiv2021

Modulation of homogeneous and isotropic turbulence in emulsions

We present a numerical study of emulsions in homogeneous and isotropic turbulence at $Re_λ=137$. The problem is addressed via Direct Numerical Simulations (DNS), where the Volume of Fluid (VOF) is used to represent the complex features of the liquid-liquid interface. We consider a mixture of two iso-density fluids, where fluid properties are varied with the goal of understanding their role in turbulence modulation, in particular the volume fraction ($0.03<α<0.5$), viscosity ratio ($0.01<μ_d/μ_c<100$) and large scale Weber number ($10.6<We_\mathcal{L}<106.5$). The analysis, performed by studying integral quantities and spectral scale-by-scale analysis, reveals that energy is consistently transported from large to small scales by the interface, and no inverse cascade is observed. Furthermore, the total surface is found to be directly proportional to the amount of energy transported, while viscosity and surface tension alter the dynamic that regulates energy transport. We also observe the $-10/3$ and $-3/2$ scaling on droplet size distributions, suggesting that the dimensional arguments which led to their derivation are verified in HIT conditions.

physics.flu-dyn↗

arXiv2021

Predator-prey plankton dynamics in turbulent wakes behind islands

Plankton constitutes the productive base of aquatic ecosystems and plays an essential role in the global carbon cycle. The impact of hydrodynamic conditions on the biological activity of plankton species can manifest in a variety of different ways and the understanding of the basic effects due to multiscale complex flows still appears incomplete. Here, we consider a predator-prey model of plankton dynamics in the presence of a turbulent flow past an idealized island, to investigate the conditions under which an algal bloom is observed, and the resulting patchiness of plankton distributions. By means of direct numerical simulations, we explore the role of the turbulent intensity and of the island shape. In particular, we focus on population variance spectra, and on their relation with the statistical features of the turbulent flow, as well as on the correlation between the spatial distribution of the planktonic species and velocity field persistent structures. We find that both the average biomass and local plankton dynamics critically depend on the relation between advective and biological time scales, confirming and extending previous results obtained in simpler flow settings. We further provide evidence that, in the present system, due to local flow action plankton accumulates in localized filamentary regions. Small turbulent scales are found to impact the statistics of plankton density fields at very fine scales, but the main global features of the population dynamics only weakly depend on the Reynolds number and are also found to be remarkably independent of the geometrical details of the obstacle.

physics.flu-dyn↗

arXiv2020

Constrained Reversible system for Navier-Stokes Turbulence: evidence for Gallavotti's equivalence conjecture

Following the Gallavotti's conjecture, Stationary states of Navier-Stokes fluids are proposed to be described equivalently by alternative equations besides the NS equation itself. We propose a model system symmetric under time-reversal based on the Navier-Stokes equations constrained to keep the Enstrophy constant. It is demonstrated through high-resolved numerical experiments that the reversible model evolves to a stationary state which reproduces quite accurately all statistical observables relevant for the physics of turbulence extracted by direct numerical simulations at different Reynolds numbers. The possibility of using reversible models to mimic turbulence dynamics is of practical importance for coarse-grained version of Navier-Stokes equations, as used in Large-eddy simulations. Furthermore, the reversible model appears mathematically simpler, since enstrophy is bounded to be constant for every Reynolds. Finally, the theoretically interest in the context of statistical mechanics is briefly discussed.

physics.flu-dyn↗

arXiv2020

Direct Numerical Simulation of bubble-induced turbulence at high Reynolds numbers

We report on a investigation of turbulent bubbly flows. Bubbles of a size larger than the dissipative scale, cannot be treated as point-wise inclusions, and generate important hydrodynamic fields in the carrier fluid when in motion. Furthermore, when the volume fraction of bubbles is large enough, the bubble motion may induce a collective agitation due to hydrodynamic interactions which display some turbulent-like features. We tackle this complex phenomenon numerically, performing direct numerical simulations (DNS) with a Volume-of-fluid (VOF) method. In the first part of the work, we perform both 2D and 3D tests in order to determine appropriate numerical and physical parameters. We then carry out a highly-resolved simulation of a 3D bubble column, with a configuration and physical parameters similar to those used in laboratory experiments. This is the largest simulation attempted for such a configuration and is possible only thanks to adaptive grid refinement. Results are compared both with experiments and previous coarse-mesh numerical simulations. In particular, the one-point Probability Density Function (PDF) of the liquid velocity fluctuations is in good quantitative agreement with experiments, notably in the vertical direction, although more extreme events are sampled in the present configuration. The spectra of the liquid kinetic energy show a clear $k^{-3}$ scaling. The mechanisms underlying the energy transfer and notably the possible presence of a cascade are unveiled by a local scale-by-scale analysis in the physical space. The comparison with previous simulations indicate to what extent simulations not fully resolved may yet give correct results, from a statistical point of view.

physics.flu-dyn↗

arXiv2020

Irreversibility and Chaos in Active Particle Suspensions

Active matter has been the object of huge amount of research in recent years for its important fundamental and applicative properties. In this paper we investigate active suspensions of micro-swimmers through direct numerical simulation, so that no approximation is made at the continuous level other than the numerical one. We consider both pusher and puller organisms, with a spherical or ellipsoidal shape. We analyse the velocity and the characteristic scales for an homogeneous two-dimensional suspension and the effective viscosity under shear. We bring evidences that the complex features displayed are related to a spontaneous breaking of the time-reversal symmetry. We show that chaos is not a key ingredient, whereas a large enough number of interacting particles and a non-spherical shape are needed to break the symmetry and are therefore at the basis of the phenomenology. Our numerical study also shows that pullers display some collective motion, though with different characteristics from pushers.

cond-mat.soft↗

arXiv2020

On the local energy flux of turbulent flows

We investigate the local energy flux rate $Π_\ell(\bf x)$ towards small scales in isotropic turbulent flows using direct numerical simulations and applying different low-pass filters. Two different filters are examined, a sharp Fourier filter and a Gaussian filter. The probability density function (pdf) of the local energy flux is calculated for the different filters and for different filtering scales. It is shown that the local energy flux is a largely fluctuating quantity taking both negative and positive values and this is more pronounced for the sharp filter. The variance, the skewness and the kurtosis of these fluctuations are shown to increase as the filtering scale is decreased. Furthermore we calculate the joint pdf of $Π_\ell(\bf x)$ with the local filtered strain rate $S_\ell$ and the enstrophy $Ω_\ell$. The flux shows a good correlation with the strain but not with the enstrophy. It is shown that its conditional mean value scales like $<{Π_\ell}>_S \propto \ell^2 S_\ell^{3} $ in support to the Smagorinsky eddy viscosity model. Nonetheless strong fluctuations exist around this value that also need to be modeled. We discuss the implications of our results for subgrid scale models, and propose new modelling directions.

physics.flu-dyn↗

arXiv2019

Lagrangian stochastic modeling of acceleration in turbulent wall-bounded flows

The Lagrangian approach is natural to study issues of turbulent dispersion and mixing. We propose in this work a general Lagrangian stochastic model including velocity and acceleration as dynamical variables for inhomogeneous turbulent flows. The model takes the form of a diffusion process and the coefficients of the model are determined via Kolmogorov theory and the requirement of consistency with the velocity-based models. It is shown that the present model generalises both the acceleration-based models for homogeneous flows and the generalised Langevin models for the velocity. The resulting closed model is applied to a channel flow at high Reynolds number and compared to experiments as well as direct numerical simulations. A hybrid approach coupling the stochastic model with a Reynolds-Averaged-Navier-Stokes (RANS) is used to obtain a self-consistent model, as commonly used in probability density function methods. Results highlight that most of the acceleration features are well represented, notably the anisotropy and the strong intermittency. These results are valuable, since the model allows to improve the modelling of boundary layers yet remaining relatively simple. It sheds also some light on the statistical mechanisms at play in the near-wall region.

physics.flu-dyn↗

arXiv2018

Universal route to thermalization in weakly-nonlinear one-dimensional chains

We apply Wave Turbulence theory to describe the dynamics on nonlinear one-dimensional chains. We consider $α$ and $β$ Fermi-Pasta-Ulam-Tsingou (FPUT) systems, and the discrete nonlinear Klein-Gordon chain. We demonstrate that resonances are responsible for the irreversible transfer of energy among the Fourier modes. We predict that all the systems thermalize for large times, and that the equipartition time scales as a power-law of the strength of the nonlinearity. Our methodology is not limited to only these systems and can be applied to the case of a finite number of modes, such as in the original FPUT experiment, or to the thermodynamic limit, i.e. when the number of modes approach infinity. In the latter limit, we perform state of the art numerical simulations and show that the results are consistent with theoretical predictions. We suggest that the route to thermalization, based only on the presence of exact resonance, has universal features. Moreover, a by-product of our analysis is the asymptotic integrability, up to four wave interactions, of the discrete nonlinear Klein-Gordon chain.

nlin.CD↗

arXiv2018

A Lagrangian probability-density-function model for collisional turbulent fluid-particle flows. II. Application to homogeneous flows

The Lagrangian probability-density-function model, proposed in Part I for dense particle-laden turbulent flows, is validated here against Eulerian-Lagrangian direct numerical simulation (EL) data for different homogeneous flows, namely statistically steady and decaying homogeneous isotropic turbulence, homogeneous-shear flow and cluster-induced turbulence (CIT). We consider the general model developed in Part I adapted to the homogeneous case together with a simplified version in which the decomposition of the phase-averaged (PA) particle-phase fluctuating energy into the spatially correlated and uncorrelated components is not used, and only total exchange of kinetic energy between phases is allowed. The simplified model employs the standard two-way coupling approach. The comparison between EL simulations and the two stochastic models in homogeneous and isotropic turbulence and in homogeneous-shear flow shows that in all cases both models are capable to reproduce rather well the flow behaviour, notably for dilute flows. The analysis of the CIT gives more insights on the physical nature of such systems and about the quality of the models. Results elucidate the fact that simple two-way coupling is sufficient to induce turbulence, even though the granular energy is not considered. Furthermore, first-order moments including velocity of the fluid seen by particles can be fairly well represented with such a simplified stochastic model. However, the decomposition into spatially correlated and uncorrelated components is found to be necessary to account for anisotropic energy exchanges. When these factors are properly accounted for as in the complete model, the agreement with the EL statistics is satisfactory up to second order.

physics.flu-dyn↗

arXiv2018

A Lagrangian probability-density-function model for collisional turbulent fluid-particle flows. I. Model derivation

Inertial particles in turbulent flows are characterised by preferential concentration and segregation and, at sufficient mass loading, dense particle clusters may spontaneously arise due to momentum coupling between the phases. These clusters, in turn, can generate and sustain turbulence in the fluid phase, which we refer to as cluster-induced turbulence. In the present theoretical work, we tackle the problem of developing a framework for the stochastic modelling of moderately dense particle-laden flows, based on a Lagrangian formalism, which naturally includes the Eulerian one. A rigorous formalism and a general model have been put forward focusing, in particular, on the two ingredients that are key in moderately dense flows, namely, two-way coupling in the carrier phase, and the decomposition of the particle-phase velocity into its spatially correlated and uncorrelated components. Specifically, this last contribution allows to identify in the stochastic model the contributions due to the correlated fluctuating energy and to the granular temperature of the particle phase, which determines the time scale for particle-particle collisions. Applications of the Lagrangian probability-density-function model developed in this work to moderately dense particle-laden flows are discussed in a companion paper.

physics.flu-dyn↗

arXiv2017

4-wave dynamics in kinetic wave turbulence

A general Hamiltonian wave system with quartic resonances is considered, in the standard kinetic limit of a continuum of weakly interacting dispersive waves with random phases. The evolution equation for the multimode characteristic function $Z$ is obtained within an "interaction representation" and a perturbation expansion in the small nonlinearity parameter. A frequency renormalization is performed to remove linear terms that do not appear in the 3-wave case. Feynman-Wyld diagrams are used to average over phases, leading to a first order differential evolution equation for $Z$. A hierarchy of equations, analogous to the Boltzmann hierarchy for low density gases is derived, which preserves in time the property of random phases and amplitudes. This amounts to a general formalism for both the $N$-mode and the 1-mode PDF equations for 4-wave turbulent systems, suitable for numerical simulations and for investigating intermittency.

physics.flu-dyn↗

arXiv2017

Compressibility, laws of nature, initial conditions and complexity

We critically analyse the point of view for which laws of nature are just a mean to compress data. Discussing some basic notions of dynamical systems and information theory, we show that the idea that the analysis of large amount of data by means of an algorithm of compression is equivalent to the knowledge one can have from scientific laws, is rather naive. In particular we discuss the subtle conceptual topic of the initial conditions of phenomena which are generally incompressible. Starting from this point, we argue that laws of nature represent more than a pure compression of data, and that the availability of large amount of data, in general, is not particularly useful to understand the behaviour of complex phenomena.

cond-mat.stat-mech↗

arXiv2017

Wave-Turbulence Theory of four-wave nonlinear interactions

The Sagdeev-Zaslavski (SZ) equation for wave turbulence is analytically derived, both in terms of generating function and of multi-point pdf, for weakly interacting waves with initial random phases. When also initial amplitudes are random, the one-point pdf equation is derived. Such analytical calculations remarkably agree with results obtained in totally different fashions. Numerical investigations of the two-dimensional nonlinear Schroedinger equation (NLSE) and of a vibrating plate prove that: (i) generic Hamiltonian 4-wave systems rapidly attain a random distribution of phases independently of the slower dynamics of the amplitudes, vindicating the hypothesis of initially random phases; (ii) relaxation of the Fourier amplitudes to the predicted stationary distribution (exponential) happens on a faster timescale than relaxation of the spectrum (Rayleigh-Jeans distribution); (iii) the pdf equation correctly describes dynamics under different forcings: the NLSE has an exponential pdf corresponding to a quasi-gaussian solution, like the vibrating plates, that also show some intermittency at very strong forcings.

cond-mat.stat-mech↗

arXiv2016

Weak versus strong wave turbulence in the MMT model

Within the spirit of fluid turbulence, we consider the one-dimensional Majda-McLaughlin-Tabak (MMT) model that describes the interactions of nonlinear dispersive waves. We perform a detailed numerical study of the direct energy cascade in the defocusing regime. In particular, we consider a configuration with large-scale forcing and small scale dissipation, and we introduce three non- dimensional parameters: the ratio between nonlinearity and dispersion, ε, and the analogues of the Reynolds number, Re, i.e. the ratio between the nonlinear and dissipative time-scales, both at large and small scales. Our numerical experiments show that (i) in the limit of small ε the spectral slope observed in the statistical steady regime corresponds to the one predicted by the Weak Wave Turbulence (WWT) theory. (ii) As the nonlinearity is increased, the WWT theory breaks down and deviations from its predictions are observed. (iii) It is shown that such departures from the WWT theoretical predictions are accompanied by the phenomenon of intermittency, typical of three dimensional fluid turbulence. We calculate the structure-function as well as the probability density function of the wave field at each scale and show that the degree of intermittency depends on ε.

physics.flu-dyn↗

arXiv2016

Lagrangian filtered density function for LES-based stochastic modelling of turbulent dispersed flows

The Eulerian-Lagrangian approach based on Large-Eddy Simulation (LES) is one of the most promising and viable numerical tools to study turbulent dispersed flows when the computational cost of Direct Numerical Simulation (DNS) becomes too expensive. The applicability of this approach is however limited if the effects of the Sub-Grid Scales (SGS) of the flow on particle dynamics are neglected. In this paper, we propose to take these effects into account by means of a Lagrangian stochastic SGS model for the equations of particle motion. The model extends to particle-laden flows the velocity-filtered density function method originally developed for reactive flows. The underlying filtered density function is simulated through a Lagrangian Monte Carlo procedure that solves for a set of Stochastic Differential Equations (SDEs) along individual particle trajectories. The resulting model is tested for the reference case of turbulent channel flow, using a hybrid algorithm in which the fluid velocity field is provided by LES and then used to advance the SDEs in time. The model consistency is assessed in the limit of particles with zero inertia, when "duplicate fields" are available from both the Eulerian LES and the Lagrangian tracking. Tests with inertial particles were performed to examine the capability of the model to capture particle preferential concentration and near-wall segregation. Upon comparison with DNS-based statistics, our results show improved accuracy and considerably reduced errors with respect to the case in which no SGS model is used in the equations of particle motion.

physics.flu-dyn↗

arXiv2015

Fluctuations of entropy production in turbulent thermal convection

We report on a numerical experiment performed to analyze fluctuations of the entropy production in turbulent thermal convection, a physical configuration that represents here a prototypical case of an out-of-equilibrium dissipative system. We estimate the entropy production from instantaneous measurements of the local temperature and velocity fields sampled along the trajectory of a large number of point-wise Lagrangian tracers. The entropy production is characterized by large fluctuations and becomes often negative. This represents a sort of "finite-time" violation of the second principle of thermodynamics, since the direction of the energy flux is opposite to that prescribed by the external gradient. We clearly show that the fluctuations of entropy production observed in the present system verify the fluctuation relation (FR), even though the system is time-irreversible.

cond-mat.stat-mech↗

arXiv2015

Elastic wave-turbulence and intermittency

Weak Wave Turbulence is a powerful theory to predict statistical observables of diverse relevant physical phenomena, such as ocean waves, magnetohydrodynamics and nonlinear optics. The theory is based upon an asymptotic closure permitted in the limit of small nonlinearity. Here, we explore the possible deviations from this mean-field framework, in terms of anomalous scaling, focusing on the case of elastic plates. We establish the picture of the possible behaviors at varying the extent of nonlinearity, and we show that the mean-field theory is appropriate when all excited scales remain dominated by linear dynamics. The other picture is non-trivial and our results suggest that, when large scales contain much energy, the cascade sustains extreme events at small scales and the system displays intermittency.

cond-mat.stat-mech↗