A Lagrangian model of copepod dynamics: Clustering by escape jumps in turbulence

Planktonic copepods are small crustaceans that have the ability to swim by quick powerful jumps. Such an aptness is used to escape from high shear regions, which may be caused either by flow per- turbations, produced by a large predator (i.e., fish larvae), or by the inherent highly turbulent dynamics of the ocean. Through a combined experimental and numerical study, we investigate the impact of jumping behaviour on the small-scale patchiness of copepods in a turbulent environment. Recorded velocity tracks of copepods displaying escape response jumps in still water are here used to define and tune a Lagrangian Copepod (LC) model. The model is further employed to simulate the behaviour of thousands of copepods in a fully developed hydrodynamic turbulent flow obtained by direct numerical simulation of the Navier-Stokes equations. First, we show that the LC velocity statistics is in qualitative agreement with available experimental observations of copepods in tur- bulence. Second, we quantify the clustering of LC, via the fractal dimension $D_2$. We show that $D_2$ can be as low as ~ 2.3 and that it critically depends on the shear-rate sensitivity of the proposed LC model, in particular it exhibits a minimum in a narrow range of shear-rate values. We further investigate the effect of jump intensity, jump orientation and geometrical aspect ratio of the copepods on the small-scale spatial distribution. At last, possible ecological implications of the observed clustering on encounter rates and mating success are discussedComment: 13 pages, 9 figure

Intermittence lagrangienne de scalaires passifs: étude des lois d'échelle multifractales

Nous considérons ici le mélange de scalaires passifs dans un cadre Lagrangien. Nous nous plaçons dans le contexte de la turbulence homogène et isotrope, et considérons la zone inertielle convective. L'approche de Kolmogorov 1941 a été généralisée dans un cadre Lagrangien pour la vitesse par Landau, et pour un scalaire passif par Inoue. L'intermittence Lagrangienne peut être caractérisée en utilisant des exposants d'invariance d'échelle pour les fonctions de structure, pour le champ de vitesse ou de scalaire passif. Dans le cadre de différentes hypothèses théoriques, nous relions ces exposants Lagrangiens de scalaire passif aux exposants Eulériens: exposants pour les scalaires passifs, et exposants joints pour les fonctions de structure. Quatre différentes relations sont obtenues, qui sont comparées entre elles, et comparées également à des données de turbulence marine obtenues à l'aide d'une bouée de petite taille sur laquelle était fixé un capteur miniature

−5/3 Kolmogorov Turbulent Behaviour and Intermittent Sustainable Energies

The massive integration of sustainable energies into electrical grids (non-interconnected or connected) is a major problem due to their stochastic character revealed by strong fluctuations at all scales. In this paper, the scaling behaviour or power law correlations and the nature of scaling behaviour of sustainable resource data such as flow velocity, atmospheric wind speed, solar global solar radiation and sustainable energy such as, wind power output, are highlighted. For the first time, Fourier power spectral densities are estimated for each dataset. We show that the power spectrum densities obtained are close to the 5/3 Kolmogorov spectrum. Furthermore, the multifractal and intermittent properties of sustainable resource and energy data have been revealed by the concavity of the scaling exponent function. The proposed analysis frame allows a full description of fluctuations of processes considered. A good knowledge of the dynamic of fluctuations is crucial to management of the integration of sustainable energies into a grid

Fluctuations and correlations of reactive scalars near chemical equilibrium in incompressible turbulence

The statistical properties of species undergoing chemical reactions in a turbulent environment are studied. We focus on the case of reversible multi-component reactions of second and higher orders, in a condition close to chemical equilibrium sustained by random large-scale reactant sources, while the turbulent flow is highly developed. In such a state a competition exists between the chemical reaction that tends to dump reactant concentration fluctuations and enhance their correlation intensity and the turbulent mixing that on the contrary increases fluctuations and remove relative correlations. We show that a unique control parameter, the Damkh\"{o}ler number ($Da_\theta$) that can be constructed from the scalar Taylor micro-scale, the reactant diffusivity and the reaction rate characterises the functional dependence of fluctuations and correlations in a variety of conditions, i.e., at changing the reaction order, the Reynolds and the Schmidt numbers. The larger is such a Damkh\"{o}ler number the more depleted are the scalar fluctuations as compared to the fluctuations of a passive scalar field in the same conditions, and vice-versa the more intense are the correlations. A saturation in this behaviour is observed beyond $Da_\theta \simeq \mathcal{O}(10)$. We provide an analytical prediction for this phenomenon which is in excellent agreement with direct numerical simulation results.Comment: 17 pages , 9 figure

Development and Environmental Conflicts in China

China has paid little attention to environmental issues over the decades, preferring to give priority to economic development. In this article, we start off with an examination of the acute and catastrophic problems in the environmental domain. We then consider environmental social conflicts these problems have generated, and which have grown exponentially in recent years. We conclude by assessing the Chinese authorities’ approach to these problems

"Pseudodiaptomus Marinus" Sato, 1913, a New Invasive Copepod in Lake Faro (Sicily): Observations on the Swimming Behaviour and the Sex-Dependent Responses to Food

Background: The calanoid copepod Pseudodiaptomus marinus Sato, 1913 is an estuarine-coastal species, living in shallow eutrophic inshore waters. It is native of the Indo-Pacific region, but in the last 50 years, it has successfully colonized new areas worldwide. P. marinus, first recorded in Lake Faro (Messina, Italy) in October 2008, is now a stable component of the zooplankton assemblage of the lake. By means of video recordings, for the first time, the swimming behaviour of males and non-ovigerous and ovigerous females of P. marinus has been studied. The individuals were filmed in the presence and absence of food to evaluate how the presence of prey might affect the swimming behaviour. Results: The swimming motion showed marked sex-dependent features and responses to the presence of food. Mechanisms through which behaviour might influence the outcome of a new colonization were analysed. The behaviour of P. marinus was then compared with that of the congeneric Pseudodiaptomus annandalei showing the typical behaviour displayed by the representatives of the genus Pseudodiaptomus of living in proximity of the bottom. Conclusions: Environmental and hydrological conditions in Lake Faro have likely provided the newly introduced P. marinus a suitable environment for settling, although normally the presence of an anoxic deep layer would be detrimental for a demersal species. In this case, the plasticity in the behaviour of P. marinus enhanced its capacity for colonising new environments. Switching from demersal to pelagic habitat or being fully planktonic allowed it to express its large individual variability in motion strategies and thus to successfully colonize the lake

Explicit predictability and dispersion scaling exponents in fully developed turbulence

We apply a simple method to provide explicit expressions for different scaling exponents in intermittent fully developed turbulence, that before were only given through a Legendre transform. This includes predictability exponents for infinitesimal and non infinitesimal perturbations, Lagrangian velocity exponents, and dispersion exponents. We obtain also new results concerning inverse statistics corresponding to exit-time moments.Comment: Physics Letters A (in press

Analysis of experimental homogeneous turbulence time series by Hilbert-Huang transform

The Empirical Mode Decomposition (EMD) method and Hilbert-Huang transform are used to analyse experimental homogeneous turbulence time series. With this method, one can decompose nonlinear timeseries into a sum of different modes, each narrow-banded. Here we consider experimental turbulent velocity time series with a large Reynolds number (Re_lambda = 720). The Fourier power spectrum reveals a wide inertial range with a classical -5/3 Kolmogorov power-law spectrum. We show that the EMD method applies very nicely to the turbulent velocity time series, with a dyadic filter bank in the inertial range. We estimate the Fourier power spectra of each mode, showing that adding more and more modes corresponds to including lower and lower frequencies. This filtering property can have interesting applications in the field of turbulence modelling. We estimate the Hilbert-Huang power spectrum of the turbulent time series and show its scaling properties, with an exponent different from -5/3

Scaling analysis of time series using empirical mode decomposition and Hilbert spectral analysis

Recently, Hilbert-Huang transform (HHT), or Empirical Mode Decomposition (EMD) have become a promising methodology to deal with nonlinear and nonstationary time series [1,2]. This corresponds to a data-driven method with very local ability, both in physical and spectral space [3]. In this work, we propose an extended version of Hilbert spectral analysis, namely arbitrary order Hilbert spectral analysis, to characterize the scale invariant properties in spectral space directly. The most innovative part of the Hilbert-Huang transform is the Empirical Mode Decomposition, which can separate the original time series into several Intrinsic Mode Functions, called IMF. The starting point of EMD is to consider the time series from real word as multi-component signal. The corresponding characteristic scale is defined as the distance between two successive maxima points (resp. minima points). The intrinsic mode function is proposed to approximate the mono-component signal, which satisfies the following two conditions : (i) the difference between the number of local extrema and the number of zero-crossings must be zero or one ; (ii) the running mean value of the envelope defined by the local maxima and the envelope defined by the local minima is zero [1,2]. Then the empirical mode decomposition algorithm is proposed to extract IMF modes from a given time series [1,2,16].Nous proposons ici une généralisation de l’analyse spectrale de Hilbert qui est effectuée dans le cadre des décompositions modales empiriques. Cette analyse spectrale de Hilbert d’ordre arbitraire permet de caractériser les propriétés d’intermittence des séries temporelles à invariance d’échelle multiple. La méthode est validée en utilisant des simulations de mouvement Brownien fractionnaire, avec différentes valeurs du paramètre H, et avec des simulations lognormales multifractales. Une application est effectuée sur des données « réelles » issues du domaine de la turbulence. La méthode proposée ici fonctionne dans l’espace amplitude-fréquence ; cette méthode est la première approche pouvant prendre en compte les exposants d’intermittence dans l’espace des fréquences. Nous montrons également que cette méthode est supérieure à l’approche utilisant les fonctions de structure lorsque la série à analyser présente une invariance d’échelle superposée à une forte composante périodique

Velocity profiles in shear-banding wormlike micelles

Using Dynamic Light Scattering in heterodyne mode, we measure velocity profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to exhibit both shear-banding and stress plateau behavior. Our data provide evidence for the simplest shear-banding scenario, according to which the effective viscosity drop in the system is due to the nucleation and growth of a highly sheared band in the gap, whose thickness linearly increases with the imposed shear rate. We discuss various details of the velocity profiles in all the regions of the flow curve and emphasize on the complex, non-Newtonian nature of the flow in the highly sheared band.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let