arXiv subjects

Alice Jaccod

Publications and source records attributed to Alice Jaccod.

4 recordsLinked 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

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↗