International audienceWe study the effects of a stable density stratification on the turbulent dynamics of thin fluid layers forced at intermediate scales. By means of a set of high-resolution numerical simulations, performed within the Boussinesq approximation, we investigate how the stratification and confinement affect the mechanisms of kinetic and potential energy transfer. The detailed analysis of the statistics of the energy-dissipation rates and energy-exchange rates and of the spectral fluxes of potential and kinetic energy shows that stratification provides a new channel for the energy transfer towards small scales which reduces the large-scale flux of kinetic energy. We also discuss the role of vortex stretching and enstrophy flux in the transfer of kinetic energy into potential energy at small scales

We derive a simple model, valid within the Boussinesq approximation, for the dynamics of small buoyant particles in stratified turbulence, in the presence of a mean linear density profile. By means of extensive direct numerical simulations, we investigate the statistical distribution of particles as a function of the two dimensionless parameters of the problem. We find that vertical confinement of particles is mainly ruled by the degree of stratification, with a weak dependence on the particle properties. In contrast, small-scale fractal clustering is found to depend on the particles relaxation time and is only slightly dependent on the flow stratification. The implications of our findings for the formation of thin phytoplankton layers are discussed. DOI: 10.1103/PhysRevFluids.1.052401Particles of density different from that of the surrounding fluid do not follow the motion of fluid particles and generate inhomogeneous distributions even in incompressible flows [1]. This phenomenon is crucial in a variety of instances, including cloud formation in the atmosphere, the dynamics of plankton in the ocean and lakes, and industrial applications [2][3][4]. The formation of inhomogeneous distributions in turbulent flows is also interesting from a theoretical point of view and, in recent years, analytical, numerical, and experimental studies have led to significant advances in the understanding of this process [5][6][7][8][9][10]. Most of the studies have considered the case of inertial particles that accumulate in regions of high vorticity (light particles) or high strain (heavy particles) [5,7,11], as a consequence of the accelerations induced by the flow. Recent works have studied the interaction between gravity and turbulent accelerations in the dynamics of heavy particles. In particular, it has been shown that turbulence can increase their settling velocity with respect to still fluid, by pushing particles in regions of downward flow [12,13]. In the presence of density fluctuations, gravity also affects the flow itself as in the case of stratified turbulence, which finds many applications in natural flows [14,15], e.g., ocean dynamics in the presence of the pycnocline resulting from temperature and salinity variations [16,17]. Little is known about the distribution of floating particles in stratified turbulence, in spite of its relevance for applications. Recent works have studied the effect of stratification on the clustering of heavy [18] and light [19] particles, the formation of tangling clustering in stratified turbulence [20,21], and the effect of a vertical confinement in homogeneous turbulence [22].One of the most remarkable examples of confinement of particles in the ocean is the formation of the so-called thin phytoplankton layers (TPLs): aggregations of phytoplankton and zooplankton at high concentration with thickness from centimeters to few meters, extending up to several kilometers horizontally and with a time scale from hours to days [23]. Among the possible mechanisms of formation of TPLs, convergenc...

We study the effect of turbulence on a sedimenting layer of particles by means of direct numerical simulations. A Lagrangian model in which particles are considered as tracers with an additional downward settling velocity is integrated together with an isotropic homogeneous turbulent flow. We study the spatial distribution of particles when they are collected on a plane at non-asymptotic times. We relate the resulting coarse-grained particle density to the history of the stretching rate along the particle trajectory and the projection of the density onto the accumulation plane and analyze the deviation from homogeneity in terms of the Reynolds number and the settling velocity. We identify two regimes that arise during the early and well-mixed stages of advection. In the former regime, more inhomogeneity in the particle distribution is introduced for decreasing settling velocity or increasing Reynolds number, while the tendencies are opposite in the latter regime. A resonant-like crossover is found between these two regimes where inhomogeneity is maximal.

We present an efficient point-particle approach to simulate reaction-diffusion processes of spherical absorbing particles in the diffusion-limited regime, as simple models of cellular uptake. The exact solution for a single absorber is used to calibrate the method, linking the numerical parameters to the physical particle radius and uptake rate. We study configurations of multiple absorbers of increasing complexity to examine the performance of the method, by comparing our simulations with available exact analytical or numerical results. We demonstrate the potentiality of the method in resolving the complex diffusive interactions, here quantified by the Sherwood number, measuring the uptake rate in terms of that of isolated absorbers. We implement the method in a pseudospectral solver that can be generalized to include fluid motion and fluid-particle interactions. As a test case of the presence of a flow, we consider the uptake rate by a particle in a linear shear flow. Overall, our method represents a powerful and flexible computational tool that can be employed to investigate many complex situations in biology, chemistry and related sciences.

We investigate numerically the dynamics and statistic of inertial particles transported by stratified turbulence, in the case of particle density intermediate in the average density profile of the fluid. In these conditions, particles tend to form a thin layer around the corresponding fluid isopycnal. The thickness of the resulting layer is determined by a balance between buoyancy (which attracts the particle to the isopycnal) and inertia (which prevents them from following it exactly). By means of extensive numerical simulations, we explore the parameter space of the system and we find that in a range of parameters particles form fractal cluster within the layer.

No abstract

Suspended particles can significantly alter the fluid properties and, in particular, can modify the transition from laminar to turbulent flow. We investigate the effect of heavy particle suspensions on the linear stability of the Kolmogorov flow by means of a multiple-scale expansion of the Eulerian model originally proposed by Saffman (J. Fluid Mech., vol. 13, issue 1, 1962, pp. 120–128). We find that, while at small Stokes numbers particles always destabilize the flow (as already predicted by Saffman in the limit of very thin particles), at sufficiently large Stokes numbers the effect is non-monotonic in the particle mass fraction and particles can both stabilize and destabilize the flow. Numerical analysis is used to validate the analytical predictions. We find that in a region of the parameter space the multiple-scale expansion overestimates the stability of the flow and that this is a consequence of the breakdown of the scale separation assumptions.

No abstract