1,720,999 research outputs found
Dynamics of spinodal decomposition in turbulent flows
No full textWhen a binary mixture is cooled below its critical temperature it undergoes a phase transition and the mixture separates into its individual components: this phenomenon is widely known as spinodal decomposition. The dynamics proceeds through different regimes all characterized by a coarsening of the domain size. We investigate numerically the dynamics of such a system when the mixture of immiscible fluids is stirred at the large scale and thus turbulent. Under turbulent conditions we find that the coarsening of the domains is arrested and a similarity with the physics of dilute turbulent emulsions is possible. In particular we show that the typical domain size can be estimated by means of the Kolmogorov-Hinze argument for the stability of droplets in turbulence
Multifractal Droplet Dynamics in Two-Dimensional, binary-fluid turbulence
No full textWe present the most extensive direct numerical simulations, attempted so far, of statistically steady, homogeneous, isotropic turbulence in two-dimensional, binary-fluid mixtures with air-drag-induced friction. We model this mixture by using the Cahn-Hilliard-Navier-Stokes equations and choose parameters, e.g., the surface tension, such that we have a droplet of the minority phase moving inside a turbulent background of the majority phase. Our study reveals that a single droplet, whose mean radius lies in the inertial range of scales, (a) enhances the the forward-cascade part of the energy spectrum of two-dimensional turbulence and (b) stretches the tails of the PDF of the Okubo-Weiss parameter . We show that the dynamics of the droplet is affected significantly by the turbulence in the fluid. In particular, the PDFs of the components of the acceleration shows wide, non-Guassian tails. We characterize the time dependence of the deformation of the droplet and show that it exhibits multifractality
Spinodal decomposition in the inverse cascade of two-dimensional, binary-fluid turbulence
No full textWe study spinodal decomposition in the inverse-cascade regime of two dimensional turbulence in symmetric, binary fluid mixtures. We show that turbulence leads to break up of domains whose size, in the inverse cascade regime, is proportional to the Hinze scale. Even more strikingly, we show that the inverse cascade of energy is blocked by the formation of domains
Universal Statistical Properties of Inertial-particle Trajectories in Three-dimensional, Homogeneous, Isotropic, Fluid Turbulence
No full textWe obtain new universal statistical properties of heavy-particle trajectories in three-dimensional, statistically steady, homogeneous, and isotropic turbulent flows by direct numerical simulations. We show that the probability distribution functions (PDFs) P(Φ), of the angle Φ between the Eulerian velocity u and the particle velocity v, at a point and time, scales as P(Φ) ∼Φ−, with a new universal exponent ≃ 4
A Lattice Boltzmann method for turbulent emulsions
No full textThe breakup of droplets in a turbulent flow is key to many natural and industrial applications. Here we present and validate a computationally efficient numerical method that allows to study turbulent emulsion for very long times. The numerical method is based on a multi-component Lattice Boltzmann method based on the Shan-Chen model and supplemented with a large scale force to stir turbulence. A special treatment to limit mobility between different fluid components is introduced and validated. We demonstrate the potential of our approach in sustaining a turbulent emulsion over extremely long integration times (necessary to collect firm turbulence statistics) and we present first results on the probability distribution function of droplets' accelerations
Statistics of population dynamics in turbulence
No full textWe study the statistical properties of population dynamics in 2d turbulent and compressible surface flows. We show that the compressible surface flow leads to a patchiness in the population density that resembles the patchiness of the Plankton population as observed from the satellite images of the ocean surface. The statistical properties of the population concentration are investigated and quantified for different growth rates, diffusivities and for different level of compressibility of the surface flow
Lagrangian and Eulerian rotating turbulence
No full textState-of-the-art direct numerical simulations of rotating turbulence at changing Reynolds and Rossby numbers are presented. Flow is also seeded with millions of particles, with and without inertia, light and heavy. We study two regimes, at high and low rotation. Heavy and light particles are injected along different axis of rotations, allowing to study the combined effects of preferential concentration in presence of Coriolis and Centripetal forces
Cumulative compressibility effects on slow reactive dynamics in turbulent flows
No full textReactions in turbulent flows, chemical reactions or combustion, are common. Typically reaction time scales are much shorter than turbulence timescales. In biological applications, as it is the case for bacterial and plankton populations living under the influence of currents in oceans and lakes, the typical lifetime can be long and thus can fall well within the inertial range of turbulence time scales. Under these conditions, turbulent transport interacts in a very complex way with the dynamics of growth and death of the individuals in the population. In the present paper, we quantitatively investigate the effect of the flow compressibility on the dynamics of populations. Small effective compressibility can be induced by several physical mechanisms, such as, e.g., by the density mismatch, by a small but finite size of microorganisms, and by gyrotaxis (an interaction between swimming and shear). We report, for the first time, how even a tiny effective compressibility can produce a dramatically large effect on global quantities like the carrying capacity of the ecosystem. We interpret our findings by means of a cumulative effect made possible by the long replication times of the organisms with respect to turbulence time scales. A statistical quantification of the fluctuations of population concentration is presented
Simulations of Boiling Systems Using a Lattice Boltzmann Method
Full text linkWe report about a numerical algorithm based on the lattice Boltzmann method and its applications for simulations of turbulent convection in multi-phase flows. We discuss the issue of 'latent heat' definition using a thermodynamically consistent pseudo-potential on the lattice. We present results of numerical simulations in 3D with and without boiling, showing the distribution of pressure, density and temperature fluctuations inside a convective cell.
Key words: Lattice Boltzmann equation, boiling, thermal convection
Convection in Multiphase Fluid Flows Using Lattice Boltzmann Methods
Full text linkWe perform high-resolution numerical simulations of convection in multiphase flows (boiling) using a
novel algorithm based on a lattice Boltzmann method
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