107 research outputs found
Stability of viscous long liquid filaments
We study the collapse of an axisymmetric liquid filament both analytically
and by means of a numerical model. The liquid filament, also known as ligament,
may either collapse stably into a single droplet or break up into multiple
droplets. The dynamics of the filament are governed by the viscosity and the
aspect ratio, and the initial perturbations of its surface. We find that the
instability of long viscous filaments can be completely explained by the
Rayleigh-Plateau instability, whereas a low viscous filament can also break up
due to end pinching. We analytically derive the transition between stable
collapse and breakup in the Ohnesorge number versus aspect ratio phase space.
Our result is confirmed by numerical simulations based on the slender jet
approximation and explains recent experimental findings by Castrejon-Pita et
al., PRL 108, 074506 (2012).Comment: 7 page
Lyapunov exponents of heavy particles in turbulence
Lyapunov exponents of heavy particles and tracers advected by homogeneous and
isotropic turbulent flows are investigated by means of direct numerical
simulations. For large values of the Stokes number, the main effect of inertia
is to reduce the chaoticity with respect to fluid tracers. Conversely, for
small inertia, a counter-intuitive increase of the first Lyapunov exponent is
observed. The flow intermittency is found to induce a Reynolds number
dependency for the statistics of the finite time Lyapunov exponents of tracers.
Such intermittency effects are found to persist at increasing inertia.Comment: 4 pages, 4 figure
Evidences of Bolgiano scaling in 3D Rayleigh-Benard convection
We present new results from high-resolution high-statistics direct numerical
simulations of a tri-dimensional convective cell. We test the fundamental
physical picture of the presence of both a Bolgiano-like and a Kolmogorov-like
regime. We find that the dimensional predictions for these two distinct regimes
(characterized respectively by an active and passive role of the temperature
field) are consistent with our measurements.Comment: 4 pages, 3 figure
Topological structure and dynamics of three-dimensional active nematics.
Topological structures are effective descriptors of the nonequilibrium dynamics of diverse many-body systems. For example, motile, point-like topological defects capture the salient features of two-dimensional active liquid crystals composed of energy-consuming anisotropic units. We dispersed force-generating microtubule bundles in a passive colloidal liquid crystal to form a three-dimensional active nematic. Light-sheet microscopy revealed the temporal evolution of the millimeter-scale structure of these active nematics with single-bundle resolution. The primary topological excitations are extended, charge-neutral disclination loops that undergo complex dynamics and recombination events. Our work suggests a framework for analyzing the nonequilibrium dynamics of bulk anisotropic systems as diverse as driven complex fluids, active metamaterials, biological tissues, and collections of robots or organisms
HIV-1 Tat Promotes Integrin-Mediated HIV Transmission to Dendritic Cells by Binding Env Spikes and Competes Neutralization by Anti-HIV Antibodies
Fluids, flowing across the scales
Fluids are everywhere and still many of their fundamental properties are not understood. The apparent simplicity of the equations describing the motion of simple fluids contrasts with the beautiful complexity of turbulence. Fluid dynamics is clearly important to our daily life: we drive, navigate and fly inside fluids. Fluids are also responsible for the transport of mass and heat, for example in the atmosphere, while fluids flowing inside our body provide the fundamental support for cell life. In recent years computers have added to experimental equipment to help disclose the phenomenology of fluid motions. In this lecture I will try to illustrate the beauty of the physics behind fluid flows and some of the research topics that are currently keeping me busy
An accurate and efficient Lagrangian sub-grid model
A computationally efficient model is introduced to account for the sub-grid scale velocities of tracer particles dispersed in statistically homogeneous and isotropic turbulent flows. The model embeds the multi-scale nature of turbulent temporal and spatial correlations, that are essential to reproduce multi-particle dispersion. It is capable to describe the Lagrangian diffusion and dispersion of temporally and spatially correlated clouds of particles. Although the model neglects intermittent corrections, we show that pair and tetrad dispersion results nicely compare with Direct Numerical Simulations of statistically isotropic and homogeneous turbulence. This is in agreement with recent observations that deviations from self-similar pair dispersion statistics are rare events
Ultimate state of thermal convection
The ultimate regime of thermal convection, the so-called Kraichnan regime [R.ÂżH. Kraichnan, Phys. Fluids 5, 1374 (1962)], hitherto has been elusive. Here numerical evidence for that regime is presented by performing simulations of the bulk of turbulence only, eliminating the thermal and kinetic boundary layers and replacing them with periodic boundary conditions
Lattice Boltzmann method at finite Knudsen numbers
A modified lattice Boltzmann model with a stochastic relaxation mechanism
mimicking virtual collisions between free-streaming particles and solid walls is introduced.
This modified scheme permits to compute plane channel flows in satisfactory agreement with analytical results over a broad spectrum of Knudsen numbers, ranging from the hydrodynamic regime, all the way to quasi-free flow regimes up to Kn ~ 30
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