Author: toschi

Population dynamics under flow

Posted on by toschi

Various recent studies have revealed amazing phenomena in the dynamics of bacterial colonies where biology meets physics, in particular statistical physics, fluid dynamics, and (soft) con- densed matter. These biological systems reveal analogies with complex fluids (isotropic- nematic phase transitions), spinodal decomposition phenomena in physics and materials sci- ence, and diffusion-limited reaction kinetics in chemistry. Moreover, gene segregation phe- nomena can be studied with tools from statistical physics (e.g., Potts-like models).

Life on Earth is invariably associated with (flowing) water. Fluid flows determine the fate of bacterial colonies and supply nutrients. Many studies focused on population dynamics in absence of fluid motion, e.g. bacteria living on a Petri dish or in a well-mixed medium. The life of plankton and cyanobacteria in oceans and lakes, however, is ruled by fluid transport, compressibility effects and particle-number fluctuations. Thus we face fundamental questions of how fluid mechanics and turbulence will affect the dynamics of bacterial colonies and their genetic evolution.

Convection in multiphase fluid flows using lattice Boltzmann methods

Posted on by toschi

Biferale, L., Perlekar, P., Sbragaglia, M. & Toschi, F. (2012). Convection in multiphase fluid flows using lattice Boltzmann methods. Physical Review Letters, 108(10):104502

DOI

We present high-resolution numerical simulations of convection in multiphase flows (boiling) using a novel algorithm based on a lattice Boltzmann method. We first study the thermodynamical and kinematic properties of the algorithm. Then, we perform a series of 3D numerical simulations changing the mean properties in the phase diagram and compare convection with and without phase coexistence at Rayleigh number . We show that in the presence of nucleating bubbles non-Oberbeck-Boussinesq effects develop, the mean temperature profile becomes asymmetric, and heat-transfer and heat-transfer fluctuations are enhanced, at all Ra studied. We also show that small-scale properties of velocity and temperature fields are strongly affected by the presence of the buoyant bubble leading to high non-Gaussian profiles in the bulk.

Inverse energy cascade in three-dimensional isotropic turbulence

Posted on by toschi

Biferale, L., Musacchio, S. & Toschi, F. (2012). Inverse energy cascade in three-dimensional isotropic turbulence. Physical Review Letters, 108(16):164501

DOI

We study the statistical properties of homogeneous and isotropic three-dimensional () turbulent flows. By introducing a novel way to make numerical investigations of Navier-Stokes equations, we show that all 3D flows in nature possess a subset of nonlinear evolution leading to a reverse energy transfer: from small to large scales. Up to now, such an inverse cascade was only observed in flows under strong rotation and in quasi-two-dimensional geometries under strong confinement. We show here that energy flux is always reversed when mirror symmetry is broken, leading to a distribution of helicity in the system with a well-defined sign at all wave numbers. Our findings broaden the range of flows where the inverse energy cascade may be detected and rationalize the role played by helicity in the energy transfer process, showing that both and properties naturally coexist in all flows in nature. The unconventional numerical methodology here proposed, based on a Galerkin decimation of helical Fourier modes, paves the road for future studies on the influence of helicity on small-scale intermittency and the nature of the nonlinear interaction in magnetohydrodynamics.