P. Prestininzi

Effects of Knudsen diffusivity on the effective reactivity of nanoporous catalyst media

Abstract We investigate the non-equilibrium hydrodynamic effects on the reactivity of a nanoporous catalytic sample. Numerical simulations using the Lattice Boltzmann Method (LBM) show that non-equilibrium effects enhance the reactivity of the porous sample, in agreement with theoretical predictions [1] . In addition, we provide a quantitative assessment of the reactivity in terms of the thickness of the reactive layer inside the nanoporous catalytic sample. Such an assessment constitutes a first step towards integrated simulations encompassing nanoscale reactivity and transport coefficients within a macroscale description of experimental relevance.

On the effect of the intrinsic viscosity in a two-layer shallow water lattice Boltzmann model of axisymmetric density currents

In this work, a numerical assessment of the suitability of a Single Relaxation Time (SRT) Lattice Boltzmann Method (LBM) model to simulate axisymmetric gravity currents is carried out. The model results are compared with both experimental data and other numerical models. The particular SRT formulation employed is known to converge, in the limit of low Knudsen number, to the two-layer 2D Shallow Water Equations (SWEs) set with a viscosity term featuring a closed theoretical formulation. Even with the lowest viscosity achievable by the method, its effect is shown to become important in most of the cases analysed, thus posing some serious constraints on possible application of the single relaxation time LBM method to simulate the lock-release generated-type gravity currents analysed here. The comparison with classical numerical models shows that the the viscous effects in the LBM model can be well reproduced employing coefficients derived from the above-mentioned theoretical formulation.

Entropic lattice pseudo-potentials for multiphase flow simulations at high Weber and Reynolds numbers

We present an entropic version of the lattice Boltzmann pseudo-potential approach for the simulation of multiphase flows. The method is shown to correctly simulate the dynamics of impinging droplets on hydrophobic surfaces and head-on and grazing collisions between droplets, at Weber and Reynolds number regimes not accessible to previous pseudo-potential methods at comparable resolution.

Curved boundaries in multi-layer Shallow Water Lattice Boltzmann Methods: bounce back versus immersed boundary

Abstract The Lattice Boltzmann Method has been successfully applied to solve Multilayer Shallow Water models for the simulation of gravity currents generated by laboratory lock exchange experiments in regular geometries. When complex boundaries are to be modelled, several approaches are feasible. In this work two types of boundary conditions are analyzed and compared in the simulation of lock exchange gravity currents impacting on vertical obstacles. The first one belongs to the common group of bounce back approaches; the second one is an adaptation of the immersed boundary technique to the Shallow Water dynamics. The analysis is carried out comparing accuracy against experimental data. Efficiency is also taken into account. Results show that the computational overhead introduced by the Immersed Boundary technique is not counterbalanced by a gain in accuracy for the considered kind of obstacles.

Lattice kinetic approach to non-equilibrium flows

We present a Lattice Boltzmann method for the simulation of a wide range of Knudsen regimes. The method is assessed in terms of normalised discharge for flow across parallel plates and three-dimensional flows in porous media. Available analytical solutions are well reproduced, supporting the the method as an appealing candidate to bridge the gap between the hydrodynamic regime and free molecular motion.