Preface to DSFD 2021

DSFD 2021,the 30th edition of the Discrete Simulation of Fluid Dynamics,took place again at University of Tuscia,Viterbo,Italy on September 13-172021.Due to the persisting uncertainties associated with the COVID-19 pandemics it was again organised virtually,like the preceding one.

Multiscale Hybrid Modeling of Proteins in Solvent:SARS-CoV2 Spike Protein as Test Case for Lattice Boltzmann-All Atom Molecular Dynamics Coupling

Physiological solvent flows surround biological structures triggering therein collective motions.Notable examples are virus/host-cell interactions and solventmediated allosteric regulation.The present work describes a multiscale approach joining the Lattice Boltzmann fluid dynamics(for solvent flows)with the all-atom atomistic molecular dynamics(for proteins)to model functional interactions between flows and molecules.We present,as an applicative scenario,the study of the SARS-CoV-2 virus spike glycoprotein protein interacting with the surrounding solvent,modeled as a mesoscopic fluid.The equilibriumproperties of the wild-type spike and of the Alpha variant in implicit solvent are described by suitable observables.The mesoscopic solvent description is critically compared to the all-atom solvent model,to quantify the advantages and limitations of the mesoscopic fluid description.

Lattice Boltzmann Simulation of Mixed Convection Heat Transfer in a Driven Cavity with Non-uniform Heating of the Bottom Wall

The goal of this article is to study numerically the mixed convection in a differentially heated lid-driven cavity with non-uniform heating of the bottom wall. The velocity field is solved by a hybrid scheme with multiple relaxation time Lattice Boltzmann(MRT-LBM) model, while the temperature field is obtained by resolution of the energy balance equation using the finite difference method(FDM). First, the model is checked and validated using data from the literature. Validation of the present results with those available in the literature shows a good agreement.A good efficiency in time simulation is confirmed. Thereafter, the model has been applied to mixed convection in a driven cavity with non-uniform heating wall at the fixed Grashof number Gr = 106. It is found that, the heat transfer is weakened as the Richardson number is augmented. For Gr = 106, we note the appearance of secondary vortices at different positions of the cavity corners.

Improved Lattice Boltzmann Without Parasitic Currents for Rayleigh-Taylor Instability

Over the last decade the Lattice Boltzmann Method (LBM) has gained significant interest as a numerical solver for multiphase flows. However most of the LBvariants proposed to date are still faced with discreteness artifacts in the form of spurious currents around fluid-fluid interfaces. In the recent past, Lee et al. have proposeda new LB scheme, based on a higher order differencing of the non-ideal forces, whichappears to virtually free of spurious currents for a number of representative situations.In this paper, we analyze the Lee method and show that, although strictly speaking, itlacks exact mass conservation, in actual simulations, the mass-breaking terms exhibita self-stabilizing dynamics which leads to their disappearance in the long-term evolution. This property is specifically demonstrated for the case of a moving droplet atlow-Weber number, and contrasted with the behaviour of the Shan-Chen model. Furthermore, the Lee scheme is for the first time applied to the problem of gravity-drivenRayleigh-Taylor instability. Direct comparison with literature data for different values of the Reynolds number, shows again satisfactory agreement. A grid-sensitivitystudy shows that, while large grids are required to converge the fine-scale details, thelarge-scale features of the flow settle-down at relatively low resolution. We concludethat the Lee method provides a viable technique for the simulation of Rayleigh-Taylorinstabilities on a significant parameter range of Reynolds and Weber numbers.