Steering in computational science: mesoscale modelling and simulation

This paper outlines the benefits of computational steering for high performance computing applications. Lattice-Boltzmann mesoscale fluid simulations of binary and ternary amphiphilic fluids in two and three dimensions are used to illustrate the substantial improvements which computational steering offers in terms of resource efficiency and time to discover new physics. We discuss details of our current steering implementations and describe their future outlook with the advent of computational grids.Comment: 40 pages, 11 figures. Accepted for publication in Contemporary Physic

Large-scale lattice Boltzmann simulations of complex fluids: advances through the advent of computational grids

During the last two years the RealityGrid project has allowed us to be one of the few scientific groups involved in the development of computational grids. Since smoothly working production grids are not yet available, we have been able to substantially influence the direction of software development and grid deployment within the project. In this paper we review our results from large scale three-dimensional lattice Boltzmann simulations performed over the last two years. We describe how the proactive use of computational steering and advanced job migration and visualization techniques enabled us to do our scientific work more efficiently. The projects reported on in this paper are studies of complex fluid flows under shear or in porous media, as well as large-scale parameter searches, and studies of the self-organisation of liquid cubic mesophases. Movies are available at http://www.ica1.uni-stuttgart.de/~jens/pub/05/05-PhilTransReview.htmlComment: 18 pages, 9 figures, 4 movies available, accepted for publication in Phil. Trans. R. Soc. London Series

A consistent and conservative diffuse-domain lattice Boltzmann method for multiphase flows in complex geometries

Modeling and simulation of multiphase flows in complex geomerties are challenging due to the complexity in describing the interface topology changes among different phases and the difficulty in implementing the boundary conditions on the irregular solid surface. In this work, we first developed a diffuse-domain (DD) based phase-field model for multiphase flows in complex geometries. In this model, the irregular fluid region is embedded into a larger and regular domain by introducing a smooth characteristic function. Then, the reduction-consistent and conservative phase-field equation for the multiphase field and the consistent and conservative Navier-Stokes equations for the flow field are reformulated as the diffuse-domain based consistent and conservative (DD-CC) equations where some additional source terms are added to reflect the effects of boundary conditions. In this case, there is no need to directly treat the complex boundary conditions on the irregular solid surface, and additionally, based on a matched asymptotic analysis, it is also shown that the DD-CC equations can converge to the original governing equations as the interface width parameter tends to zero. Furthermore, to solve the DD-CC equations, we proposed a novel and simple lattice Boltzmann (LB) method with a Hermite-moment-based collision matrix which can not only keep consistent and conservation properties, but also improve the numerical stability with a flexible parameter. With the help of the direct Taylor expansion, the macroscopic DD-CC equations can be recovered correctly from the present LB method. Finally, to test the capacity of LB method, several benchmarks and complex problems are considered, and the numerical results show that the present LB method is accurate and efficient for the multiphase flows in complex geomerties.Comment: 22 pages, 9 figure

Electrokinetic Lattice Boltzmann solver coupled to Molecular Dynamics: application to polymer translocation

We develop a theoretical and computational approach to deal with systems that involve a disparate range of spatio-temporal scales, such as those comprised of colloidal particles or polymers moving in a fluidic molecular environment. Our approach is based on a multiscale modeling that combines the slow dynamics of the large particles with the fast dynamics of the solvent into a unique framework. The former is numerically solved via Molecular Dynamics and the latter via a multi-component Lattice Boltzmann. The two techniques are coupled together to allow for a seamless exchange of information between the descriptions. Being based on a kinetic multi-component description of the fluid species, the scheme is flexible in modeling charge flow within complex geometries and ranging from large to vanishing salt concentration. The details of the scheme are presented and the method is applied to the problem of translocation of a charged polymer through a nanopores. In the end, we discuss the advantages and complexities of the approach

A thermodynamically consistent diffuse interface model for the wetting phenomenon of miscible and immiscible ternary fluids

The wetting effect has attracted great scientific interest because of its natural significance as well as technical applications. Previous models mostly focus on one-component fluids or binary immiscible liquid mixtures. Modelling of the wetting phenomenon for multicomponent and multiphase fluids is a knotty issue. In this work, we present a thermodynamically consistent diffuse interface model to describe the wetting effect for ternary fluids, as an extension of Cahn\u27s theory for binary fluids. In particular, we consider both immiscible and miscible ternary fluids. For miscible fluids, we validate the equilibrium contact angle and the thermodynamic pressure with Young\u27s law and the Young–Laplace equation, respectively. Distinct flow patterns for dynamic wetting are presented when the surface tension and the viscous force dominate the wetting effect. For immiscible ternary fluids, we manipulate the wettability of two contact droplets deposited on a solid substrate according to three scenarios: (I) both droplets are hydrophilic; (II) a hydrophilic droplet in contact with a hydrophobic one; (III) both droplets are hydrophobic. The contact angles at each triple junction from the simulations are compared with Young\u27s contact angle and Neumann\u27s triangle rule. Simulations for the validation of our work are performed in two and three dimensions. In addition, we model the evaporation process of a ternary droplet and obtain the same power law as that of previous experiments. Our model allows one to relate the interfacial energies with surface composition, enabling the modelling of the coffee-ring phenomenon in further perspective

Spreading and engulfment of a viscoelastic film onto a Newtonian droplet

We use the conservative phase-field lattice Boltzmann method to investigate the dynamics when a Newtonian droplet comes in contact with an immiscible viscoelastic liquid film. The dynamics of the three liquid phases are explored through numerical simulations, with a focus on illustrating the contact line dynamics and the viscoelastic effects described by the Oldroyd-B model. The droplet dynamics are contrasted with the case of a Newtonian fluid film. The simulations demonstrate that when the film is viscoelastic, the droplet dynamics become insensitive to the film thickness when the polymer viscosity and relaxation time are large. A viscoelastic ridge forms at the moving contact line, which evolves with a power-law dependence on time. By rescaling the interface profile of the ridge using its height and width, it appears to collapse onto a similar shape. Our findings reveal a strong correlation between the viscoelastic stress and the interface shape near the contact line

Binary fluid flow simulations with free energy lattice Boltzmann methods

We use free energy lattice Boltzmann methods to simulate shear and extensional flows of a binary fluid in two and three dimensions. To this end, two classical configurations are digitally twinned, namely a parallel-band device for binary shear flow and a four-roller apparatus for binary extensional flow. The free energy lattice Boltzmann method and the test cases are implemented in the open-source parallel C++ framework OpenLB and evaluated for several non-dimensional numbers. Characteristic deformations are captured, where breakup mechanisms occur for critical capillary regimes. Though the known mass leakage for small droplet-domain ratios and large Cahn numbers is observed, suitable mesh sizes show good agreement to analytical predictions and reference results

Ternary Flow Simulation Based on The Conservative Phase Field Lattice Boltzmann Method

In this thesis, we numerically investigated multi-phase fluid dynamics (2 and 3-phase flow) by solving the Navier-Stokes equations coupled with the conservative phase field (CPF) equations using the Lattice Boltzmann method (LBM). To effectively simulate the large-scale multi-phase physics, we developed an open-source software, IMEXLBM, which can be easily parallelized on both CPUs and GPUs without significant modifications to the code. We first validated various parts of this software and then used this method to study the interaction of rising bubbles with a static oil droplet as well as the engulfment of the water droplet on solids coated with a layer of oil. The accuracy of the LBM implementation of IMEXLBM was validated with simulations on both CPUs and GPUs of the 3D flow around a sphere. The efficiency and structure of the software were studied using simulations of a 3D Taylor-Green vortex on GPUs: We validated our results against previous studies and then investigated both strong scaling, i.e., the decrease of the computational time for a given problem on an increasing number of processors, and weak scaling, i.e., the evolution of the computational time when increasing the number of processors at a fixed load on each processor. We then moved on to the validation of the CPF implementation. We studied the effect of mobility, a non-physical numerical parameter, by simulating the deformation of a droplet in a simple shear flow. We proposed to use a non-standard variable for mobility in order to reduce the dependency of the results on this parameter. Then, we tested three different surface tension force v formulations, namely (1) the continuum surface tension force formulation (CSF), (2) the stress form surface tension force formulation, and (3) the potential form surface tension force formulation. Simulations of static droplets led us to choose the CSF for the rest of the thesis. Finally, we validated the three-phase flow implementation by studying the evolution of two immiscible droplets initially in contact and with various spreading factors. We constructed a phase diagram of the final morphology as a function of the spreading factor and were able to obtain the three expected morphologies; separated, partially engulfed, and completely engulfed. After having extensively validated our numerical implementation, we moved on to study two new ternary flow problems. First, we studied the interaction between a rising bubble and a stationary droplet. Our results indicated that the bubble-droplet aggregate with double emulsion morphology can minimize the distortion of the bubble-droplet aggregate and achieve a greater terminal velocity than the aggregate with partially engulfed morphology. Second, we investigated the engulfment of a droplet on solids coated by thin and thick fluid films. When an aqueous drop contacts an immiscible oil film, it displays complex interfacial dynamics. When the spreading factor is positive, upon contact the oil spreads onto the drop’s liquid-air interface, first forming a liquid bridge whose curvature drives an apparent drop spreading motion and later fully coating the drop. Inertially and viscously limited dynamics were explored using the Ohnesorge number Oh as a function of /, the ratio between the initial drop radius R and the film height . Both regimes showed that the radial growth of the liquid bridge is insensitive to the film height, and scales with time as ∼ 0.5 for ℎ ≪ 1 and ℎ ≫ 1 as ∼ 0.4 . Contrary to the growth of the liquid bridge, however, we found that the late time engulfment dynamics and final interface profiles were significantly affected by