Phase field lattice Boltzmann method for liquid-gas flows in complex geometries with efficient and consistent wetting boundary treatment
Abstract
This study investigates the application of wetting boundary conditions for modelling flows in complex curved geometries, such as rough fractures. It implements and analyses two common variants of the wetting boundary condition within the three-dimensional (3D) phase field lattice Boltzmann method. It provides a straightforward and novel extension of the geometrical approach to curved three-dimensional surfaces. It additionally implements surface-energy approach. A novel interpolation-based mitigation of the staircase approximation for curved boundaries is then developed and consistently applied to both wetting boundary conditions. The objectives of simplicity and parallel compute efficiency in implementation are emphasised. Through detailed validation on a series of 3D benchmark cases involving curved surfaces, such as droplet spread on a sphere, capillary intrusion, and droplet impact on a sphere, the behaviour of the wetting boundary conditions are validated and the differences between methods are highlighted. To demonstrate the applicability of the proposed approach in complex geometries with varying surface curvatures, two-phase flow through a synthetic rough fracture is presented. The suitability of the methods for complex simulations is also verified by comparing the computational performance between all investigated methods using this fracture flow test case. The present work thus contributes to the field of multiphase flow modelling with the lattice Boltzmann method in realistic applications where addressing the impact of complex geometries is essential.
Enhancing data locality of the conjugate gradient method for high-order matrix-free finite-element implementations
Abstract
This work investigates a variant of the conjugate gradient (CG) method and embeds it into the context of high-order finite-element schemes with fast matrix-free operator evaluation and cheap preconditioners like the matrix diagonal. Relying on a data-dependency analysis and appropriate enumeration of degrees of freedom, we interleave the vector updates and inner products in a CG iteration with the matrix-vector product with only minor organizational overhead. As a result, around 90% of the vector entries of the three active vectors of the CG method are transferred from slow RAM memory exactly once per iteration, with all additional access hitting fast cache memory. Node-level performance analyses and scaling studies on up to 147k cores show that the CG method with the proposed performance optimizations is around two times faster than a standard CG solver as well as optimized pipelined CG and s-step CG methods for large sizes that exceed processor caches, and provides similar performance near the strong scaling limit.
Towards a Statistical Framework to Upscale Fracture Flows with Quantifiable Uncertainty
Abstract
Modelling of single- and multi-phase flow in fractured subsurface systems is critical in applications relating to the environment, energy, and resource extraction. However, techniques for quantifying the effect of fracture characteristics such as roughness and wettability, as well as fluid flow regimes, on hydraulic properties (e.g., relative permeability) are not described in the literature. Often fractures are approximated as parallel plates (at some level of locality), ignoring intrinsic roughness and decorrelation between surfaces. This leads to the well-known Cubic or Local Cubic Law for single-phase flow, of which many modifications have been proposed to cater for various pore-scale flow phenomena. Although there is a wide range of available literature working within such a methodology, the realisation of a general, robust model based on expected properties of the fracture surface remains a challenge. In comparison, two-phase fracture flows see significantly less development in the literature, with only few studies characterising permeability-saturation curves as a function of fracture properties. Roughness and wettability impact pore-scale flow and can lead to earlier transition between various displacement regimes and govern the relative permeability of the fluid phases present in a fracture. This work aims to develop a framework for studying the impact of fracture-scale phenomena and upscaling it to the level of Discrete Fracture Networks, and then into field-scale analysis. This talk will provide examples of the studies conducted for single- and two-phase flow at the fracture-scale and discuss the methodology of upscaling the observed behavior while quantifying the uncertainty introduced by fracture characteristics (e.g., topology, wettability).
preCICE v2: A sustainable and user-friendly coupling library
Abstract
preCICE is a free/open-source coupling library. It enables creating partitioned multi-physics simulations by gluing together separate software packages. This paper summarizes the development efforts in preCICE of the past five years. During this time span, we have turned the software from a working prototype – sophisticated numerical coupling methods and scalability on ten thousands of compute cores – to a sustainable and user-friendly software project with a steadily-growing community. Today, we know through forum discussions, conferences, workshops, and publications of more than 100 research groups using preCICE. We cover the fundamentals of the software alongside a performance and accuracy analysis of different data mapping methods. Afterwards, we describe ready-to-use integration with widely-used external simulation software packages, tests, and continuous integration from unit to system level, and community building measures, drawing an overview of the current preCICE ecosystem.