Localized Parallel Algorithm for Bubble Coalescence in Free Surface Lattice-Boltzmann Method

Abstract

The lattice Boltzmann method is a popular method from computational fluid dynamics. An extension of this method handling liquid flows with free surfaces can be used to simulate bubbly flows. It is based on a volume-of-fluids approach and an explicit tracking of the interface, including a reconstruction of the curvature to model surface tension. When this algorithm is parallelized, complicated data exchange is required, in particular when bubbles extend across several subdomains and when topological changes occur through coalescence of bubbles. In a previous implementation this was handled by using all-to-all MPI communication in each time step, restricting the scalability of the simulations to a moderate parallelism on a small number of processors. In this paper, a new parallel bubble merge algorithm will be introduced that communicates updates of the bubble status only locally in a restricted neighborhood. This results in better scalability and is suitable for massive parallelism. The algorithm has been implemented in the lattice Boltzmann software framework waLBerla, resulting in parallel efficiency of 90% on up to 4080 cores.

WaLBerla: HPC software design for computational engineering simulations

“…Performance results for WaLBerla can be found in [45]. Details on the performance and scalability of the free-surface implementation up to 4000 cores can be found in [1] and on the particulate flow implementation in [4].…”

“…For the free-surface flows, we developed a model [21] similar to the volume of fluid approach, which also includes surface tension and wetting phenomena. The model has been successfully parallelized [1], and it has been used to study metal foams [22], which led to a deeper understanding of the foaming process of metal foams. Our current research focuses on the liquid water transport in the porous gas-diffusion layer of fuel cells.…”

“…This can be utilized for instance to solve an implicit equation iteratively, using an error bound as recursion criteria. A more complex example is the parallel algorithm to handle bubble coalescence in the free-surface application [1], where a beforehand unknown number of communication steps have to be performed. The number of communication steps is unknown for our local communication pattern, as several bubbles spanning over several processes can coalesce at the same time into one bubble.…”

“…The implementation has also been carried out within several cooperations and by master students. Form top left to lower right, we have included free-surface flows [1], free-surface flows with floating objects [2], flows through porous media, clotting processes in blood vessels [3], particulate flows for several million volumetric particles [4] on up to 8192 cores, and a fluctuating Lattice-Boltzmann [5] for nanoscale flows where thermal fluctuations are important.…”

Numerical simulation of adsorption and bubble interaction in protein foams using a lattice Boltzmann method

“…[23][24][25] This model tracks the actual volume of such gas regions and the initial volume from the beginning of the simulation.…”