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The MPI intercommunication framework was used for coupling of two lattice-Boltzmann solvers with suspended particles, which model advection and diffusion respectively of these particles in a carrier fluid. Simulation domain was divided into two parts, one with advection and diffusion, and the other with diffusion only (no macroscopic flow). Particles were exchanged between these domains at their common boundary by a direct process to process communication. By analysing weak and strong scaling, it was shown that the linear scaling characteristics of the lattice-Boltzmann solvers were not compromised by their coupling.

In this Letter, we study the collective behaviour of a large number of self-propelled microswimmers immersed in a fluid. Using unprecedently large-scale lattice Boltzmann simulations, we reproduce the transition to bacterial turbulence. We show that, even well below the transition, swimmers move in a correlated fashion that cannot be described by a mean-field approach. We develop a novel kinetic theory that captures these correlations and is non-perturbative in the swimmer density. To provide an experimentally accessible measure of correlations, we calculate the diffusivity of passive tracers and reveal its non-trivial density dependence. The theory is in quantitative agreement with the lattice Boltzmann simulations and captures the asymmetry between pusher and puller swimmers below the transition to turbulence., Accepted for publication in Phys. Rev. Lett

Obtaining a good load balance is a significant challenge in scaling up lattice-Boltzmann simulations of realistic sparse problems to the exascale. Here we analyze the effect of weighted decomposition on the performance of the HemeLB lattice-Boltzmann simulation environment, when applied to sparse domains. Prior to domain decomposition, we assign wall and in/outlet sites with increased weights which reflect their increased computational cost. We combine our weighted decomposition with a second optimization, which is to sort the lattice sites according to a space filling curve. We tested these strategies on a sparse bifurcation and very sparse aneurysm geometry, and find that using weights reduces calculation load imbalance by up to 85%, although the overall communication overhead is higher than some of our runs., 11 pages, 8 figures, 1 table, accepted for the EASC2014 conference

The Discrete Simulation of Fluid Dynamics (DSFD) series of conferences originated with the 1986 Los Alamos conference organised by Gary D. Doolen. Since that time, the DSFD conferences have emerged as the premier forum for researchers in the field, and many exciting new discoveries in lattice models of fluid dynamics have been announced at DSFD conferences. Topics emphasised at these meetings include lattice Boltzmann schemes, dissipative particle dynamics, smoothed-particle hydrodynamics, direct simulation Monte Carlo, molecular dynamics, quantum Monte Carlo methods, multiparticle collision dynamics and hybrid methods. At the 23rd edition of DSFD in Paris 2014, we were given the privilege of organising the 24th running of the meeting series in Edinburgh, from 13th to 17th July 2015 at the Royal Society of Edinburgh, which names Adam Smith, Joseph Black and Benjamin Franklin among its founding fellows. Edinburgh is Scotland’s truly cosmopolitan capital. Apart from its amazing castle and Arthur’s Seat (an ancient volcanic plug), parts of the city centre are UNESCO World Heritage Sites. Numerous famous scientists have been born or worked in Edinburgh, for example Alexander Graham Bell, Max Born, Charles Darwin, Peter Higgs or James Clerk Maxwell. We were delighted to host a total of 164 attendees, 60 of whom were students, from 21 countries. We enjoyed twelve invited talks and 100 contributed talks. There were 29 posters and 16 flash presentations. We had, of course, a great deal of help in preparing the meeting, particularly from The University of Edinburgh, our colleagues on the Scientific Committee, the conference venue the Royal Society of Edinburgh, and this Journal for the publication of this Special Issue. We are also grateful to the following for their financial support, without which arranging the meeting would have been impossible: Porous Media, Processes and Mathematics; SoftComp; the British Heart Foundation; Sphere Fluidics; Electric Ant Lab; The American Physical Society; UK- COMES, the UK Consortium on Mesoscale Engineering Sciences; SUPA, the Scottish 1 Universities Physics Alliance; our colleagues from the Institute for Condensed Matter and Complex Systems at the School of Physics and Astronomy through their EPSRC Programme Grant; Cray, the Supercomputer Company, and NVIDIA. We are very pleased that with this Special Issue the Journal of Computational Science has given us the opportunity to present the latest developments in theory and application of discrete simulation methods for fluid dynamics. Topics discussed at the conference include the theory of lattice Boltzmann, e.g. the mathematical lattice structure [14], mixed quadratures for finite Knudsen number flows [1], optimisation of pseudopotential models [6], phase-field models for multiphase flows [18] and axisymmetric multiphase models [13]. Other presenters focussed on various lattice Boltzmann applications, such as the simulation of harbour seal vibrissae [10], diffusivity and reactivity of nanoporous catalyst media [9], rare cell capture in obstacle arrays [3] and the PowerFLOW solver for multi-component engineering applications [12]. A specific emphasis was put on droplet dynamics through various simulation methods, including droplet dynamics in confinement [4], bounce regime of droplet collisions [19], contact angle hysteresis of microdroplets on structured surfaces [2] and phase-field simulations of partially wetted and textured surfaces [16]. One of the conference topics was multiscale problems in fluid dynamics, for instance the coupling for lattice Boltzmann and conventional Navier-Stokes solvers [11], a hybrid molecular-dynamics/fluctuating-hydrodynamics model [5] and direct-simulation-Monte- Carlo/lattice-Boltzmann mapping for gas flows [15]. Finally, the conference featured a number of multi-physics applications, including inertial dilute particle flow with an Euler- Euler lattice Boltzmann method [17], lattice Boltzmann for light simulation and radiative transport through absorbing and scattering media [8] and jet electro-hydrodynamics [7]. We hope these contributions will be of interest and value to readers from a wide range of subject areas and form a reference for future development of discrete simulation methods of fluid dynamics.

We investigate the performance of the HemeLB lattice-Boltzmann simulator for cerebrovascular blood flow, aimed at providing timely and clinically relevant assistance to neurosurgeons. HemeLB is optimised for sparse geometries, supports interactive use, and scales well to 32,768 cores for problems with ~81 million lattice sites. We obtain a maximum performance of 29.5 billion site updates per second, with only an 11% slowdown for highly sparse problems (5% fluid fraction). We present steering and visualisation performance measurements and provide a model which allows users to predict the performance, thereby determining how to run simulations with maximum accuracy within time constraints., Accepted by the Journal of Computational Science. 33 pages, 16 figures, 7 tables