A locally corrected multiblob method with hydrodynamically matched grids for the Stokes mobility problem

• Published in: Journal of computational physics 2023-08, Vol.487, p.112172, Article 112172
• Main Authors: Broms, Anna, Sandberg, Mattias, Tornberg, Anna-Karin
• Format: Article
• Language: English
• Subjects: Accuracy; Axisymmetry; Grid optimisation; Numerical Analysis; Numerisk analys; Pair-correction; Rigid multiblob; Stokes flow
• ISSN: 0021-9991; 1090-2716; 1090-2716
• DOI: 10.1016/j.jcp.2023.112172

Inexpensive numerical methods are key to enabling simulations of systems of a large number of particles of different shapes in Stokes flow and several approximate methods have been introduced for this purpose. We study the accuracy of the multiblob method for solving the Stokes mobility problem in free space, where the 3D geometry of a particle surface is discretised with spherical blobs and the pair-wise interaction between blobs is described by the RPY-tensor. The paper aims to investigate and improve on the magnitude of the error in the solution velocities of the Stokes mobility problem using a combination of two different techniques: an optimally chosen grid of blobs and a pair-correction inspired by Stokesian dynamics. Different optimisation strategies to determine a grid with a given number of blobs are presented with the aim of matching the hydrodynamic response of a single accurately described ideal particle, alone in the fluid. It is essential to obtain small errors in this self-interaction, as they determine the basic error level in a system of well-separated particles. With an optimised grid, reasonable accuracy can be obtained even with coarse blob-resolutions of the particle surfaces. The error in the self-interaction is however sensitive to the exact choice of grid parameters and simply hand-picking a suitable geometry of blobs can lead to errors several orders of magnitude larger in size. The pair-correction is local and cheap to apply, and reduces the error for moderately separated particles and particles in close proximity. Two different types of geometries are considered: spheres and axisymmetric rods with smooth caps. The error in solutions to mobility problems is quantified for particles of varying inter-particle distances for systems containing a few particles, comparing to an accurate solution based on a second kind BIE-formulation where the quadrature error is controlled by employing quadrature by expansion (QBX). •Rigid rods and spheres are studied in Stokes flow in 3D free space.•The accuracy of the multiblob method is improved at no extra cost.•An optimal grid of blobs matches the hydrodynamic interaction of a model particle.•A self-interaction error dominant in the far-field is reduced with the optimal grid.•Pair-corrections of Stokesian dynamics type reduce errors in near-field.