14th World Congress on Computational Mechanics (WCCM) ECCOMAS Congress 2020) Virtual Congress: 11-–15 January 2021 F. Chinesta, R. Abgrall, O. Allix and M. Kaliske (Eds) AN IB-LBM FOR FSI PROBLEMS INVOLVING VISCOELASTIC FLUIDS AND COMPLEX GEOMETRIES Jingtao MA 1 , Zhen Wang 2 , Yi Sui 2 , John Young 1 , Joseph C.S. Lai 1 and Fang-Bao Tian 1 1 School of Engineering and Information Technology, University of New South Wales, Canberra, ACT 2600, Australia 2 School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, UK Key words: Immersed-boundary method, lattice Boltzmann method, fluid-structure interaction, vis- coelastic fluids Abstract. An immersed boundary-lattice Boltzmann method (IB-LBM) for fluid-structure interaction (FSI) problems involving viscoelastic fluids and complex geometries with a few validation cases is pre- sented in this paper. In this method, the lattice Boltzmann method is used to solve the fluid dynamics and the constitutive equations of viscoelastic fluids. An artificial damping is introduced to enhance numeri- cal stability in solving the constitutive equations. A hybrid of the finite difference method (2D and 3D rigid particles) and the finite element method (3D capsule) is employed to solve the structural dynamics. The interaction between the solid structure and the fluid is achieved by an immersed boundary method. The present method and models are validated by several cases including 2D Oldroyd-B channel flow, 2D lid-driven cavity flow, 2D Oldroyd-B flow over a confined cylinder, a 2D rigid particle migration in an Oldryod-B Couette flow, a spherical particle rotation in an Oldroyd-B shear flow, a spherical particle settling in a Newtonian fluid, and the deformation of a spherical capsule in a long channel filled with a Newtonian fluid. 1 Introduction Non-Newtonian fluids can be involved in many industrial and biological systems, and typical examples of non-Newtonian fluids are polymer solutions, shampoo, paints, food and mucus layer. In many scenarios, the interaction between non-Newtonian fluids and solid structures can be found. For example, a free swimmer (e.g., microorganism or sperm) may encounter complex fluids (non-Newotnian fluids, e.g., substrates or cervical fluid) [1, 2, 3, 4, 5]. Sometimes, it is difficult to conduct the experimental study on fluid-structure interaction (FSI) problems due to the small scale of the structure (e.g., microorganisms in the order of μm). In this case, the numerical method can be a suitable tool to investigate these problems. Many previous numerical investigations on FSI problems involving non-Newtonian fluids have been based on body-conformal grids, and methods of these investigations normally include the arbitrary La- grangian Eulerian-finite element method (ALE-FEM) [6, 7] and the deforming-spatial-domain/stabilized space-time (DSD/SST) method [8, 9, 10]. The disadvantage of these methods is obvious: the mesh needs to be regenerated to ensure the quality of mesh when large deformation/displacement is involved [11, 12, 13]. In contrast, IB-LBM (based on the non-conformal mesh) provides a convenient alternative for 1