SUMMARYIn this study, we assess several interface schemes for stationary complex boundary flows under the direct-forcing immersed boundary-lattice Boltzmann methods (IB-LBM) based on a split-forcing lattice Boltzmann equation (LBE). Our strategy is to couple various interface schemes, which were adopted in the previous direct-forcing immersed boundary methods (IBM), with the split-forcing LBE, which enables us to directly use the direct-forcing concept in the lattice Boltzmann calculation algorithm with a second-order accuracy without involving the Navier-Stokes equation. In this study, we investigate not only common diffuse interface schemes but also a sharp interface scheme. For the diffuse interface scheme, we consider explicit and implicit interface schemes. In the calculation of velocity interpolation and force distribution, we use the 2-and 4-point discrete delta functions, which give the second-order approximation. For the sharp interface scheme, we deal with the exterior sharp interface scheme, where we impose the force density on exterior (solid) nodes nearest to the boundary. All tested schemes show a second-order overall accuracy when the simulation results of the Taylor-Green decaying vortex are compared with the analytical solutions. It is also confirmed that for stationary complex boundary flows, the sharper the interface scheme, the more accurate the results are. In the simulation of flows past a circular cylinder, the results from each interface scheme are comparable to those from other corresponding numerical schemes.

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