General curved boundary treatment for two- and three-dimensional stationary and moving walls in flow and nonflow lattice Boltzmann simulations.

The aim of this study is to introduce a general approach to implement curved boundaries in lattice Boltzmann simulations. The main idea is to determine boundary values by extrapolating macroscopic properties from some reference points inside the computational domain. The introduced approach is based on a unified extrapolation equation that can be employed for any macroscopic value (flow and nonflow properties) in arbitrary two- and three-dimensional geometries. In the case of nonflow simulations, the present treatment can easily apply Dirichlet and Neumann boundary conditions. By introducing a point cloud description of geometry, the new treatment can handle any complex geometry that is modeled by a CAD program. The application of the new treatment is also extended to moving boundaries, by developing a novel force calculation method. The proposed boundary treatment is tested against several well-established problems and the order of accuracy of solutions is evaluated. Numerical results show that the present treatment is of second-order accuracy with respect to the grid spacing in flow simulations, and it leads to a significant enhancement in nonflow simulations as well.