In 3D cases, an iterative approach with the FieldSplit preconditioner was adopted with the symmetric block Gauss-Seidel algorithm for the charge transport problem and Schur complement approximation for the fluid flow. For the charge transport equation, a parallel algebraic multigrid (AMG) preconditioner, namely BoomerAMG, in conjunction with the generalized minimal residual solver (GMRES) was employed. For the flow problem, an AMG preconditioner was used for the velocity block, and a Jacobi preconditioner was assigned to the pressure block.