A computational model based on a large-eddy simulation (LES) technique was proposed to estimate turbulent mixing and pressure drop in subchannels with grid spacers. For an efficient treatment of this complex geometry, improvements were made to the LES technique coupled with an immersed boundary method: A one-equation dynamic subgrid scale model was introduced to account for the complex geometry without any artificial modification; the higher order accuracy was maintained by a consistent treatment of boundary conditions for velocity and pressure on solid walls. Computations were carried out for each of the convolute and periodic arrangements with two-step inclinations of the mixing vanes. The results reasonably reproduced the geometric effect in the turbulent mixing and drag coefficients for the flow, including unsteady separation and multiple vortices. The present computational model is useful for designing grid spacers: By coarser mesh, one can screen several candidates for spacer design; by finer mesh, more quantitative analysis is possible.