For residual projection, we use a one-way average operator. Assume that x is the dominant direction. The residual at the grid points corresponding to the coarse grid points is averaged with respect to its two neighboring grid points in the x direction only. No averaging is performed with respect to the y direction. Letting ri, j be the residual at fine grid point (i, j ), and r ̄i ̄, j the corresponding coarse grid residual, we have i = 2i ̄, and