Nonphysical time is introduced and the lower - upper symmetric successive over - relaxation ( lu - ssor ) scheme is modified in navier - stokes equations . an application to natural unsteady viscous flow is presented to check for the capability of the procedure 在原控制方程中引入伪时间导数项,对上下对称逐次超松弛( lu - ssor )格式进行了修正。